15: Distributed Computing

Historically, programming across multiple machines has been error-prone, difficult, and complex.

The programmer had to know many details about the network and sometimes even the hardware. You usually needed to understand the various “layers” of the networking protocol, and there were a lot of different functions in each different networking library concerned with connecting, packing, and unpacking blocks of information; shipping those blocks back and forth; and handshaking. It was a daunting task. [ Add Comment ]

However, the basic idea of distributed computing is not so difficult, and is abstracted very nicely in the Java libraries. You want to:

Each topic will be given a light introduction in this chapter. Please note that each subject is voluminous and by itself the subject of entire books, so this chapter is only meant to familiarize you with the topics, not make you an expert (however, you can go a long way with the information presented here on network programming, servlets and JSPs). [ Add Comment ]

One of Java’s great strengths is painless networking. The Java network library designers have made it quite similar to reading and writing files, except that the “file” exists on a remote machine and the remote machine can decide exactly what it wants to do about the information you’re requesting or sending. As much as possible, the underlying details of networking have been abstracted away and taken care of within the JVM and local machine installation of Java. The programming model you use is that of a file; in fact, you actually wrap the network connection (a “socket”) with stream objects, so you end up using the same method calls as you do with all other streams. In addition, Java’s built-in multithreading is exceptionally handy when dealing with another networking issue: handling multiple connections at once. [ Add Comment ]

This section introduces Java’s networking support using easy-to-understand examples. [ Add Comment ]

Of course, in order to tell one machine from another and to make sure that you are connected with a particular machine, there must be some way of uniquely identifying machines on a network. Early networks were satisfied to provide unique names for machines within the local network. However, Java works within the Internet, which requires a way to uniquely identify a machine from all the others in the world. This is accomplished with the IP (Internet Protocol) address which can exist in two forms“: [ Add Comment ]

In both cases, the IP address is represented internally as a 32-bit number[72] (so each of the quad numbers cannot exceed 255), and you can get a special Java object to represent this number from either of the forms above by using the static InetAddress.getByName( ) method that’s in java.net. The result is an object of type InetAddress that you can use to build a “socket,” as you will see later. [ Add Comment ]

As a simple example of using InetAddress.getByName( ), consider what happens if you have a dial-up Internet service provider (ISP). Each time you dial up, you are assigned a temporary IP address. But while you’re connected, your IP address has the same validity as any other IP address on the Internet. If someone connects to your machine using your IP address then they can connect to a Web server or FTP server that you have running on your machine. Of course, they need to know your IP address, and since a new one is assigned each time you dial up, how can you find out what it is? [ Add Comment ]

The following program uses InetAddress.getByName( ) to produce your IP address. To use it, you must know the name of your computer. On Windows 95/98, go to “Settings,” “Control Panel,” “Network,” and then select the “Identification” tab. “Computer name” is the name to put on the command line.

In this case, the machine is called “peppy.” So, once I’ve connected to my ISP I run the program:

I get back a message like this (of course, the address is different each time):

If I tell my friend this address and I have a Web server running on my computer, he can connect to it by going to the URL http://199.190.87.75 (only as long as I continue to stay connected during that session). This can sometimes be a handy way to distribute information to someone else, or to test out a Web site configuration before posting it to a “real” server. [ Add Comment ]

The whole point of a network is to allow two machines to connect and talk to each other. Once the two machines have found each other they can have a nice, two-way conversation. But how do they find each other? It’s like getting lost in an amusement park: one machine has to stay in one place and listen while the other machine says, “Hey, where are you?” [ Add Comment ]

The machine that “stays in one place” is called the server, and the one that seeks is called the client. This distinction is important only while the client is trying to connect to the server. Once they’ve connected, it becomes a two-way communication process and it doesn’t matter anymore that one happened to take the role of server and the other happened to take the role of the client. [ Add Comment ]

So the job of the server is to listen for a connection, and that’s performed by the special server object that you create. The job of the client is to try to make a connection to a server, and this is performed by the special client object you create. Once the connection is made, you’ll see that at both server and client ends, the connection is magically turned into an I/O stream object, and from then on you can treat the connection as if you were reading from and writing to a file. Thus, after the connection is made you will just use the familiar I/O commands from Chapter 11. This is one of the nice features of Java networking. [ Add Comment ]

For many reasons, you might not have a client machine, a server machine, and a network available to test your programs. You might be performing exercises in a classroom situation, or you could be writing programs that aren’t yet stable enough to put onto the network. The creators of the Internet Protocol were aware of this issue, and they created a special address called localhost to be the “local loopback” IP address for testing without a network. The generic way to produce this address in Java is:

If you hand getByName( ) a null, it defaults to using the localhost. The InetAddress is what you use to refer to the particular machine, and you must produce this before you can go any further. You can’t manipulate the contents of an InetAddress (but you can print them out, as you’ll see in the next example). The only way you can create an InetAddress is through one of that class’s overloaded static member methods getByName( ) (which is what you’ll usually use), getAllByName( ), or getLocalHost( ). [ Add Comment ]

You can also produce the local loopback address by handing it the string localhost:

(assuming “localhost” is configured in your machine’s “hosts” table), or by using its dotted quad form to name the reserved IP number for the loopback:

All three forms produce the same result. [ Add Comment ]

An IP address isn’t enough to identify a unique server, since many servers can exist on one machine. Each IP machine also contains ports, and when you’re setting up a client or a server you must choose a port where both client and server agree to connect; if you’re meeting someone, the IP address is the neighborhood and the port is the bar. [ Add Comment ]

The port is not a physical location in a machine, but a software abstraction (mainly for bookkeeping purposes). The client program knows how to connect to the machine via its IP address, but how does it connect to a desired service (potentially one of many on that machine)? That’s where the port numbers come in as a second level of addressing. The idea is that if you ask for a particular port, you’re requesting the service that’s associated with the port number. The time of day is a simple example of a service. Typically, each service is associated with a unique port number on a given server machine. It’s up to the client to know ahead of time which port number the desired service is running on. [ Add Comment ]

The system services reserve the use of ports 1 through 1024, so you shouldn’t use those or any other port that you know to be in use. The first choice for examples in this book will be port 8080 (in memory of the venerable old 8-bit Intel 8080 chip in my first computer, a CP/M machine). [ Add Comment ]

The socket is the software abstraction used to represent the “terminals” of a connection between two machines. For a given connection, there’s a socket on each machine, and you can imagine a hypothetical “cable” running between the two machines with each end of the “cable” plugged into a socket. Of course, the physical hardware and cabling between machines is completely unknown. The whole point of the abstraction is that we don’t have to know more than is necessary. [ Add Comment ]

In Java, you create a socket to make the connection to the other machine, then you get an InputStream and OutputStream (or, with the appropriate converters, Reader and Writer) from the socket in order to be able to treat the connection as an I/O stream object. There are two stream-based socket classes: a ServerSocket that a server uses to “listen” for incoming connections and a Socket that a client uses in order to initiate a connection. Once a client makes a socket connection, the ServerSocket returns (via the accept( ) method) a corresponding Socket through which communications will take place on the server side. From then on, you have a true Socket to Socket connection and you treat both ends the same way because they are the same. At this point, you use the methods getInputStream( ) and getOutputStream( ) to produce the corresponding InputStream and OutputStream objects from each Socket. These must be wrapped inside buffers and formatting classes just like any other stream object described in Chapter 11. [ Add Comment ]

The use of the term ServerSocket would seem to be another example of a confusing naming scheme in the Java libraries. You might think ServerSocket would be better named “ServerConnector” or something without the word “Socket” in it. You might also think that ServerSocket and Socket should both be inherited from some common base class. Indeed, the two classes do have several methods in common, but not enough to give them a common base class. Instead, ServerSocket’s job is to wait until some other machine connects to it, then to return an actual Socket. This is why ServerSocket seems to be a bit misnamed, since its job isn’t really to be a socket but instead to make a Socket object when someone else connects to it. [ Add Comment ]

However, the ServerSocket does create a physical “server” or listening socket on the host machine. This socket listens for incoming connections and then returns an “established” socket (with the local and remote endpoints defined) via the accept( ) method. The confusing part is that both of these sockets (listening and established) are associated with the same server socket. The listening socket can accept only new connection requests and not data packets. So while ServerSocket doesn’t make much sense programmatically, it does “physically.” [ Add Comment ]

When you create a ServerSocket, you give it only a port number. You don’t have to give it an IP address because it’s already on the machine it represents. When you create a Socket, however, you must give both the IP address and the port number where you’re trying to connect. (However, the Socket that comes back from ServerSocket.accept( ) already contains all this information.) [ Add Comment ]

This example makes the simplest use of servers and clients using sockets. All the server does is wait for a connection, then uses the Socket produced by that connection to create an InputStream and OutputStream. These are converted to a Reader and a Writer, then wrapped in a BufferedReader and a PrintWriter. After that, everything it reads from the BufferedReader it echoes to the PrintWriter until it receives the line “END,” at which time it closes the connection. [ Add Comment ]

The client makes the connection to the server, then creates an OutputStream and performs the same wrapping as in the server. Lines of text are sent through the resulting PrintWriter. The client also creates an InputStream (again, with appropriate conversions and wrapping) to hear what the server is saying (which, in this case, is just the words echoed back). [ Add Comment ]

Both the server and client use the same port number and the client uses the local loopback address to connect to the server on the same machine so you don’t have to test it over a network. (For some configurations, you might need to be connected to a network for the programs to work, even if you aren’t communicating over that network.) [ Add Comment ]

Here is the server:

You can see that the ServerSocket just needs a port number, not an IP address (since it’s running on this machine!). When you call accept( ), the method blocks until some client tries to connect to it. That is, it’s there waiting for a connection, but other processes can run (see Chapter 14). When a connection is made, accept( ) returns with a Socket object representing that connection. [ Add Comment ]

The responsibility for cleaning up the sockets is crafted carefully here. If the ServerSocket constructor fails, the program just quits (notice we must assume that the constructor for ServerSocket doesn’t leave any open network sockets lying around if it fails). For this case, main( ) throws IOException so a try block is not necessary. If the ServerSocket constructor is successful then all other method calls must be guarded in a try-finally block to ensure that, no matter how the block is left, the ServerSocket is properly closed. [ Add Comment ]

The same logic is used for the Socket returned by accept( ). If accept( ) fails, then we must assume that the Socket doesn’t exist or hold any resources, so it doesn’t need to be cleaned up. If it’s successful, however, the following statements must be in a try-finally block so that if they fail the Socket will still be cleaned up. Care is required here because sockets use important nonmemory resources, so you must be diligent in order to clean them up (since there is no destructor in Java to do it for you). [ Add Comment ]

Both the ServerSocket and the Socket produced by accept( ) are printed to System.out. This means that their toString( ) methods are automatically called. These produce:

Shortly, you’ll see how these fit together with what the client is doing. [ Add Comment ]

The next part of the program looks just like opening files for reading and writing except that the InputStream and OutputStream are created from the Socket object. Both the InputStream and OutputStream objects are converted to Reader and Writer objects using the “converter” classes InputStreamReader and OutputStreamWriter, respectively. You could also have used the Java 1.0 InputStream and OutputStream classes directly, but with output there’s a distinct advantage to using the Writer approach. This appears with PrintWriter, which has an overloaded constructor that takes a second argument, a boolean flag that indicates whether to automatically flush the output at the end of each println( ) (but not print( )) statement. Every time you write to out, its buffer must be flushed so the information goes out over the network. Flushing is important for this particular example because the client and server each wait for a line from the other party before proceeding. If flushing doesn’t occur, the information will not be put onto the network until the buffer is full, which causes lots of problems in this example. [ Add Comment ]

When writing network programs you need to be careful about using automatic flushing. Every time you flush the buffer a packet must be created and sent. In this case, that’s exactly what we want, since if the packet containing the line isn’t sent then the handshaking back and forth between server and client will stop. Put another way, the end of a line is the end of a message. But in many cases, messages aren’t delimited by lines so it’s much more efficient to not use auto flushing and instead let the built-in buffering decide when to build and send a packet. This way, larger packets can be sent and the process will be faster. [ Add Comment ]

Note that, like virtually all streams you open, these are buffered. There’s an exercise at the end of this chapter to show you what happens if you don’t buffer the streams (things get slow). [ Add Comment ]

The infinite while loop reads lines from the BufferedReader in and writes information to System.out and to the PrintWriter out. Note that in and out could be any streams, they just happen to be connected to the network. [ Add Comment ]

When the client sends the line consisting of “END,” the program breaks out of the loop and closes the Socket.

Here’s the client:

In main( ) you can see all three ways to produce the InetAddress of the local loopback IP address: using null, localhost, or the explicit reserved address 127.0.0.1. Of course, if you want to connect to a machine across a network you substitute that machine’s IP address. When the InetAddress addr is printed (via the automatic call to its toString( ) method) the result is: [ Add Comment ]

By handing getByName( ) a null, it defaulted to finding the localhost, and that produced the special address 127.0.0.1. [ Add Comment ]

Note that the Socket called socket is created with both the InetAddress and the port number. To understand what it means when you print one of these Socket objects, remember that an Internet connection is determined uniquely by these four pieces of data: clientHost, clientPortNumber, serverHost, and serverPortNumber. When the server comes up, it takes up its assigned port (8080) on the localhost (127.0.0.1). When the client comes up, it is allocated to the next available port on its machine, 1077 in this case, which also happens to be on the same machine (127.0.0.1) as the server. Now, in order for data to move between the client and server, each side has to know where to send it. Therefore, during the process of connecting to the “known” server, the client sends a “return address” so the server knows where to send its data. This is what you see in the example output for the server side:

This means that the server just accepted a connection from 127.0.0.1 on port 1077 while listening on its local port (8080). On the client side: [ Add Comment ]

which means that the client made a connection to 127.0.0.1 on port 8080 using the local port 1077. [ Add Comment ]

You’ll notice that every time you start up the client anew, the local port number is incremented. It starts at 1025 (one past the reserved block of ports) and keeps going up until you reboot the machine, at which point it starts at 1025 again. (On UNIX machines, once the upper limit of the socket range is reached, the numbers will wrap around to the lowest available number again.) [ Add Comment ]

Once the Socket object has been created, the process of turning it into a BufferedReader and PrintWriter is the same as in the server (again, in both cases you start with a Socket). Here, the client initiates the conversation by sending the string “howdy” followed by a number. Note that the buffer must again be flushed (which happens automatically via the second argument to the PrintWriter constructor). If the buffer isn’t flushed, the whole conversation will hang because the initial “howdy” will never get sent (the buffer isn’t full enough to cause the send to happen automatically). Each line that is sent back from the server is written to System.out to verify that everything is working correctly. To terminate the conversation, the agreed-upon “END” is sent. If the client simply hangs up, then the server throws an exception. [ Add Comment ]

You can see that the same care is taken here to ensure that the network resources represented by the Socket are properly cleaned up, using a try-finally block. [ Add Comment ]

Sockets produce a “dedicated” connection that persists until it is explicitly disconnected. (The dedicated connection can still be disconnected unexplicitly if one side, or an intermediary link, of the connection crashes.) This means the two parties are locked in communication and the connection is constantly open. This seems like a logical approach to networking, but it puts an extra load on the network. Later in this chapter you’ll see a different approach to networking, in which the connections are only temporary. [ Add Comment ]

The JabberServer works, but it can handle only one client at a time. In a typical server, you’ll want to be able to deal with many clients at once. The answer is multithreading, and in languages that don’t directly support multithreading this means all sorts of complications. In Chapter 14 you saw that multithreading in Java is about as simple as possible, considering that multithreading is a rather complex topic. Because threading in Java is reasonably straightforward, making a server that handles multiple clients is relatively easy. [ Add Comment ]

The basic scheme is to make a single ServerSocket in the server and call accept( ) to wait for a new connection. When accept( ) returns, you take the resulting Socket and use it to create a new thread whose job is to serve that particular client. Then you call accept( ) again to wait for a new client. [ Add Comment ]

In the following server code, you can see that it looks similar to the JabberServer.java example except that all of the operations to serve a particular client have been moved inside a separate thread class:

The ServeOneJabber thread takes the Socket object that’s produced by accept( ) in main( ) every time a new client makes a connection. Then, as before, it creates a BufferedReader and auto-flushed PrintWriter object using the Socket. Finally, it calls the special Thread method start( ), which performs thread initialization and then calls run( ). This performs the same kind of action as in the previous example: reading something from the socket and then echoing it back until it reads the special “END” signal. [ Add Comment ]

The responsibility for cleaning up the socket must again be carefully designed. In this case, the socket is created outside of the ServeOneJabber so the responsibility can be shared. If the ServeOneJabber constructor fails, it will just throw the exception to the caller, who will then clean up the thread. But if the constructor succeeds, then the ServeOneJabber object takes over responsibility for cleaning up the thread, in its run( ). [ Add Comment ]

Notice the simplicity of the MultiJabberServer. As before, a ServerSocket is created and accept( ) is called to allow a new connection. But this time, the return value of accept( ) (a Socket) is passed to the constructor for ServeOneJabber, which creates a new thread to handle that connection. When the connection is terminated, the thread simply goes away. [ Add Comment ]

If the creation of the ServerSocket fails, the exception is again thrown through main( ). But if the creation succeeds, the outer try-finally guarantees its cleanup. The inner try-catch guards only against the failure of the ServeOneJabber constructor; if the constructor succeeds, then the ServeOneJabber thread will close the associated socket. [ Add Comment ]

To test that the server really does handle multiple clients, the following program creates many clients (using threads) that connect to the same server. The maximum number of threads allowed is determined by the final int MAX_THREADS.

The JabberClientThread constructor takes an InetAddress and uses it to open a Socket. You’re probably starting to see the pattern: the Socket is always used to create some kind of Reader and/or Writer (or InputStream and/or OutputStream) object, which is the only way that the Socket can be used. (You can, of course, write a class or two to automate this process instead of doing all the typing if it becomes painful.) Again, start( ) performs thread initialization and calls run( ). Here, messages are sent to the server and information from the server is echoed to the screen. However, the thread has a limited lifetime and eventually completes. Note that the socket is cleaned up if the constructor fails after the socket is created but before the constructor completes. Otherwise the responsibility for calling close( ) for the socket is relegated to the run( ) method. [ Add Comment ]

The threadcount keeps track of how many JabberClientThread objects currently exist. It is incremented as part of the constructor and decremented as run( ) exits (which means the thread is terminating). In MultiJabberClient.main( ), you can see that the number of threads is tested, and if there are too many, no more are created. Then the method sleeps. This way, some threads will eventually terminate and more can be created. You can experiment with MAX_THREADS to see where your particular system begins to have trouble with too many connections. [ Add Comment ]

The examples you’ve seen so far use the Transmission Control Protocol (TCP, also known as stream-based sockets), which is designed for ultimate reliability and guarantees that the data will get there. It allows retransmission of lost data, it provides multiple paths through different routers in case one goes down, and bytes are delivered in the order they are sent. All this control and reliability comes at a cost: TCP has a high overhead. [ Add Comment ]

There’s a second protocol, called User Datagram Protocol (UDP), which doesn’t guarantee that the packets will be delivered and doesn’t guarantee that they will arrive in the order they were sent. It’s called an “unreliable protocol” (TCP is a “reliable protocol”), which sounds bad, but because it’s much faster it can be useful. There are some applications, such as an audio signal, in which it isn’t so critical if a few packets are dropped here or there but speed is vital. Or consider a time-of-day server, where it really doesn’t matter if one of the messages is lost. Also, some applications might be able to fire off a UDP message to a server and can then assume, if there is no response in a reasonable period of time, that the message was lost. [ Add Comment ]

Typically, you’ll do most of your direct network programming with TCP, and only occasionally will you use UDP. There’s a more complete treatment of UDP, including an example, in the first edition of this book (available on the CD ROM bound into this book, or as a free download from www.BruceEckel.com). [ Add Comment ]

It’s possible for an applet to cause the display of any URL through the Web browser the applet is running within. You can do this with the following line:

in which u is the URL object. Here’s a simple example that redirects you to another Web page. Although you’re just redirected to an HTML page, you could also redirect to the output of a CGI program. [ Add Comment ]

The beauty of the URL class is how much it shields you from. You can connect to Web servers without knowing much at all about what’s going on under the covers. [ Add Comment ]

A variation on the above program reads a file located on the server. In this case, the file is specified by the client:

The creation of the URL object is similar to the previous example—getDocumentBase( ) is the starting point as before, but this time the name of the file is read from the JTextField. Once the URL object is created, its String version is placed in the JTextArea so we can see what it looks like. Then an InputStream is procured from the URL, which in this case will simply produce a stream of the characters in the file. After converting to a Reader and buffering, each line is read and appended to the JTextArea. Note that the JTextArea has been placed inside a JScrollPane so that scrolling is handled automatically. [ Add Comment ]

There’s actually a lot more to networking than can be covered in this introductory treatment. Java networking also provides fairly extensive support for URLs, including protocol handlers for different types of content that can be discovered at an Internet site. You can find other Java networking features fully and carefully described in Java Network Programming by Elliotte Rusty Harold (O’Reilly, 1997). [ Add Comment ]

It has been estimated that half of all software development involves client/server operations. A great promise of Java has been the ability to build platform-independent client/server database applications. This has come to fruition with Java DataBase Connectivity (JDBC). [ Add Comment ]

One of the major problems with databases has been the feature wars between the database companies. There is a “standard” database language, Structured Query Language (SQL-92), but you must usually know which database vendor you’re working with despite the standard. JDBC is designed to be platform-independent, so you don’t need to worry about the database you’re using while you’re programming. However, it’s still possible to make vendor-specific calls from JDBC so you aren’t restricted from doing what you must. [ Add Comment ]

One place where programmers may need to use SQL type names is in the SQL TABLE CREATE statement when they are creating a new database table and defining the SQL type for each column. Unfortunately there are significant variations between SQL types supported by different database products. Different databases that support SQL types with the same semantics and structure may give those types different names. Most major databases support an SQL data type for large binary values: in Oracle this type is called a LONG RAW, Sybase calls it IMAGE, Informix calls it BYTE, and DB2 calls it LONG VARCHAR FOR BIT DATA. Therefore, if database portability is a goal you should try to use only generic SQL type identifiers. [ Add Comment ]

Portability is an issue when writing for a book where readers may be testing the examples with all kinds of unknown data stores. I have tried to write these examples to be as portable as possible. You should also notice that the database-specific code has been isolated in order to centralize any changes that you may need to perform to get the examples operational in your environment. [ Add Comment ]

JDBC, like many of the APIs in Java, is designed for simplicity. The method calls you make correspond to the logical operations you’d think of doing when gathering data from a database: connect to the database, create a statement and execute the query, and look at the result set. [ Add Comment ]

To allow this platform independence, JDBC provides a driver manager that dynamically maintains all the driver objects that your database queries will need. So if you have three different kinds of vendor databases to connect to, you’ll need three different driver objects. The driver objects register themselves with the driver manager at the time of loading, and you can force the loading using Class.forName( ). [ Add Comment ]

To open a database, you must create a “database URL” that specifies:

All this information is combined into one string, the “database URL.” For example, to connect through the ODBC subprotocol to a database identified as “people,” the database URL could be: [ Add Comment ]

If you’re connecting across a network, the database URL will contain the connection information identifying the remote machine and can become a bit intimidating. Here is an example of a CloudScape database being called from a remote client utilizing RMI: [ Add Comment ]

This database URL is really two jdbc calls in one. The first part “jdbc:rmi://192.168.170.27:1099/” uses RMI to make the connection to the remote database engine listening on port 1099 at IP Address 192.168.170.27. The second part of the URL, “jdbc:cloudscape:db” conveys the more typical settings using the subprotocol and database name but this will only happen after the first section has made the connection via RMI to the remote machine. [ Add Comment ]

When you’re ready to connect to the database, call the static method DriverManager.getConnection( ) and pass it the database URL, the user name, and a password to get into the database. You get back a Connection object that you can then use to query and manipulate the database. [ Add Comment ]

The following example opens a database of contact information and looks for a person’s last name as given on the command line. It selects only the names of people that have email addresses, then prints out all the ones that match the given last name:

You can see the creation of the database URL as previously described. In this example, there is no password protection on the database so the user name and password are empty strings. [ Add Comment ]

Once the connection is made with DriverManager.getConnection( ), you can use the resulting Connection object to create a Statement object using the createStatement( ) method. With the resulting Statement, you can call executeQuery( ), passing in a string containing an SQL-92 standard SQL statement. (You’ll see shortly how you can generate this statement automatically, so you don’t have to know much about SQL.) [ Add Comment ]

The executeQuery( ) method returns a ResultSet object, which is an iterator: the next( ) method moves the iterator to the next record in the statement, or returns false if the end of the result set has been reached. You’ll always get a ResultSet object back from executeQuery( ) even if a query results in an empty set (that is, an exception is not thrown). Note that you must call next( ) once before trying to read any record data. If the result set is empty, this first call to next( ) will return false. For each record in the result set, you can select the fields using (among other approaches) the field name as a string. Also note that the capitalization of the field name is ignored—it doesn’t matter with an SQL database. You determine the type you’ll get back by calling getInt( ), getString( ), getFloat( ), etc. At this point, you’ve got your database data in Java native format and can do whatever you want with it using ordinary Java code. [ Add Comment ]

With JDBC, understanding the code is relatively simple. The confusing part is making it work on your particular system. The reason this is confusing is that it requires you to figure out how to get your JDBC driver to load properly, and how to set up a database using your database administration software. [ Add Comment ]

Of course, this process can vary radically from machine to machine, but the process I used to make it work under 32-bit Windows might give you clues to help you attack your own situation. [ Add Comment ]

The program above contains the statement:

This implies a directory structure, which is deceiving. With this particular installation of JDK 1.1, there was no file called JdbcOdbcDriver.class, so if you looked at this example and went searching for it you’d be frustrated. Other published examples use a pseudo name, such as “myDriver.ClassName,” which is less than helpful. In fact, the load statement above for the jdbc-odbc driver (the only one that actually comes with the JDK) appears in only a few places in the online documentation (in particular, a page labeled “JDBC-ODBC Bridge Driver”). If the load statement above doesn’t work, then the name might have been changed as part of a Java version change, so you should hunt through the documentation again. [ Add Comment ]

If the load statement is wrong, you’ll get an exception at this point. To test whether your driver load statement is working correctly, comment out the code after the statement and up to the catch clause; if the program throws no exceptions it means that the driver is loading properly. [ Add Comment ]

Again, this is specific to 32-bit Windows; you might need to do some research to figure it out for your own platform. [ Add Comment ]

First, open the control panel. You might find two icons that say “ODBC.” You must use the one that says “32bit ODBC,” since the other one is for backward compatibility with 16-bit ODBC software and will produce no results for JDBC. When you open the “32bit ODBC” icon, you’ll see a tabbed dialog with a number of tabs, including “User DSN,” “System DSN,” “File DSN,” etc., in which “DSN” means “Data Source Name.” It turns out that for the JDBC-ODBC bridge, the only place where it’s important to set up your database is “System DSN,” but you’ll also want to test your configuration and create queries, and for that you’ll also need to set up your database in “File DSN.” This will allow the Microsoft Query tool (that comes with Microsoft Office) to find the database. Note that other query tools are also available from other vendors. [ Add Comment ]

The most interesting database is one that you’re already using. Standard ODBC supports a number of different file formats including such venerable workhorses as DBase. However, it also includes the simple “comma-separated ASCII” format, which virtually every data tool has the ability to write. In my case, I just took my “people” database that I’ve been maintaining for years using various contact-management tools and exported it as a comma-separated ASCII file (these typically have an extension of .csv). In the “System DSN” section I chose “Add,” chose the text driver to handle my comma-separated ASCII file, and then un-checked “use current directory” to allow me to specify the directory where I exported the data file. [ Add Comment ]

You’ll notice when you do this that you don’t actually specify a file, only a directory. That’s because a database is typically represented as a collection of files under a single directory (although it could be represented in other forms as well). Each file usually contains a single table, and the SQL statements can produce results that are culled from multiple tables in the database (this is called a join). A database that contains only a single table (like my “people” database) is usually called a flat-file database. Most problems that go beyond the simple storage and retrieval of data generally require multiple tables that must be related by joins to produce the desired results, and these are called relational databases. [ Add Comment ]

To test the configuration you’ll need a way to discover whether the database is visible from a program that queries it. Of course, you can simply run the JDBC program example above, up to and including the statement:

If an exception is thrown, your configuration was incorrect. [ Add Comment ]

However, it’s useful to get a query-generation tool involved at this point. I used Microsoft Query that came with Microsoft Office, but you might prefer something else. The query tool must know where the database is, and Microsoft Query required that I go to the ODBC Administrator’s “File DSN” tab and add a new entry there, again specifying the text driver and the directory where my database lives. You can name the entry anything you want, but it’s helpful to use the same name you used in “System DSN.” [ Add Comment ]

Once you’ve done this, you will see that your database is available when you create a new query using your query tool. [ Add Comment ]

The query that I created using Microsoft Query not only showed me that my database was there and in good order, but it also automatically created the SQL code that I needed to insert into my Java program. I wanted a query that would search for records that had the last name that was typed on the command line when starting the Java program. So as a starting point, I searched for a specific last name, “Eckel.” I also wanted to display only those names that had email addresses associated with them. The steps I took to create this query were:

The result of this query will show you whether you’re getting what you want. [ Add Comment ]

Now you can press the SQL button and without any research on your part, up will pop the correct SQL code, ready for you to cut and paste. For this query, it looked like this:

Especially with more complicated queries it’s easy to get things wrong, but by using a query tool you can interactively test your queries and automatically generate the correct code. It’s hard to argue the case for doing this by hand. [ Add Comment ]

You’ll notice that the code above looks different from what’s used in the program. That’s because the query tool uses full qualification for all of the names, even when there’s only one table involved. (When more than one table is involved, the qualification prevents collisions between columns from different tables that have the same names.) Since this query involves only one table, you can optionally remove the “people” qualifier from most of the names, like this: [ Add Comment ]

In addition, you don’t want this program to be hard coded to look for only one name. Instead, it should hunt for the name given as the command-line argument. Making these changes and turning the SQL statement into a dynamically-created String produces: [ Add Comment ]

SQL has another way to insert names into a query called stored procedures, which is used for speed. But for much of your database experimentation and for your first cut, building your own query strings in Java is fine. [ Add Comment ]

You can see from this example that by using the tools currently available—in particular the query-building tool—database programming with SQL and JDBC can be quite straightforward. [ Add Comment ]

It’s more useful to leave the lookup program running all the time and simply switch to it and type in a name whenever you want to look someone up. The following program creates the lookup program as an application/applet, and it also adds name completion so the data will show up without forcing you to type the entire last name:

Much of the database logic is the same, but you can see that a DocumentListener is added to listen to the JTextField (see the javax.swing.JTextField entry in the Java HTML documentation from java.sun.com for details), so that whenever you type a new character it first tries to do a name completion by looking up the last name in the database and using the first one that shows up. (It places it in the completion JLabel, and uses that as the lookup text.) This way, as soon as you’ve typed enough characters for the program to uniquely find the name you’re looking for, you can stop. [ Add Comment ]

When you browse the online documentation for JDBC it can seem daunting. In particular, in the DatabaseMetaData interface—which is just huge, contrary to most of the interfaces you see in Java—there are methods such as dataDefinitionCausesTransactionCommit( ), getMaxColumnNameLength( ), getMaxStatementLength( ), storesMixedCaseQuotedIdentifiers( ), supportsANSI92IntermediateSQL( ), supportsLimitedOuterJoins( ), and so on. What’s this all about? [ Add Comment ]

As mentioned earlier, databases have seemed from their inception to be in a constant state of turmoil, primarily because the demand for database applications, and thus database tools, is so great. Only recently has there been any convergence on the common language of SQL (and there are plenty of other database languages in common use). But even with an SQL “standard” there are so many variations on that theme that JDBC must provide the large DatabaseMetaData interface so that your code can discover the capabilities of the particular “standard” SQL database that it’s currently connected to. In short, you can write simple, transportable SQL, but if you want to optimize speed your coding will multiply tremendously as you investigate the capabilities of a particular vendor’s database. [ Add Comment ]

This, of course, is not Java’s fault. The discrepancies between database products are just something that JDBC tries to help compensate for. But bear in mind that your life will be easier if you can either write generic queries and not worry quite as much about performance, or, if you must tune for performance, know the platform you’re writing for so you don’t need to write all that investigation code. [ Add Comment ]

A more interesting example[73] involves a multitable database that resides on a server. Here, the database is meant to provide a repository for community activities and to allow people to sign up for these events, so it is called the Community Interests Database (CID). This example will only provide an overview of the database and its implementation, and is not intended to be an in-depth tutorial on database development. There are numerous books, seminars, and software packages that will help you in the design and development of a database. [ Add Comment ]

In addition, this example presumes the prior installation of an SQL database on a server (although it could also be run on a local machine), and the interrogation and discovery of an appropriate JDBC driver for that database. Several free SQL databases are available, and some are even automatically installed with various flavors of Linux. You are responsible for making the choice of database and locating the JDBC driver; the example here is based on an SQL database system called “Cloudscape.” [ Add Comment ]

To keep changes in the connection information simple, the database driver, database URL, user name, and password are placed in a separate class:

In this example, there is no password protection on the database so the user name and password are empty strings. [ Add Comment ]

The database consists of a set of tables that have a structure as shown here:

“Members” contains community member information, “Events” and “Locations” contain information about the activities and where they take place, and “Evtmems” connects events and members that would like to attend that event. You can see that a data member in one table produces a key in another table. [ Add Comment ]

The following class contains the SQL strings that will create these database tables (refer to an SQL guide for an explanation of the SQL code): [ Add Comment ]

The following program uses the CIDConnect and CIDSQL information to load the JDBC driver, make a connection to the database, and then create the table structure diagrammed above. To connect with the database, you call the static method DriverManager.getConnection( ), passing it the database URL, the user name, and a password to get into the database. You get back a Connection object that you can use to query and manipulate the database. Once the connection is made you can simply push the SQL to the database, in this case by marching through the CIDSQL array. However, the first time this program is run, the “drop table” command will fail, causing an exception, which is caught, reported, and then ignored. The reason for the “drop table” command is to allow easy experimentation: you can modify the SQL that defines the tables and then rerun the program, causing the old tables to be replaced by the new. [ Add Comment ]

In this example, it makes sense to let the exceptions be thrown out to the console:

Note that all changes in the database can be controlled by changing Strings in the CIDSQL table, without modifying CIDCreateTables. [ Add Comment ]

executeUpdate( ) will usually return the number of rows that were affected by the SQL statement. executeUpdate( ) is more commonly used to execute INSERT, UPDATE, or DELETE statements that modify one or more rows. For statements such as CREATE TABLE, DROP TABLE, and CREATE INDEX, executeUpdate( ) always returns zero. [ Add Comment ]

To test the database, it is loaded with some sample data. This requires a series of INSERTs followed by a SELECT to produce result set. To make additions and changes to the test data easy, the test data is set up as a two-dimensional array of Objects, and the executeInsert( ) method can then use the information in one row of the table to create the appropriate SQL command.

The TestSet class contains a default set of data that is produced if you use the default constructor; however, you can also create a TestSet object using an alternate data set with the second constructor. The set of data is held in a two-dimensional array of Object because it can be any type, including String or numerical types. The executeInsert( ) method uses RTTI to distinguish between String data (which must be quoted) and non-String data as it builds the SQL command from the data. After printing this command to the console, executeUpdate( ) is used to send it to the database. [ Add Comment ]

The constructor for LoadDB makes the connection, and load( ) steps through the data and calls executeInsert( ) for each record. cleanup( ) closes the statement and the connection; to guarantee that this is called, it is placed inside a finally clause. [ Add Comment ]

Once the database is loaded, an executeQuery( ) statement produces a sample result set. Since the query combines several tables, it is an example of a join. [ Add Comment ]

There is more JDBC information available in the electronic documents that come as part of the Java distribution from Sun. In addition, you can find more in the book JDBC Database Access with Java (Hamilton, Cattel, and Fisher, Addison-Wesley, 1997). Other JDBC books appear regularly. [ Add Comment ]

Client access from the Internet or corporate intranets is a sure way to allow many users to access data and resources easily[74]. This type of access is based on clients using the World Wide Web standards of Hypertext Markup Language (HTML) and Hypertext Transfer Protocol (HTTP). The Servlet API set abstracts a common solution framework for responding to HTTP requests. [ Add Comment ]

Traditionally, the way to handle a problem such as allowing an Internet client to update a database is to create an HTML page with text fields and a “submit” button. The user types the appropriate information into the text fields and presses the “submit” button. The data is submitted along with a URL that tells the server what to do with the data by specifying the location of a Common Gateway Interface (CGI) program that the server runs, providing the program with the data as it is invoked. The CGI program is typically written in Perl, Python, C, C++, or any language that can read from standard input and write to standard output. That’s all that is provided by the Web server: the CGI program is invoked, and standard streams (or, optionally for input, an environment variable) are used for input and output. The CGI program is responsible for everything else. First it looks at the data and decides whether the format is correct. If not, the CGI program must produce HTML to describe the problem; this page is handed to the Web server (via standard output from the CGI program), which sends it back to the user. The user must usually back up a page and try again. If the data is correct, the CGI program processes the data in an appropriate way, perhaps adding it to a database. It must then produce an appropriate HTML page for the Web server to return to the user. [ Add Comment ]

It would be ideal to go to a completely Java-based solution to this problem—an applet on the client side to validate and send the data, and a servlet on the server side to receive and process the data. Unfortunately, although applets are a proven technology with plenty of support, they have been problematic to use on the Web because you cannot rely on a particular version of Java being available on a client’s Web browser; in fact, you can’t rely on a Web browser supporting Java at all! In an intranet, you can require that certain support be available, which allows a lot more flexibility in what you can do, but on the Web the safest approach is to handle all the processing on the server side and deliver plain HTML to the client. That way, no client will be denied the use of your site because they do not have the proper software installed. [ Add Comment ]

Because servlets provide an excellent solution for server-side programming support, they are one of the most popular reasons for moving to Java. Not only do they provide a framework that replaces CGI programming (and eliminates a number of thorny CGI problems), but all your code has the platform portability gained from using Java, and you have access to all the Java APIs (except, of course, the ones that produce GUIs, like Swing). [ Add Comment ]

The architecture of the servlet API is that of a classic service provider with a service( ) method through which all client requests will be sent by the servlet container software, and life cycle methods init( ) and destroy( ), which are called only when the servlet is loaded and unloaded (this happens rarely). [ Add Comment ]

getServletConfig( )’s sole purpose is to return a ServletConfig object that contains initialization and startup parameters for this servlet. getServletInfo( ) returns a string containing information about the servlet, such as author, version, and copyright. [ Add Comment ]

The GenericServlet class is a shell implementation of this interface and is typically not used. The HttpServlet class is an extension of GenericServlet and is designed specifically to handle the HTTP protocol— HttpServlet is the one that you’ll use most of the time. [ Add Comment ]

The most convenient attribute of the servlet API is the auxiliary objects that come along with the HttpServlet class to support it. If you look at the service( ) method in the Servlet interface, you’ll see it has two parameters: ServletRequest and ServletResponse. With the HttpServlet class these two object are extended for HTTP: HttpServletRequest and HttpServletResponse. Here’s a simple example that shows the use of HttpServletResponse:

ServletsRule is about as simple as a servlet can get. The servlet is initialized only once by calling its init( ) method, on loading the servlet after the servlet container is first booted up. When a client makes a request to a URL that happens to represent a servlet, the servlet container intercepts this request and makes a call to the service( ) method, after setting up the HttpServletRequest and HttpServletResponse objects. [ Add Comment ]

The main responsibility of the service( ) method is to interact with the HTTP request that the client has sent, and to build an HTTP response based on the attributes contained within the request. ServletsRule only manipulates the response object without looking at what the client may have sent. [ Add Comment ]

After setting the content type of the response (which must always be done before the Writer or OutputStream is procured), the getWriter( ) method of the response object produces a PrintWriter object, which is used for writing character-based response data (alternatively, getOutputStream( ) produces an OutputStream, used for binary response, which is only utilized in more specialized solutions). [ Add Comment ]

The rest of the program simply sends HTML back to the client (it’s assumed you understand HTML, so that part is not explained) as a sequence of Strings. However, notice the inclusion of the “hit counter” represented by the variable i. This is automatically converted to a String in the print( ) statement. [ Add Comment ]

When you run the program, you’ll notice that the value of i is retained between requests to the servlet. This is an essential property of servlets: since only one servlet of a particular class is loaded into the container, and it is never unloaded (unless the servlet container is terminated, which is something that only normally happens if you reboot the server computer), any fields of that servlet class effectively become persistent objects! This means that you can effortlessly maintain values between servlet requests, whereas with CGI you had to write values to disk in order to preserve them, which required a fair amount of fooling around to get it right, and resulted in a non-cross-platform solution. [ Add Comment ]

Of course, sometimes the Web server, and thus the servlet container, must be rebooted as part of maintenance or during a power failure. To avoid losing any persistent information, the servlet’s init( ) and destroy( ) methods are automatically called whenever the servlet is loaded or unloaded, giving you the opportunity to save data during shutdown, and restore it after rebooting. The servlet container calls the destroy( ) method as it is terminating itself, so you always get an opportunity to save valuable data as long as the server machine is configured in an intelligent way. [ Add Comment ]

There’s one other issue when using HttpServlet. This class provides doGet( ) and doPost( ) methods that differentiate between a CGI “GET” submission from the client, and a CGI “POST.” GET and POST vary only in the details of the way that they submit the data, which is something that I personally would prefer to ignore. However, most published information that I’ve seen seems to favor the creation of separate doGet( ) and doPost( ) methods instead of a single generic service( ) method, which handles both cases. This favoritism seems quite common, but I’ve never seen it explained in a fashion that leads me to believe that it’s anything more than inertia from CGI programmers who are used to paying attention to whether a GET or POST is being used. So in the spirit of “doing the simplest thing that could possibly work,”[75] I will just use the service( ) method in these examples, and let it care about GETs vs. POSTs. However, keep in mind that I might have missed something and so there may in fact be a good reason to use doGet( ) and doPost( ) instead. [ Add Comment ]

Whenever a form is submitted to a servlet, the HttpServletRequest comes preloaded with all the form data, stored as key-value pairs. If you know the names of the fields, you can just use them directly with the getParameter( ) method to look up the values. You can also get an Enumeration (the old form of the Iterator) to the field names, as is shown in the following example. This example also demonstrates how a single servlet can be used to produce the page that contains the form, and to respond to the page (a better solution will be seen later, with JSPs). If the Enumeration is empty, there are no fields; this means no form was submitted. In this case, the form is produced, and the submit button will re-call the same servlet. If fields do exist, however, they are displayed.

One drawback you’ll notice here is that Java does not seem to be designed with string processing in mind—the formatting of the return page is painful because of line breaks, escaping quote marks, and the “+” signs necessary to build String objects. With a larger HTML page it becomes unreasonable to code it directly into Java. One solution is to keep the page as a separate text file, then open it and hand it to the Web server. If you have to perform any kind of substitution to the contents of the page, it’s not much better since Java has treated string processing so poorly. In these cases you’re probably better off using a more appropriate solution (Python would be my choice; there’s a version that embeds itself in Java called JPython) to generate the response page. [ Add Comment ]

The servlet container has a pool of threads that it will dispatch to handle client requests. It is quite likely that two clients arriving at the same time could be processing through your service( ) at the same time. Therefore the service( ) method must written in a thread-safe manner. Any access to common resources (files, databases) will need to be guarded by using the synchronized keyword. [ Add Comment ]

The following simple example puts a synchronized clause around the thread’s sleep( ) method. This will block all other threads until the allotted time (five seconds) is all used up. When testing this you should start several browser instances and hit this servlet as quickly as possible in each one—you’ll see that each one has to wait until its turn comes up.

It is also possible to synchronize the entire servlet by putting the synchronized keyword in front of the service( ) method. In fact, the only reason to use the synchronized clause instead is if the critical section is in an execution path that might not get executed. In that case, you might as well avoid the overhead of synchronizing every time by using a synchronized clause. Otherwise, all the threads will have to wait anyway so you might as well synchronize the whole method. [ Add Comment ]

HTTP is a “sessionless” protocol, so you cannot tell from one server hit to another if you’ve got the same person repeatedly querying your site, or if it is a completely different person. A great deal of effort has gone into mechanisms that will allow Web developers to track sessions. Companies could not do e-commerce without keeping track of a client and the items they have put into their shopping cart, for example. [ Add Comment ]

There are several methods of session tracking, but the most common method is with persistent “cookies,” which are an integral part of the Internet standards. The HTTP Working Group of the Internet Engineering Task Force has written cookies into the official standard in RFC 2109 (ds.internic.net/rfc/rfc2109.txt or check www.cookiecentral.com). [ Add Comment ]

A cookie is nothing more than a small piece of information sent by a Web server to a browser. The browser stores the cookie on the local disk, and whenever another call is made to the URL that the cookie is associated with, the cookie is quietly sent along with the call, thus providing the desired information back to that server (generally, providing some way that the server can be told that it’s you calling). Clients can, however, turn off the browser’s ability to accept cookies. If your site must track a client who has turned off cookies, then another method of session tracking (URL rewriting or hidden form fields) must be incorporated by hand, since the session tracking capabilities built into the servlet API are designed around cookies. [ Add Comment ]

The servlet API (version 2.0 and up) provides the Cookie class. This class incorporates all the HTTP header details and allows the setting of various cookie attributes. Using the cookie is simply a matter of adding it to the response object. The constructor takes a cookie name as the first argument and a value as the second. Cookies are added to the response object before you send any content.

Cookies are recovered by calling the getCookies( ) method of the HttpServletRequest object, which returns an array of cookie objects.

You can then call getValue( ) for each cookie, to produce a String containing the cookie contents. In the above example, getValue("TIJava") will produce a String containing “2000.” [ Add Comment ]

A session is one or more page requests by a client to a Web site during a defined period of time. If you buy groceries online, for example, you want a session to be confined to the period from when you first add an item to “my shopping cart” to the point where you check out. Each item you add to the shopping cart will result in a new HTTP connection, which has no knowledge of previous connections or items in the shopping cart. To compensate for this lack of information, the mechanics supplied by the cookie specification allow your servlet to perform session tracking. [ Add Comment ]

A servlet Session object lives on the server side of the communication channel; its goal is to capture useful data about this client as the client moves through and interacts with your Web site. This data may be pertinent for the present session, such as items in the shopping cart, or it may be data such as authentication information that was entered when the client first entered your Web site, and which should not have to be reentered during a particular set of transactions. [ Add Comment ]

The Session class of the servlet API uses the Cookie class to do its work. However, all the Session object needs is some kind of unique identifier stored on the client and passed to the server. Web sites may also use the other types of session tracking but these mechanisms will be more difficult to implement as they are not encapsulated into the servlet API (that is, you must write them by hand to deal with the situation when the client has disabled cookies). [ Add Comment ]

Here’s an example that implements session tracking with the servlet API:

Inside the service( ) method, getSession( ) is called for the request object, which returns the Session object associated with this request. The Session object does not travel across the network, but instead it lives on the server and is associated with a client and its requests. [ Add Comment ]

getSession( ) comes in two versions: no parameter, as used here, and getSession(boolean). getSession(true) is equivalent to getSession( ). The only reason for the boolean is to state whether you want the session object created if it is not found. getSession(true) is the most likely call, hence getSession( ). [ Add Comment ]

The Session object, if it is not new, will give us details about the client from previous visits. If the Session object is new then the program will start to gather information about this client’s activities on this visit. Capturing this client information is done through the setAttribute( ) and getAttribute( ) methods of the session object. [ Add Comment ]

The Session object uses a simple name-value pairing for loading information. The name is a String, and the value can be any object derived from java.lang.Object. SessionPeek keeps track of how many times the client has been back during this session. This is done with an Integer object named sesspeek.cntr. If the name is not found an Integer is created with value of one, otherwise an Integer is created with the incremented value of the previously held Integer. The new Integer is placed into the Session object. If you use same key in a setAttribute( ) call, then the new object overwrites the old one. The incremented counter is used to display the number of times that the client has visited during this session. [ Add Comment ]

getAttributeNames( ) is related to getAttribute( ) and setAttribute( ); it returns an enumeration of the names of the objects that are bound to the Session object. A while loop in SessionPeek shows this method in action. [ Add Comment ]

You may wonder how long a Session object hangs around. The answer depends on the servlet container you are using; they usually default to 30 minutes (1800 seconds), which is what you should see from the ServletPeek call to getMaxInactiveInterval( ). Tests seem to produce mixed results between servlet containers. Sometimes the Session object can hang around overnight, but I have never seen a case where the Session object disappears in less than the time specified by the inactive interval. You can try this by setting the inactive interval with setMaxInactiveInterval( ) to 5 seconds and see if your Session object hangs around or if it is cleaned up at the appropriate time. This may be an attribute you will want to investigate while choosing a servlet container. [ Add Comment ]

If you are not already working with an application server that handles Sun’s servlet and JSP technologies for you, you may download the Tomcat implementation of Java servlets and JSPs, which is a free, open-source implementation of servlets, and is the official reference implementation sanctioned by Sun. It can be found at jakarta.apache.org. [ Add Comment ]

Follow the instructions for installing the Tomcat implementation, then edit the server.xml file to point to the location in your directory tree where your servlets will be placed. Once you start up the Tomcat program you can test your servlet programs. [ Add Comment ]

This has only been a brief introduction to servlets; there are entire books on the subject. However, this introduction should give you enough ideas to get you started. In addition, many of the ideas in the next section are backward compatible with servlets. [ Add Comment ]

Java Server Pages (JSP) is a standard Java extension that is defined on top of the servlet Extensions. The goal of JSPs is the simplified creation and management of dynamic Web pages. [ Add Comment ]

The previously mentioned, freely available Tomcat reference implementation from jakarta.apache.org automatically supports JSPs. [ Add Comment ]

JSPs allow you to combine the HTML of a Web page with pieces of Java code in the same document. The Java code is surrounded by special tags that tell the JSP container that it should use the code to generate a servlet, or part of one. The benefit of JSPs is that you can maintain a single document that represents both the page and the Java code that enables it. The downside is that the maintainer of the JSP page must be skilled in both HTML and Java (however, GUI builder environments for JSPs should be forthcoming). [ Add Comment ]

The first time a JSP is loaded by the JSP container (which is typically associated with, or even part of, a Web server), the servlet code necessary to fulfill the JSP tags is automatically generated, compiled, and loaded into the servlet container. The static portions of the HTML page are produced by sending static String objects to write( ). The dynamic portions are included directly into the servlet. [ Add Comment ]

From then on, as long as the JSP source for the page is not modified, it behaves as if it were a static HTML page with associated servlets (all the HTML code is actually generated by the servlet, however). If you modify the source code for the JSP, it is automatically recompiled and reloaded the next time that page is requested. Of course, because of all this dynamism you’ll see a slow response for the first-time access to a JSP. However, since a JSP is usually used much more than it is changed, you will normally not be affected by this delay. [ Add Comment ]

The structure of a JSP page is a cross between a servlet and an HTML page. The JSP tags begin and end with angle brackets, just like HTML tags, but the tags also include percent signs, so all JSP tags are denoted by

The leading percent sign may be followed by other characters that determine the precise type of JSP code in the tag. [ Add Comment ]

Here’s an extremely simple JSP example that uses a standard Java library call to get the current time in milliseconds, which is then divided by 1000 to produce the time in seconds. Since a JSP expression (the <%= ) is used, the result of the calculation is coerced into a String and placed on the generated Web page:

In the JSP examples in this book, the first and last lines are not included in the actual code file that is extracted and placed in the book’s source-code tree. [ Add Comment ]

When the client creates a request for the JSP page, the Web server must have been configured to relay the request to the JSP container, which then invokes the page. As mentioned above, the first time the page is invoked, the components specified by the page are generated and compiled by the JSP container as one or more servlets. In the above example, the servlet will contain code to configure the HttpServletResponse object, produce a PrintWriter object (which is always named out), and then turn the time calculation into a String which is sent to out. As you can see, all this is accomplished with a very succinct statement, but the average HTML programmer/Web designer will not have the skills to write such code. [ Add Comment ]

Servlets include classes that provide convenient utilities, such as HttpServletRequest, HttpServletResponse, Session, etc. Objects of these classes are built into the JSP specification and automatically available for use in your JSP without writing any extra lines of code. The implicit objects in a JSP are detailed in the table below.

The scope of each object can vary significantly. For example, the session object has a scope which exceeds that of a page, as it many span several client requests and pages. The application object can provide services to a group of JSP pages that together represent a Web application.

Directives are messages to the JSP container and are denoted by the “@”: [ Add Comment ]

Directives do not send anything to the out stream, but they are important in setting up your JSP page’s attributes and dependencies with the JSP container. For example, the line: [ Add Comment ]

says that the scripting language being used within the JSP page is Java. In fact, the JSP specification only describes the semantics of scripts for the language attribute equal to “Java.” The intent of this directive is to build flexibility into the JSP technology. In the future, if you were to choose another language, say Python (a good scripting choice), then that language would have to support the Java Run-time Environment by exposing the Java technology object model to the scripting environment, especially the implicit variables defined above, JavaBeans properties, and public methods. [ Add Comment ]

The most important directive is the page directive. It defines a number of page dependent attributes and communicates these attributes to the JSP container. These attributes include: language, extends, import, session, buffer, autoFlush, isThreadSafe, info and errorPage. For example:

This line first indicates that the page requires participation in an HTTP session. Since we have not set the language directive the JSP container defaults to using Java and the implicit script language variable named session is of type javax.servlet.http.HttpSession. If the directive had been false then the implicit variable session would be unavailable. If the session variable is not specified, then it defaults to “true.” [ Add Comment ]

The import attribute describes the types that are available to the scripting environment. This attribute is used just as it would be in the Java programming language, i.e., a comma-separated list of ordinary import expressions. This list is imported by the translated JSP page implementation and is available to the scripting environment. Again, this is currently only defined when the value of the language directive is “java.” [ Add Comment ]

Once the directives have been used to set up the scripting environment you can utilize the scripting language elements. JSP 1.1 has three scripting language elements—declarations, scriptlets, and expressions. A declaration will declare elements, a scriptlet is a statement fragment, and an expression is a complete language expression. In JSP each scripting element begins with a “<%”. The syntax for each is:

White space is optional after “<%!”, “<%”, “<%=”, and before “%>.” [ Add Comment ]

All these tags are based upon XML; you could even say that a JSP page can be mapped to a XML document. The XML equivalent syntax for the scripting elements above would be:

In addition, there are two types of comments:

The first form allows you to add comments to JSP source pages that will not appear in any form in the HTML that is sent to the client. Of course, the second form of comment is not specific to JSPs—it’s just an ordinary HTML comment. What’s interesting is that you can insert JSP code inside an HTML comment and the comment will be produced in the resulting page, including the result from the JSP code. [ Add Comment ]

Declarations are used to declare variables and methods in the scripting language (currently Java only) used in a JSP page. The declaration must be a complete Java statement and cannot produce any output in the out stream. In the Hello.jsp example below, the declarations for the variables loadTime, loadDate and hitCount are all complete Java statements that declare and initialize new variables.

When you run this program you’ll see that the variables loadTime, loadDate and hitCount hold their values between hits to the page, so they are clearly fields and not local variables. [ Add Comment ]

At the end of the example is a scriptlet that writes “Goodbye” to the Web server console and “Cheerio” to the implicit JspWriter object out. Scriptlets can contain any code fragments that are valid Java statements. Scriptlets are executed at request-processing time. When all the scriptlet fragments in a given JSP are combined in the order they appear in the JSP page, they should yield a valid statement as defined by the Java programming language. Whether or not they produce any output into the out stream depends upon the code in the scriptlet. You should be aware that scriptlets can produce side effects by modifying the objects that are visible to them. [ Add Comment ]

JSP expressions can found intermingled with the HTML in the middle section of Hello.jsp. Expressions must be complete Java statements, which are evaluated, coerced to a String, and sent to out. If the result of the expression cannot be coerced to a String then a ClassCastException is thrown. [ Add Comment ]

The following example is similar to one shown earlier in the servlet section. The first time you hit the page it detects that you have no fields and returns a page containing a form, using the same code as in the servlet example, but in JSP format. When you submit the form with the filled-in fields to the same JSP URL, it detects the fields and displays them. This is a nice technique because it allows you to have both the page containing the form for the user to fill out and the response code for that page in a single file, thus making it easier to create and maintain.

The most interesting feature of this example is that it demonstrates how scriptlet code can be intermixed with HTML code, even to the point of generating HTML within a Java for loop. This is especially convenient for building any kind of form where repetitive HTML code would otherwise be required. [ Add Comment ]

By poking around in the HTML documentation for servlets and JSPs, you will find features that report information about the servlet or JSP that is currently running. The following example displays a few of these pieces of data.

This example also shows the use of both embedded HTML and writing to out in order to output to the resulting HTML page. [ Add Comment ]

The first piece of information produced is the name of the servlet, which will probably just be “JSP” but it depends on your implementation. You can also discover the current version of the servlet container by using the application object. Finally, after setting a session attribute, the “attribute names” in a particular scope are displayed. You don’t use the scopes very much in most JSP programming; they were just shown here to add interest to the example. There are four attribute scopes, as follows: The page scope (scope 1), the request scope (scope 2), the session scope (scope 3—here, the only element available in session scope is “My dog,” added right before the for loop), and the application scope (scope 4), based upon the ServletContext object. There is one ServletContext per “Web application” per Java Virtual Machine. (A “Web application” is a collection of servlets and content installed under a specific subset of the server’s URL namespace such as /catalog. This is generally set up using a configuration file.) At the application scope you will see objects that represent paths for the working directory and temporary directory. [ Add Comment ]

Sessions were introduced in the prior section on servlets, and are also available within JSPs. The following example exercises the session object and allows you to manipulate the amount of time before the session becomes invalid.

The session object is provided by default so it is available without any extra coding. The calls to getID( ), getCreationTime( ) and getMaxInactiveInterval( ) are used to display information about this session object. [ Add Comment ]

When you first bring up this session you will see a MaxInactiveInterval of, for example, 1800 seconds (30 minutes). This will depend on the way your JSP/servlet container is configured. The MaxInactiveInterval is shortened to 5 seconds to make things interesting. If you refresh the page before the 5 second interval expires, then you’ll see:

But if you wait longer than that, “Ralph” will become null. [ Add Comment ]

To see how the session information can be carried through to other pages, and also to see the effect of invalidating a session object versus just letting it expire, two other JSPs are created. The first one (reached by pressing the “invalidate” button in SessionObject.jsp) reads the session information and then explicitly invalidates that session:

To experiment with this, refresh SessionObject.jsp, then immediately click the “invalidate” button to bring you to SessionObject2.jsp. At this point you will still see “Ralph,” and right away (before the 5-second interval has expired), refresh SessionObject2.jsp to see that the session has been forcefully invalidated and “Ralph” has disappeared. [ Add Comment ]

If you go back to SessionObject.jsp, refresh the page so you have a new 5-second interval, then press the “Keep Around” button, it will take you to the following page, SessionObject3.jsp, which does NOT invalidate the session:

Because this page doesn’t invalidate the session, “Ralph” will hang around as long as you keep refreshing the page before the 5 second time interval expires. This is not unlike a “Tomagotchi” pet—as long as you play with “Ralph” he will stick around, otherwise he expires. [ Add Comment ]

Since each browser stores cookies in its own way, you may see different behaviors with different browsers (not reassuring, but it might be some kind of bug that could be fixed by the time you read this). Also, you may experience different results if you shut down the browser and restart it, rather than just visiting a different page and then returning to Cookies.jsp. Note that using session objects seems to be more robust than directly using cookies. [ Add Comment ]

After displaying the session identifier, each cookie in the array of cookies that comes in with the request object is displayed, along with its maximum age. The maximum age is changed and displayed again to verify the new value, then a new cookie is added to the response. However, your browser may seem to ignore the maximum age; it’s worth playing with this program and modifying the maximum age value to see the behavior under different browsers. [ Add Comment ]

This section has only been a brief coverage of JSPs, and yet even with what was covered here (along with the Java you’ve learned in the rest of the book, and your own knowledge of HTML) you can begin to write sophisticated web pages via JSPs. The JSP syntax isn’t meant to be particularly deep or complicated, so if you understand what was presented in this section you’re ready to be productive with JSPs. You can find further information in most current books on servlets, or at java.sun.com. [ Add Comment ]

It’s especially nice to have JSPs available, even if your goal is only to produce servlets. You’ll discover that if you have a question about the behavior of a servlet feature, it’s much easier and faster to write a JSP test program to answer that question than it is to write a servlet. Part of the benefit comes from having to write less code and being able to mix the display HTML in with the Java code, but the leverage becomes especially obvious when you see that the JSP Container handles all the recompilation and reloading of the JSP for you whenever the source is changed. [ Add Comment ]

As terrific as JSPs are, however, it’s worth keeping in mind that JSP creation requires a higher level of skill than just programming in Java or just creating Web pages. In addition, debugging a broken JSP page is not as easy as debugging a Java program, as (currently) the error messages are more obscure. This should change as development systems improve, but we may also see other technologies built on top of Java and the Web that are better adapted to the skills of the web site designer. [ Add Comment ]

Traditional approaches to executing code on other machines across a network have been confusing as well as tedious and error-prone to implement. The nicest way to think about this problem is that some object happens to live on another machine, and that you can send a message to the remote object and get a result as if the object lived on your local machine. This simplification is exactly what Java Remote Method Invocation (RMI) allows you to do. This section walks you through the steps necessary to create your own RMI objects. [ Add Comment ]

RMI makes heavy use of interfaces. When you want to create a remote object, you mask the underlying implementation by passing around an interface. Thus, when the client gets a reference to a remote object, what they really get is an interface reference, which happens to connect to some local stub code that talks across the network. But you don’t think about this, you just send messages via your interface reference. [ Add Comment ]

When you create a remote interface, you must follow these guidelines:

Here’s a simple remote interface that represents an accurate time service: [ Add Comment ]

It looks like any other interface except that it extends Remote and all of its methods throw RemoteException. Remember that all the methods of an interface and all of its methods are automatically public. [ Add Comment ]

The server must contain a class that extends UnicastRemoteObject and implements the remote interface. This class can also have additional methods, but only the methods in the remote interface are available to the client, of course, since the client will get only a reference to the interface, not the class that implements it. [ Add Comment ]

You must explicitly define the constructor for the remote object even if you’re only defining a default constructor that calls the base-class constructor. You must write it out since it must throw RemoteException. [ Add Comment ]

Here’s the implementation of the remote interface PerfectTimeI:

Here, main( ) handles all the details of setting up the server. When you’re serving RMI objects, at some point in your program you must: [ Add Comment ]

Here, you see a call to the static method Naming.bind( ). However, this call requires that the registry be running as a separate process on the computer. The name of the registry server is rmiregistry, and under 32-bit Windows you say: [ Add Comment ]

to start it in the background. On Unix, the command is:

Like many network programs, the rmiregistry is located at the IP address of whatever machine started it up, but it must also be listening at a port. If you invoke the rmiregistry as above, with no argument, the registry’s port will default to 1099. If you want it to be at some other port, you add an argument on the command line to specify the port. For this example, the port is located at 2005, so the rmiregistry should be started like this under 32-bit Windows: [ Add Comment ]

or for Unix:

The information about the port must also be given to the bind( ) command, as well as the IP address of the machine where the registry is located. But this brings up what can be a frustrating problem if you’re expecting to test RMI programs locally the way the network programs have been tested so far in this chapter. In the JDK 1.1.1 release, there are a couple of problems:[76] [ Add Comment ]

With all this in mind, the bind( ) command becomes: [ Add Comment ]

If you are using the default port 1099, you don’t need to specify a port, so you could say: [ Add Comment ]

You should be able to perform local testing by leaving off the IP address and using only the identifier: [ Add Comment ]

The name for the service is arbitrary; it happens to be PerfectTime here, just like the name of the class, but you could call it anything you want. The important thing is that it’s a unique name in the registry that the client knows to look for to procure the remote object. If the name is already in the registry, you’ll get an AlreadyBoundException. To prevent this, you can always use rebind( ) instead of bind( ), since rebind( ) either adds a new entry or replaces the one that’s already there. [ Add Comment ]

Even though main( ) exits, your object has been created and registered so it’s kept alive by the registry, waiting for a client to come along and request it. As long as the rmiregistry is running and you don’t call Naming.unbind( ) on your name, the object will be there. For this reason, when you’re developing your code you need to shut down the rmiregistry and restart it when you compile a new version of your remote object. [ Add Comment ]

You aren’t forced to start up rmiregistry as an external process. If you know that your application is the only one that’s going to use the registry, you can start it up inside your program with the line:

Like before, 2005 is the port number we happen to be using in this example. This is the equivalent of running rmiregistry 2005 from a command line, but it can often be more convenient when you’re developing RMI code since it eliminates the extra steps of starting and stopping the registry. Once you’ve executed this code, you can bind( ) using Naming as before. [ Add Comment ]

If you compile and run PerfectTime.java, it won’t work even if you have the rmiregistry running correctly. That’s because the framework for RMI isn’t all there yet. You must first create the stubs and skeletons that provide the network connection operations and allow you to pretend that the remote object is just another local object on your machine. [ Add Comment ]

What’s going on behind the scenes is complex. Any objects that you pass into or return from a remote object must implement Serializable (if you want to pass remote references instead of the entire objects, the object arguments can implement Remote), so you can imagine that the stubs and skeletons are automatically performing serialization and deserialization as they “marshal” all of the arguments across the network and return the result. Fortunately, you don’t have to know any of this, but you do have to create the stubs and skeletons. This is a simple process: you invoke the rmic tool on your compiled code, and it creates the necessary files. So the only requirement is that another step be added to your compilation process. [ Add Comment ]

The rmic tool is particular about packages and classpaths, however. PerfectTime.java is in the package c15.rmi, and even if you invoke rmic in the same directory in which PerfectTime.class is located, rmic won’t find the file, since it searches the classpath. So you must specify the location off the class path, like so:

You don’t have to be in the directory containing PerfectTime.class when you execute this command, but the results will be placed in the current directory. [ Add Comment ]

When rmic runs successfully, you’ll have two new classes in the directory:

corresponding to the stub and skeleton. Now you’re ready to get the server and client to talk to each other. [ Add Comment ]

The whole point of RMI is to make the use of remote objects simple. The only extra thing that you must do in your client program is to look up and fetch the remote interface from the server. From then on, it’s just regular Java programming: sending messages to objects. Here’s the program that uses PerfectTime:

The ID string is the same as the one used to register the object with Naming, and the first part represents the URL and port number. Since you’re using a URL, you can also specify a machine on the Internet. [ Add Comment ]

What comes back from Naming.lookup( ) must be cast to the remote interface, not to the class. If you use the class instead, you’ll get an exception. [ Add Comment ]

You can see in the method call

that once you have a reference to the remote object, programming with it is indistinguishable from programming with a local object (with one difference: remote methods throw RemoteException). [ Add Comment ]

In large, distributed applications, your needs might not be satisfied by the preceding approaches. For example, you might want to interface with legacy data stores, or you might need services from a server object regardless of its physical location. These situations require some form of Remote Procedure Call (RPC), and possibly language independence. This is where CORBA can help. [ Add Comment ]

CORBA is not a language feature; it’s an integration technology. It’s a specification that vendors can follow to implement CORBA-compliant integration products. CORBA is part of the OMG’s effort to define a standard framework for distributed, language-independent object interoperability. [ Add Comment ]

CORBA supplies the ability to make remote procedure calls into Java objects and non-Java objects, and to interface with legacy systems in a location-transparent way. Java adds networking support and a nice object-oriented language for building graphical and non-graphical applications. The Java and OMG object model map nicely to each other; for example, both Java and CORBA implement the interface concept and a reference object model. [ Add Comment ]

The object interoperability specification developed by the OMG is commonly referred to as the Object Management Architecture (OMA). The OMA defines two components: the Core Object Model and the OMA Reference Architecture. The Core Object Model states the basic concepts of object, interface, operation, and so on. (CORBA is a refinement of the Core Object Model.) The OMA Reference Architecture defines an underlying infrastructure of services and mechanisms that allow objects to interoperate. The OMA Reference Architecture includes the Object Request Broker (ORB), Object Services (also known as CORBA services), and common facilities. [ Add Comment ]

The ORB is the communication bus by which objects can request services from other objects, regardless of their physical location. This means that what looks like a method call in the client code is actually a complex operation. First, a connection with the server object must exist, and to create a connection the ORB must know where the server implementation code resides. Once the connection is established, the method arguments must be marshaled, i.e. converted in a binary stream to be sent across a network. Other information that must be sent are the server machine name, the server process, and the identity of the server object inside that process. Finally, this information is sent through a low-level wire protocol, the information is decoded on the server side, and the call is executed. The ORB hides all of this complexity from the programmer and makes the operation almost as simple as calling a method on local object. [ Add Comment ]

There is no specification for how an ORB Core should be implemented, but to provide a basic compatibility among different vendors’ ORBs, the OMG defines a set of services that are accessible through standard interfaces. [ Add Comment ]

CORBA is designed for language transparency: a client object can call methods on a server object of different class, regardless of the language they are implemented with. Of course, the client object must know the names and signatures of methods that the server object exposes. This is where IDL comes in. The CORBA IDL is a language-neutral way to specify data types, attributes, operations, interfaces, and more. The IDL syntax is similar to the C++ or Java syntax. The following table shows the correspondence between some of the concepts common to three languages that can be specified through CORBA IDL:

The inheritance concept is supported as well, using the colon operator as in C++. The programmer writes an IDL description of the attributes, methods, and interfaces that are implemented and used by the server and clients. The IDL is then compiled by a vendor-provided IDL/Java compiler, which reads the IDL source and generates Java code.

The IDL compiler is an extremely useful tool: it doesn’t just generate a Java source equivalent of the IDL, it also generates the code that will be used to marshal method arguments and to make remote calls. This code, called the stub and skeleton code, is organized in multiple Java source files and is usually part of the same Java package. [ Add Comment ]

The naming service is one of the fundamental CORBA services. A CORBA object is accessed through a reference, a piece of information that’s not meaningful for the human reader. But references can be assigned programmer-defined, string names. This operation is known as stringifying the reference, and one of the OMA components, the Naming Service, is devoted to performing string-to-object and object-to-string conversion and mapping. Since the Naming Service acts as a telephone directory that both servers and clients can consult and manipulate, it runs as a separate process. Creating an object-to-string mapping is called binding an object, and removing the mapping is called unbinding. Getting an object reference passing a string is called resolving the name. [ Add Comment ]

For example, on startup, a server application could create a server object, bind the object into the name service, and then wait for clients to make requests. A client first obtains a server object reference, resolving the string name, and then can make calls into the server using the reference. [ Add Comment ]

Again, the Naming Service specification is part of CORBA, but the application that implements it is provided by the ORB vendor. The way you get access to the Naming Service functionality can vary from vendor to vendor. [ Add Comment ]

The code shown here will not be elaborate because different ORBs have different ways to access CORBA services, so examples are vendor specific. (The example below uses JavaIDL, a free product from Sun that comes with a light-weight ORB, a naming service, and an IDL-to-Java compiler.) In addition, since Java is young and still evolving, not all CORBA features are present in the various Java/CORBA products. [ Add Comment ]

We want to implement a server, running on some machine, that can be queried for the exact time. We also want to implement a client that asks for the exact time. In this case we’ll be implementing both programs in Java, but we could also use two different languages (which often happens in real situations). [ Add Comment ]

The first step is to write an IDL description of the services provided. This is usually done by the server programmer, who is then free to implement the server in any language in which a CORBA IDL compiler exists. The IDL file is distributed to the client side programmer and becomes the bridge between languages. [ Add Comment ]

The example below shows the IDL description of our ExactTime server:

This is a declaration of the ExactTime interface inside the remotetime namespace. The interface is made up of one single method that gives back the current time in string format. [ Add Comment ]

The second step is to compile the IDL to create the Java stub and skeleton code that we’ll use for implementing the client and the server. The tool that comes with the JavaIDL product is idltojava:

This will automatically generate code for both the stub and the skeleton. Idltojava generates a Java package named after the IDL module, remotetime, and the generated Java files are put in the remotetime subdirectory. _ExactTimeImplBase.java is the skeleton that we’ll use to implement the server object, and _ExactTimeStub.java will be used for the client. There are Java representations of the IDL interface in ExactTime.java and a couple of other support files used, for example, to facilitate access to the naming service operations. [ Add Comment ]

Below you can see the code for the server side. The server object implementation is in the ExactTimeServer class. The RemoteTimeServer is the application that creates a server object, registers it with the ORB, gives a name to the object reference, and then sits quietly waiting for client requests.

As you can see, implementing the server object is simple; it’s a regular Java class that inherits from the skeleton code generated by the IDL compiler. Things get a bit more complicated when it comes to interacting with the ORB and other CORBA services. [ Add Comment ]

This is a short description of what the JavaIDL-related code is doing (primarily ignoring the part of the CORBA code that is vendor dependent). The first line in main( ) starts up the ORB, and of course, this is because our server object will need to interact with it. Right after the ORB initialization, a server object is created. Actually, the right term would be a transient servant object: an object that receives requests from clients, and whose lifetime is the same as the process that creates it. Once the transient servant object is created, it is registered with the ORB, which means that the ORB knows of its existence and can now forward requests to it. [ Add Comment ]

Up to this point, all we have is timeServerObjRef, an object reference that is known only inside the current server process. The next step will be to assign a stringified name to this servant object; clients will use that name to locate the servant object. We accomplish this operation using the Naming Service. First, we need an object reference to the Naming Service; the call to resolve_initial_references( ) takes the stringified object reference of the Naming Service that is “NameService,” in JavaIDL, and returns an object reference. This is cast to a specific NamingContext reference using the narrow( ) method. We can use now the naming services. [ Add Comment ]

To bind the servant object with a stringified object reference, we first create a NameComponent object, initialized with “ExactTime,” the name string we want to bind to the servant object. Then, using the rebind( ) method, the stringified reference is bound to the object reference. We use rebind( ) to assign a reference, even if it already exists, whereas bind( ) raises an exception if the reference already exists. A name is made up in CORBA by a sequence of NameContexts—that’s why we use an array to bind the name to the object reference. [ Add Comment ]

The servant object is finally ready for use by clients. At this point, the server process enters a wait state. Again, this is because it is a transient servant, so its lifetime is confined to the server process. JavaIDL does not currently support persistent objects—objects that survive the execution of the process that creates them. [ Add Comment ]

Now that we have an idea of what the server code is doing, let’s look at the client code:

The first few lines do the same as they do in the server process: the ORB is initialized and a reference to the naming service server is resolved. Next, we need an object reference for the servant object, so we pass the stringified object reference to the resolve( ) method, and we cast the result into an ExactTime interface reference using the narrow( ) method. Finally, we call getTime( ). [ Add Comment ]

Finally we have a server and a client application ready to interoperate. You’ve seen that both need the naming service to bind and resolve stringified object references. You must start the naming service process before running either the server or the client. In JavaIDL, the naming service is a Java application that comes with the product package, but it can be different with other products. The JavaIDL naming service runs inside an instance of the JVM and listens by default to network port 900. [ Add Comment ]

Now you are ready to start your server and client application (in this order, since our server is transient). If everything is set up correctly, what you’ll get is a single output line on the client console window, giving you the current time. Of course, this might be not very exciting by itself, but you should take one thing into account: even if they are on the same physical machine, the client and the server application are running inside different virtual machines and they can communicate via an underlying integration layer, the ORB and the Naming Service. [ Add Comment ]

This is a simple example, designed to work without a network, but an ORB is usually configured for location transparency. When the server and the client are on different machines, the ORB can resolve remote stringified references using a component known as the Implementation Repository. Although the Implementation Repository is part of CORBA, there is almost no specification, so it differs from vendor to vendor. [ Add Comment ]

As you can see, there is much more to CORBA than what has been covered here, but you should get the basic idea. If you want more information about CORBA, the place to start is the OMG Web site, at www.omg.org. There you’ll find documentation, white papers, proceedings, and references to other CORBA sources and products. [ Add Comment ]

Java applets can act as CORBA clients. This way, an applet can access remote information and services exposed as CORBA objects. But an applet can connect only with the server from which it was downloaded, so all the CORBA objects the applet interacts with must be on that server. This is the opposite of what CORBA tries to do: give you complete location transparency. [ Add Comment ]

This is an issue of network security. If you’re on an intranet, one solution is to loosen the security restrictions on the browser. Or, set up a firewall policy for connecting with external servers. [ Add Comment ]

Some Java ORB products offer proprietary solutions to this problem. For example, some implement what is called HTTP Tunneling, while others have their special firewall features. [ Add Comment ]

This is too complex a topic to be covered in an appendix, but it is definitely something you should be aware of. [ Add Comment ]

You saw that one of the main CORBA features is RPC support, which allows your local objects to call methods in remote objects. Of course, there already is a native Java feature that does exactly the same thing: RMI (see Chapter 15). While RMI makes RPC possible between Java objects, CORBA makes RPC possible between objects implemented in any language. It’s a big difference. [ Add Comment ]

However, RMI can be used to call services on remote, non-Java code. All you need is some kind of wrapper Java object around the non-Java code on the server side. The wrapper object connects externally to Java clients via RMI, and internally connects to the non-Java code using one of the techniques shown above, such as JNI or J/Direct. [ Add Comment ]

This approach requires you to write a kind of integration layer, which is exactly what CORBA does for you, but then you don’t need a third-party ORB. [ Add Comment ]

Suppose[77] you need to develop a multi-tiered application to view and update records in a database through a Web interface. You can write a database application using JDBC, a Web interface using JSP/servlets, and a distributed system using CORBA/RMI. But what extra considerations must you make when developing a distributed object system rather than just knowing API’s? Here are the issues: [ Add Comment ]

Performance: The distributed objects you create must perform well, as they could potentially service many clients at a time. You’ll need to use optimization techniques such as caching as well as pooling resources like database connections. You’ll also have to manage the lifecycle of your distributed objects. [ Add Comment ]

Scalability: The distributed objects must also be scalable. Scalability in a distributed application means that the number of instances of your distributed objects can be increased and moved onto additional machines without modifying any code. [ Add Comment ]

Security: A distributed object must often manage the authorization of the clients that access it. Ideally, you can add new users and roles to it without recompilation. [ Add Comment ]

Distributed Transactions: A distributed object should be able to reference distributed transactions transparently. For example, if you are working with two separated databases, you should be able to update them simultaneously within the same transaction and roll them both back if a certain criteria is not met. [ Add Comment ]

Reusability: The ideal distributed object can be effortlessly moved onto another vendors’ application server. It would be nice if you could resell a distributed object component without making special modifications, or buy someone else’s component and use it without having to recompile or rewrite it. [ Add Comment ]

Availability: If one of the machines in the system goes down, clients should automatically fail-over to backup copies of the objects running on other machines. [ Add Comment ]

These considerations, in addition the business problem that you set out to solve, can make for a daunting development project. However, all the issues except for your business problem are redundant—solutions must be reinvented for every distributed business application. [ Add Comment ]

Sun, along with other leading distributed object vendors, realized that sooner or later every development team would be reinventing these particular solutions, so they created the Enterprise JavaBeans specification (EJB). EJB describes a server-side component model that tackles all of the considerations mentioned above using a standard approach that allows developers to create business components called EJBs that are isolated from low-level “plumbing” code and that focus solely on providing business logic. Because EJB’s are defined in a standard way, they can vendor independent. [ Add Comment ]

Because of the similarity in names, there is much confusion about the relationship between the JavaBeans component model and the Enterprise JavaBeans specification. While both the JavaBeans and Enterprise JavaBeans specifications share the same objectives in promoting reuse and portability of Java code between development and deployment tools with the use of standard design patterns, the motives behind each specification are geared to solve different problems. [ Add Comment ]

The standards defined in the JavaBeans component model are designed for creating reusable components that are typically used in IDE development tools and are commonly, although not exclusively, visual components. [ Add Comment ]

The Enterprise JavaBeans specification defines a component model for developing server side java code. Because EJBs can potentially run on many different server-side platforms—including mainframes that do not have visual displays—An EJB cannot make use of graphical libraries such as AWT or Swing. [ Add Comment ]

The Enterprise JavaBeans specification describes a server-side component model. It defines six roles that are used to perform the tasks in development and deployment as well as defining the components of the system. These roles are used in the development, deployment and running of a distributed system. Vendors, administrators and developers play the various roles, to allow the partitioning of technical and domain knowledge. The vendor provides a technically sound framework and the developers create domain-specific components; for example, an “accounting” component. The same party can perform one or many roles. The roles defined in the EJB specification are summarized in the following table:

EJB components are elements of reusable business logic that adhere to strict standards and design patterns as defined in the EJB specification. This allows the components to be portable. It also allows other services—such as security, caching and distributed transactions—to be performed on behalf of the components. An Enterprise Bean Provider is responsible for developing EJB components. [ Add Comment ]

The EJB Container is a run-time environment that contains and runs EJB components and provides a set of standard services to those components. The EJB Container’s responsibilities are tightly defined by the specification to allow for vendor neutrality. The EJB container provides the low-level “plumbing” of EJB, including distributed transactions, security, lifecycle management of beans, caching, threading and session management. The EJB Container Provider is responsible for providing an EJB Container. [ Add Comment ]

An EJB Server is defined as an Application Server that contains and runs one or more EJB Containers. The EJB Server Provider is responsible for providing an EJB Server. You can generally assume that the EJB Container and EJB Server are the same. [ Add Comment ]

Java Naming and Directory Interface (JNDI) is used in Enterprise JavaBeans as the naming service for EJB Components on the network and other container services such as transactions. JNDI maps very closely to other naming and directory standards such as CORBA CosNaming and can actually be implemeted as a wrapper on top of it. [ Add Comment ]

JTA/JTS is used in Enterprise JavaBeans as the transactional API. An Enterprise Bean Provider can use the JTS to create transaction code, although the EJB Container commonly implements transactions in EJB on the EJB components’ behalf. The deployer can define the transactional attributes of an EJB component at deployment time. The EJB Container is responsible for handling the transaction whether it is local or distributed. The JTS specification is the Java mapping to the CORBA OTS (Object Transaction Service) [ Add Comment ]

The EJB specification defines interoperability with CORBA through compatibility with CORBA protocols. This is achieved by mapping EJB services such as JTS and JNDI to corresponding CORBA services and the implementation of RMI on top of the CORBA protocol IIOP. [ Add Comment ]

Use of CORBA and RMI/IIOP in Enterprise JavaBeans is implemented in the EJB Container and is the responsibility of the EJB Container provider. Use of CORBA and RMI/IIOP in the EJB Container is hidden from the EJB Component itself. This means that the Enterprise Bean Provider can write their EJB Component and deploy it into any EJB Container without any regard of which communication protocol is being used. [ Add Comment ]

An EJB consists of a number of pieces, including the Bean itself, the implementation of some interfaces, and an information file. Everything is packaged together into a special jar file. [ Add Comment ]

The Enterprise Bean is a Java class that the Enterprise Bean Provider develops. It implements an Enterprise Bean interface and provides the implementation of the business methods that the component is to perform. The class does not implement any authorization, authentication, multithreading, or transactional code. [ Add Comment ]

Every Enterprise Bean that is created must have an associated Home interface. The Home interface is used as a factory for your EJB. Clients use the Home interface to find an instance of your EJB or create a new instance of your EJB. [ Add Comment ]

The Remote interface is a Java Interface that reflects the methods of your Enterprise Bean that you wish to expose to the outside world. The Remote interface plays a similar role to a CORBA IDL interface. [ Add Comment ]

The deployment descriptor is an XML file that contains information about your EJB. Using XML allows the deployer to easily change attributes about your EJB. The configurable attributes defined in the deployment descriptor include:

The EJB-Jar file is a normal java jar file that contains your EJB, Home and Remote interfaces, as well as the deployment descriptor. [ Add Comment ]

Once you have an EJB-Jar file containing the Bean, the Home and Remote interfaces, and the deployment descriptor, you can fit all of the pieces together and in the process understand why the Home and Remote interfaces are needed and how the EJB Container uses them. [ Add Comment ]

The EJB Container implements the Home and Remote interfaces that are in the EJB-Jar file. As mentioned earlier, the Home interface provides methods to create and find your EJB. This means that the EJB Container is responsible for the lifecycle management of your EJB. This level of indirection allows for optimizations to occur. For example, 5 clients might simultaneously request the creation of an EJB through a Home Interface, and the EJB Container would respond by creating only one EJB and sharing it between all 5 clients. This is achieved through the Remote Interface, which is also implemented by the EJB Container. The implemented Remote object plays the role of a proxy object to the EJB. [ Add Comment ]

All calls to the EJB are ‘proxied’ through the EJB Container via the Home and Remote interfaces. This indirection is the reason why the EJB container can control security and transactional behavior. [ Add Comment ]

The Enterprise JavaBeans specification defines different types of EJBs that have different characteristics and behaviors. Two categories of EJBs have been defined in the specification: Session Beans and Entity Beans, and each categoriy has variations. [ Add Comment ]

Session Beans are used to represent Use-Cases or Workflow on behalf of a client. They represent operations on persistent data, but not the persistent data itself. There are two types of Session Beans, Stateless and Stateful. All Session Beans must implement the javax.ejb.SessionBean interface. The EJB Container governs the life of a Session Bean. [ Add Comment ]

Stateless Session Beans are the simplest type of EJB component to implement. They do not maintain any conversational state with clients between method invocations so they are easily reusable on the server side and because they can be cached, they scale well on demand. When using Stateless Session Beans, all state must be stored outside of the EJB. [ Add Comment ]

Stateful Session Beans maintain state between invocations. They have a one-to-one logical mapping to a client and can maintain state within themselves. The EJB Container is responsible for pooling and caching of Stateful Session Beans, which is achieved through Passivation and Activation. If the EJB Container crashes, data for all Stateful Session Beans could be lost. Some high-end EJB Containers provide recovery for Stateful Session Beans. [ Add Comment ]

Entity Beans are components that represent persistent data and behavior of this data. Entity Beans can be shared among multiple clients, the same way that data in a database can be shared. The EJB Container is responsible for caching Entity Beans and for maintaining the integrity of the Entity Beans. The life of an Entity Bean outlives the EJB Container, so if an EJB Container crashes, the Entity Bean is still expected to be available when the EJB Container again becomes available. [ Add Comment ]

There are two types of Entity Beans: those with Container Managed persistence and those with Bean-Managed persistence. [ Add Comment ]

Container Managed Persistence (CMP). A CMP Entity Bean has its persistence implemented on its behalf by the EJB Container. Through attributes specified in the deployment descriptor, the EJB Container will map the Entity Bean’s attributes to some persistent store (usually—but not always—a database). CMP reduces the development time for the EJB, as well as dramatically reducing the amount of code required. [ Add Comment ]

Bean Managed Persistence (BMP). A BMP Entity Bean has its persistence implemented by the Enterprise Bean Provider. The Enterprise Bean Provider is responsible for implementing the logic required to create a new EJB, update some attributes of the EJBS, delete an EJB and find an EJB from persistent store. This usually involves writing JDBC code to interact with a database or other persistent store. With BMP, the developer is in full control of how the Entity Bean persistence is managed. [ Add Comment ]

BMP also gives flexibility where a CMP implementation may not be available. For example, if you wanted to create an EJB that wrapped some code on an existing mainframe system, you could write your persistence using CORBA. [ Add Comment ]

As an example, the “Perfect Time” example from the previous RMI section will be implemented as an EJB component. The example will be a simple Stateless Session Bean. [ Add Comment ]

As mentioned earlier, EJB components consist of at least one class (the EJB) and two interfaces: the Remote and Home interfaces. When you create a Remote interface for an EJB , you must follow these guidelines: [ Add Comment ]

The Home interface is the factory where the component will be created. It can define create methods, to create instances of EJBs, or finder methods, which locate existing EJBs and are used for Entity Beans only. When you create a Home interface for an EJB , you must follow these guidelines:

The standard naming convention for Home interfaces is to take the Remote interface name and append “Home” to the end. Here is the Home interface for the PerfectTime EJB: [ Add Comment ]

You can now implement the business logic. When you create your EJB implementation class, you must follow these guidelines, (note that you should consult the EJB specification for a complete list of guidelines when developing Enterprise JavaBeans): [ Add Comment ]

Because this is a simple example, the EJB methods (ejbCreate( ), ejbRemove( ), ejbActivate( ), ejbPassivate( ) ) are all empty. These methods are invoked by the EJB Container and are used to control the state of the component. The setSessionContext( ) method passes a javax.ejb.SessionContext object which contains information about the component’s context, such as the current transaction and security information. [ Add Comment ]

After we have created the Enterprise JavaBean, we then need to create a deployment descriptor. The deployment descriptor is an XML file that describes the EJB component. The deployment descriptor should be stored in a file called ejb-jar.xml. [ Add Comment ]

You can see the Component, the Remote interface and the Home interface defined inside the <session> tag of this deployment descriptor. Deployment descriptors may be automatically generated using EJB development tools. [ Add Comment ]

Along with the standard ejb-jar.xml deployment descriptor, the EJB specification states that any vendor specific tags should be stored in a separate file. This is to achieve high portability between components and different brands of EJB containers. [ Add Comment ]

The files must be archived inside a standard Java Archive (JAR) file. The deployment descriptors should be placed inside the /META-INF sub-directory of the Jar file. [ Add Comment ]

Once the EJB component is defined in the deployment descriptor, the deployer should then deploy the EJB component into the EJB Container. At the time of this writing, the deployment process was quite “GUI intensive” and specific to each individual EJB Container, so this overview does not document that process. Every EJB Container, however will have a documented process for deploying an EJB. [ Add Comment ]

Because an EJB component is a distributed object, the deployment process should also create some client stubs for calling the EJB component. These classes should be placed on the classpath of the client application. Because EJB components can be implemented on top of RMI-IIOP (CORBA) or RMI-JRMP, the stubs generated could vary between EJB Containers; nevertheless they are generated classes. [ Add Comment ]

When a client program wishes to invoke an EJB, it must look up the EJB component inside JNDI and obtain a reference to the home interface of the EJB component. The Home interface is used to create an instance of the EJB. [ Add Comment ]

In this example the client program is a simple Java program, but you should remember that it could just as easily be a servlet, a JSP or even a CORBA or RMI distributed object.

The sequence of the example is explained in the comments. Note the use of the narrow( ) method to perform a kind of casting of the object before a Java cast is performed. This is very similar to what happens in CORBA. Also note that the Home object becomes a factory for PerfectTime objects. [ Add Comment ]

The Enterprise JavaBeans specification is a dramatic step forward in the standardization and simplification of distributed object computing. It is a major piece of the Java 2 Enterprise Edition (J2EE) platform and is receiving much support from the distributed object community. Many tools are currently available or will be available in the near future to help accelerate the development of EJB components. [ Add Comment ]

This overview was only a brief tour of EJBs. For more information about the EJB specification you should see the official Enterprise JavaBeans home page at java.sun.com/products/ejb/, where you can download the latest specification and the J2EE reference implementation. These can be used to develop and deploy your own EJB components. [ Add Comment ]

This section[78] gives an overview of Sun Microsystems’s Jini technology. It describes some Jini nuts and bolts and shows how Jini’s architecture helps to raise the level of abstraction in distributed systems programming, effectively turning network programming into object-oriented programming. [ Add Comment ]

Traditionally, operating systems have been designed with the assumption that a computer will have a processor, some memory, and a disk. When you boot a computer, the first thing it does is look for a disk. If it doesn’t find a disk, it can’t function as a computer. Increasingly, however, computers are appearing in a different guise: as embedded devices that have a processor, some memory, and a network connection—but no disk. The first thing a cell phone does when you boot it up, for example, is look for the telephone network. If it doesn’t find the network, it can’t function as a cell phone. This trend in the hardware environment, from disk-centric to network-centric, will affect how we organize the software—and that’s where Jini comes in. [ Add Comment ]

Jini is an attempt to rethink computer architecture, given the rising importance of the network and the proliferation of processors in devices that have no disk drive. These devices, which will come from many different vendors, will need to interact over a network. The network itself will be very dynamic—devices and services will be added and removed regularly. Jini provides mechanisms to enable smooth adding, removal, and finding of devices and services on the network. In addition, Jini provides a programming model that makes it easier for programmers to get their devices talking to each other. [ Add Comment ]

Building on top of Java, object serialization, and RMI (which together enable objects to move around the network from virtual machine to virtual machine) Jini attempts to extend the benefits of object-oriented programming to the network. Instead of requiring device vendors to agree on the network protocols through which their devices can interact, Jini enables the devices to talk to each other through interfaces to objects. [ Add Comment ]

Jini is a set of APIs and network protocols that can help you build and deploy distributed systems that are organized as federations of services. A service can be anything that sits on the network and is ready to perform a useful function. Hardware devices, software, communications channels—even human users themselves—can be services. A Jini-enabled disk drive, for example, could offer a “storage” service. A Jini-enabled printer could offer a “printing” service. A federation of services, then, is a set of services, currently available on the network, that a client (meaning a program, service, or user) can bring together to help it accomplish some goal. [ Add Comment ]

To perform a task, a client enlists the help of services. For example, a client program might upload pictures from the image storage service in a digital camera, download the pictures to a persistent storage service offered by a disk drive, and send a page of thumbnail-sized versions of the images to the printing service of a color printer. In this example, the client program builds a distributed system consisting of itself, the image storage service, the persistent storage service, and the color-printing service. The client and services of this distributed system work together to perform the task: to offload and store images from a digital camera and print a page of thumbnails. [ Add Comment ]

The idea behind the word federation is that the Jini view of the network doesn’t involve a central controlling authority. Because no one service is in charge, the set of all services available on the network form a federation—a group composed of equal peers. Instead of a central authority, Jini’s run-time infrastructure merely provides a way for clients and services to find each other (via a lookup service, which stores a directory of currently available services). After services locate each other, they are on their own. The client and its enlisted services perform their task independently of the Jini run-time infrastructure. If the Jini lookup service crashes, any distributed systems brought together via the lookup service before it crashed can continue their work. Jini even includes a network protocol that clients can use to find services in the absence of a lookup service. [ Add Comment ]

Jini defines a run-time infrastructure that resides on the network and provides mechanisms that enable you to add, remove, locate, and access services. The run-time infrastructure resides in three places: in lookup services that sit on the network, in the service providers (such as Jini-enabled devices), and in clients. Lookup services are the central organizing mechanism for Jini-based systems. When new services become available on the network, they register themselves with a lookup service. When clients wish to locate a service to assist with some task, they consult a lookup service. [ Add Comment ]

The run-time infrastructure uses one network-level protocol, called discovery, and two object-level protocols, called join and lookup. Discovery enables clients and services to locate lookup services. Join enables a service to register itself in a lookup service. Lookup enables a client to query for services that can help accomplish its goals. [ Add Comment ]

Discovery works like this: Imagine you have a Jini-enabled disk drive that offers a persistent storage service. As soon as you connect the drive to the network, it broadcasts a presence announcement by dropping a multicast packet onto a well-known port. Included in the presence announcement is an IP address and port number where the disk drive can be contacted by a lookup service. [ Add Comment ]

Lookup services monitor the well-known port for presence announcement packets. When a lookup service receives a presence announcement, it opens and inspects the packet. The packet contains information that enables the lookup service to determine whether or not it should contact the sender of the packet. If so, it contacts the sender directly by making a TCP connection to the IP address and port number extracted from the packet. Using RMI, the lookup service sends an object, called a service registrar, across the network to the originator of the packet. The purpose of the service registrar object is to facilitate further communication with the lookup service. By invoking methods on this object, the sender of the announcement packet can perform join and lookup on the lookup service. In the case of the disk drive, the lookup service would make a TCP connection to the disk drive and would send it a service registrar object, through which the disk drive would then register its persistent storage service via the join process. [ Add Comment ]

Once a service provider has a service registrar object, the end product of discovery, it is ready to do a join—to become part of the federation of services that are registered in the lookup service. To do a join, the service provider invokes the register( ) method on the service registrar object, passing as a parameter an object called a service item, a bundle of objects that describe the service. The register( ) method sends a copy of the service item up to the lookup service, where the service item is stored. Once this has completed, the service provider has finished the join process: its service has become registered in the lookup service. [ Add Comment ]

The service item is a container for several objects, including an object called a service object, which clients can use to interact with the service. The service item can also include any number of attributes, which can be any object. Some potential attributes are icons, classes that provide GUIs for the service, and objects that give more information about the service. [ Add Comment ]

Service objects usually implement one or more interfaces through which clients interact with the service. For example, a lookup service is a Jini service, and its service object is the service registrar. The register( ) method invoked by service providers during join is declared in the ServiceRegistrar interface (a member of the net.jini.core.lookup package), which all service registrar objects implement. Clients and service providers talk to the lookup service through the service registrar object by invoking methods declared in the ServiceRegistrar interface. Likewise, a disk drive would provide a service object that implemented some well-known storage service interface. Clients would look up and interact with the disk drive by this storage service interface. [ Add Comment ]

Once a service has registered with a lookup service via the join process, that service is available for use by clients who query that lookup service. To build a distributed system of services that will work together to perform some task, a client must locate and enlist the help of the individual services. To find a service, clients query lookup services via a process called lookup. [ Add Comment ]

To perform a lookup, a client invokes the lookup( ) method on a service registrar object. (A client, like a service provider, gets a service registrar through the previously-described process of discovery.) The client passes as an argument to lookup( ) a service template, an object that serves as search criteria for the query. The service template can include a reference to an array of Class objects. These Class objects indicate to the lookup service the Java type (or types) of the service object desired by the client. The service template can also include a service ID, which uniquely identifies a service, and attributes, which must exactly match the attributes uploaded by the service provider in the service item. The service template can also contain wildcards for any of these fields. A wildcard in the service ID field, for example, will match any service ID. The lookup( ) method sends the service template to the lookup service, which performs the query and sends back zero to any matching service objects. The client gets a reference to the matching service objects as the return value of the lookup( ) method. [ Add Comment ]

In the general case, a client looks up a service by Java type, usually an interface. For example, if a client needed to use a printer, it would compose a service template that included a Class object for a well-known interface to printer services. All printer services would implement this well-known interface. The lookup service would return a service object (or objects) that implemented this interface. Attributes can be included in the service template to narrow the number of matches for such a type-based search. The client would use the printer service by invoking methods from the well-known printer service interface on the service object. [ Add Comment ]

Jini’s architecture brings object-oriented programming to the network by enabling network services to take advantage of one of the fundamentals of objects: the separation of interface and implementation. For example, a service object can grant clients access to the service in many ways. The object can actually represent the entire service, which is downloaded to the client during lookup and then executed locally. Alternatively, the service object can serve merely as a proxy to a remote server. Then when the client invokes methods on the service object, it sends the requests across the network to the server, which does the real work. A third option is for the local service object and a remote server to each do part of the work. [ Add Comment ]

One important consequence of Jini’s architecture is that the network protocol used to communicate between a proxy service object and a remote server does not need to be known to the client. As illustrated in the figure below, the network protocol is part of the service’s implementation. This protocol is a private matter decided upon by the developer of the service. The client can communicate with the service via this private protocol because the service injects some of its own code (the service object) into the client’s address space. The injected service object could communicate with the service via RMI, CORBA, DCOM, some home-brewed protocol built on top of sockets and streams, or anything else. The client simply doesn’t need to care about network protocols, because it can talk to the well-known interface that the service object implements. The service object takes care of any necessary communication on the network. [ Add Comment ]

Different implementations of the same service interface can use completely different approaches and network protocols. A service can use specialized hardware to fulfill client requests, or it can do all its work in software. In fact, the implementation approach taken by a single service can evolve over time. The client can be sure it has a service object that understands the current implementation of the service, because the client receives the service object (by way of the lookup service) from the service provider itself. To the client, a service looks like the well-known interface, regardless of how the service is implemented. [ Add Comment ]

Jini attempts to raise the level of abstraction for distributed systems programming, from the network protocol level to the object interface level. In the emerging proliferation of embedded devices connected to networks, many pieces of a distributed system may come from different vendors. Jini makes it unnecessary for vendors of devices to agree on network level protocols that allow their devices to interact. Instead, vendors must agree on Java interfaces through which their devices can interact. The processes of discovery, join, and lookup, provided by the Jini run-time infrastructure, enable devices to locate each other on the network. Once they locate each other, devices can communicate with each other through Java interfaces. [ Add Comment ]

Along with Jini for local device networks, this chapter has introduced some, but not all, of the components that Sun refers to as J2EE: the Java 2 Enterprise Edition. The goal of J2EE is to build create a set of tools that allows the Java developer to build server-based applications much more quickly than before, and in a platform-independent way. It’s not only difficult and time-consuming to build such applications, but it’s especially hard to build them so that they can be easily ported to other platforms, and also to keep the business logic separated from the underlying details of the implementation. J2EE provides a framework to assist in creating server-based applications; these applications are in demand now, and that demand appears to be increasing. [ Add Comment ]

Solutions to selected exercises can be found in the electronic document The Thinking in Java Annotated Solution Guide, available for a small fee from www.BruceEckel.com.

[72] This means a maximum of just over four billion numbers, which is rapidly running out. The new standard for IP addresses will use a 128-bit number, which should produce enough unique IP addresses for the foreseeable future.

[73] Created by Dave Bartlett.

[74] Dave Bartlett was instrumental in the development of this material, and also the JSP section.

[75] A primary tenet of Extreme Programming (XP). See www.xprogramming.com.

[76] Many brain cells died in agony to discover this information.

[77] This section was contributed by Robert Castaneda, with help from Dave Bartlett.

[78] This section was contributed by Bill Venners (www.artima.com).