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.

However, the concept of networking is not so difficult. You want to get some information from that machine over there and move it to this machine here, or vice versa. It’s 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.

One of Java’s great strengths is painless networking. 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.

This chapter introduces Java’s networking support using easy-to-understand examples.

Of course, in order to tell one machine from another and to make sure that you are connected with the machine you want, 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 that can exist in two forms:

In both cases, the IP address is represented internally as a 32-bit number[68] (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.

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 it’s assigned each time you dial up, how can you find out what it is?

The following program uses InetAddress.getByName( ) to produce your IP address. To use it, you must know the name of your computer. It has been tested only on Windows 95, but there you can 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, he can log onto my personal Web server 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.

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?”

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.

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 just magically turned into an IO 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 IO commands from Chapter 11. This is one of the nice features of Java networking.

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 static member methods getByName( ) (which is what you’ll usually use), getAllByName( ), or getLocalHost( ).

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

or by using its dotted quad form to name the reserved IP number for the loopback:

All three forms produce the same result.

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.

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 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.

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).

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.

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 IO 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 server side Socket through which direct communications will take place. 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.

The use of the term ServerSocket would seem to be another example of a confusing name 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.

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.”

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. (On the other hand, the Socket that comes back from ServerSocket.accept( ) already contains all this information.)

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. After that, everything it reads from the InputStream it echoes to the OutputStream until it receives the line END, at which time it closes the connection.

The client makes the connection to the server, then creates an OutputStream. Lines of text are sent through the OutputStream. The client also creates an InputStream to hear what the server is saying (which, in this case, is just the words echoed back).

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.)

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.

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.

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 non-memory resources, so you must be diligent in order to clean them up (since there is no destructor in Java to do it for you).

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.

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 Java 1.1 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.

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.

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

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

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:

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

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:

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

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.)

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.

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.

Sockets produce a “dedicated” connection that persists until it is explicitly disconnected. (The dedicated connection can still be disconnected un-explicitly 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 the chapter you’ll see a different approach to networking, in which the connections are only temporary.

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.

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.

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.

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( ).

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.

If the creation of the ServerSocket fails, the exception is again thrown through main( ). But if it 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.

To test that the server really does handle multiple clients, the following program creates many clients (using threads) that connect to the same server. Each thread has a limited lifetime, and when it goes away, that leaves space for the creation of a new thread. The maximum number of threads allowed is determined by the final int maxthreads. You’ll notice that this value is rather critical, since if you make it too high the threads seem to run out of resources and the program mysteriously fails.

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.

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.

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.

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.

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 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).

Now that you understand the basics you should be realizing that servlets are excellent for server-side Web development. Elegant and straight forward, they do just about everything for you - right? Well, almost. Remember early we said all client requests drive through the service method? This is the well-used, high traffic corridor of the servlet and more than one client request may come through at the same time. The servlet engine 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 beprocessing through your service( ) or doGet( ) or doPost( ) methods at the same time. Therefore the service( ) methods and other methods called by HttpServlet.service( ) (e.g., doGet( ), doPost( ), doHead( ),etc.) need to be written in a thread-safe manner. Any common resources (files, databases) that will be used by your client requests will need to be synchronized.

ThreadServlet is a simple example that simply synchronizes around the threads sleep( ) method. This will hold up all threads until the allotted time (5000 ms) is all used up. When testing this you should start several browsers instances and hit this servlet as quickly as possible.

The servlet API comes with more than just the classes that implement the servlet interface, GenericServlet and HttpServlet and the Request and Response objects. The design of HTTP is such that it is a 'sessionless' protocol. A great deal of effort has gone into mechanisms that will allow Web developers to track sessions. How could companies do e-commerce if you couldn't keep track of client and the items they have put into their shopping cart? You couldn't! This may be great for privacy advocates but it does little to help create robust, commerce driven Web sites.

There are several methods of session tracking but the most common method is with persistant 'cookies'. The term cookie sounds cute and could be perceived as a session tracking solution that was baked up in someone's garage. The fact is that cookies are an integral part of the Internet standards. The HTTP Working Group of the Internet Engineering Task Force has written cookies in the official standard in RFC 2109 (http://ds.internic.net/rfc/rfc2109.txt or check http://www.cookiecentral.com).

A cookie is nothing more than a small piece of information sent by a Web server to a browser. The browser stores the cookie locally and all calls to the server from that browser will contain the cookie as an identifier. The cookie therefore acts to uniquely identify the client with each hit of this Web server. It should be noted that clients can turn off the browsers ability to accept cookies. If your site still need to be able to session track this type of client then another method of session tracking (URL rewriting or hidden form fields) will have to be incorporated. The session tracking capabilities built into the Servlet API are designed around cookies.

The Servlet API (version 2.0 and up) provides the javax.servlet.http.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 creating it using the constructor and 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 then received by calling the getCookies( ) method of the HttpServletRequest object which returns an array of cookie objects.

A session in the world of HTTP and the Internet is one or more page requests by a client to a Web site during a defined period of time. If I am buying my groceries on-line, I want a session to be confined to the period from when I first add an item to my shopping cart to the point where I checkout. Each item I add to the shopping cart will be a new connection in the HTTP world, they have no knowledge of previous connections or items in the shopping cart. The mechanics supplied by the Cookie specification allows us to perform 'session tracking'.

You should understand that a cookie is an object that encapsulates that small bit of information that will be stored on the client side. A Servlet Session object lives on the server side of the communication channel and its goal is to capture data about this client that would be useful 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 information you asked the client to enter such as authentication information entered when the client first entered your Web site and which should not have to be re-enter before a set time of inactivity.

The Session class of the Servlet API uses the Cookie class but really all the session object needs is a unique identifier stored on the client and passed to the server. Usually this is a cookie and that is the mechanism we will cover here. 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.

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 you can send a message to that object and get a result as if the object lived on your local machine. This simplification is exactly what Java 1.1 Remote Method Invocation (RMI) allows you to do. This section walks you through the steps necessary to create your own RMI objects.

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.

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

This section[71] 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.

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 cellphone 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 cellphone. This trend in the hardware environment, from disk-centric to network-centric, will affect how we organize our software—and that's where Jini comes in.

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.

Building on top of Java, object serialization, and RMI (which 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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, will enable devices to locate each other on the network. Once they locate each other, devices will be able to communicate with each other through Java interfaces.

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).