Introduction

There is much excitement about the Internet and the World Wide Web. The Internet ties the information world together. The World Wide Web makes the Internet easy to use and gives it the flair and sizzle of multimedia. Organizations see the Internet and the Web as crucial to their information-systems strategies. Java provides a number of built-in networking capabilities that make it easy to develop Internet-based and Web-based applications. Java can enable programs to search the world for information and to collaborate with programs running on other computers internationally, nationally or just within an organization. Java can enable applets and applications to communicate with one another (subject to security constraints).

Networking is a massive and complex topic. Computer science and computer engineering students will typically take a full-semester, upper-level course in computer networking and continue with further study at the graduate level. Java is often used as an implementation vehicle in computer networking courses. In Java How to Program, Sixth Edition, we introduce a portion of Java's networking concepts and capabilities. We discuss more advanced networking capabilities in our book Advanced Java 2 Platform How to Program.

Java's fundamental networking capabilities are declared by classes and interfaces of package java.net, through which Java offers stream-based communications that enable applications to view networking as streams of data. The classes and interfaces of package java.net also offer packet-based communications for transmitting individual packets of informationcommonly used to transmit audio and video over the Internet. In this chapter, we show how to create and manipulate sockets and how to communicate with packets and streams of data.

Our discussion of networking focuses on both sides of the client-server relationship. The client requests that some action be performed, and the server performs the action and responds to the client. A common implementation of the request-response model is between Web browsers and Web servers. When a user selects a Web site to browse through a browser (the client application), a request is sent to the appropriate Web server (the server application). The server normally responds to the client by sending an appropriate HTML Web page.

We introduce Java's socket-based communications, which enable applications to view networking as if it were file I/Oa program can read from a socket or write to a socket as simply as reading from a file or writing to a file. The socket is simply a software construct that represents one endpoint of a connection. We show how to create and manipulate stream sockets and datagram sockets.

With stream sockets, a process establishes a connection to another process. While the connection is in place, data flows between the processes in continuous streams. Stream sockets are said to provide a connection-oriented service. The protocol used for transmission is the popular TCP (Transmission Control Protocol).

With datagram sockets, individual packets of information are transmitted. This is not appropriate for everyday programmers, because the protocol usedUDP, the User Datagram Protocolis a connectionless service, and thus does not guarantee that packets arrive in any particular order. With UDP, packets can even be lost or duplicated. Significant extra programming is required on the programmer's part to deal with these problems (if the programmer chooses to do so). UDP is most appropriate for network applications that do not require the error checking and reliability of TCP. Stream sockets and the TCP protocol will be more desirable for the vast majority of Java programmers.

Performance Tip 24.1

Connectionless services generally offer greater performance but less reliability than connection-oriented services.

Portability Tip 24.1

TCP, UDP and related protocols enable a great variety of heterogeneous computer systems (i.e., computer systems with different processors and different operating systems) to intercommunicate.

The chapter includes a case study in which we implement a client/server chat application similar to the instant-messaging services popular on the Web today. The application incorporates many networking techniques introduced in this chapter. It also introduces multicasting, in which a server can publish information and clients can subscribe to that information. Each time the server publishes more information, all subscribers receive it. Throughout the examples of this chapter, we will see that many of the networking details are handled by the Java APIs.