Basic Knowledge/Tutorial Programming About Socket in JAVA

The network is the soul of Java. Most of what is interesting about Java centers around the potential for dynamic, networked applications. As Java's networking APIs have matured, Java has also become the language of choice for implementing traditional client-server applications and services.

The classes of java.net fall into two general categories: the Sockets API for working with low-level Internet protocols and higher-level, web-oriented APIs that work with uniform resource locators (URLs). Figure 1 shows the java.net package.





Java's Sockets API provides access to the standard network protocols used for communications between hosts on the Internet. Sockets are the mechanism underlying all other kinds of portable networked communications. Sockets are the lowest-level toolyou can use sockets for any kind of communications between client and server or peer applications on the Net, but you have to implement your own application-level protocols for handling and interpreting the data. Higher-level networking tools, such as remote method invocation, HTTP, and web services are implemented on top of sockets.

Java RMI is a powerful tool that leverages Java object serialization, allowing you to transparently work with objects on remote machines almost as if they were local. With RMI, it is easy to write distributed applications in which clients and servers work with each other's data as full-fledged Java objects rather than raw streams or packets of data. Technologies like Enterprise JavaBeans build on RMI and extend its semantics, so it is very important to understand the concepts involved.

When writing network applications, it's common to talk about clients and servers. The distinction is increasingly vague, but the side that initiates the conversation is usually considered the client. The side that accepts the request is usually the server. In the case where two peer applications use sockets to talk, the distinction is less important, but for simplicity we'll use this definition.

For our purposes, the most important difference between a client and a server is that a client can create a socket to initiate a conversation with a server application at any time while a server must be prepared in advance to listen for incoming conversations. The java.net.Socket class represents one side of an individual socket connection on both the client and server. In addition, the server uses the java.net.ServerSocket class to listen for new connections from clients. In most cases, an application acting as a server creates a ServerSocket object and waits, blocked in a call to its accept( ) method, until a connection arrives. When it arrives, the accept( ) method creates a Socket object the server uses to communicate with the client. A server may carry on conversations with multiple clients at once; in this case, there is still only a single ServerSocket, but the server has multiple Socket objectsone associated with each client, as shown in Figure2.

At the socket level, a client needs two pieces of information to locate and connect to a server on the Internet: a hostname (used to find the host's network address) and a port number. The port number is an identifier that differentiates between multiple clients or servers on the same host. A server application listens on a prearranged port while waiting for connections. Clients select the port number assigned to the service they want to access. If you think of the host computers as hotels and the applications as

guests, the ports are like the guests' room numbers. For one person to call another, he or she must know the other party's hotel name and room number.