Java Network programming Question:
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To create a Socket, you need to know the Internet host to which you want to connect?


To create a Socket, you need to know the Internet host to which you want to connect. When you're writing a server, you don't know in advance who will contact you, and even if you did, you wouldn't know when that host wanted to contact you. In other words, servers are like receptionists who sit by the phone and wait for incoming calls. They don't know who will call or when, only that when the phone rings, they have to pick it up and talk to whoever is there. We can't program that behavior with the Socket class alone. Granted, there's no reason that clients written in Java have to talk to Java servers--in fact, a client doesn't care what language the server was written in or what platform it runs on. However, if Java didn't let us write servers, there would be a glaring hole in its capabilities.

Fortunately, there's no such hole. Java provides a ServerSocket class to allow programmers to write servers. Basically, a server socket's job is to sit by the phone and wait for incoming calls. More technically, a ServerSocket runs on the server and listens for incoming TCP connections. Each ServerSocket listens on a particular port on the server machine. When a client Socket on a remote host attempts to connect to that port, the server wakes up, negotiates the connection between the client and the server, and opens a regular Socket between the two hosts. In other words, server sockets wait for connections while client sockets initiate connections. Once the server socket has set up the connection, the server uses a regular Socket object to send data to the client. Data always travels over the regular socket.

The ServerSocket Class

The ServerSocket class contains everything you need to write servers in Java. It has constructors that create new ServerSocket objects, methods that listen for connections on a specified port, and methods that return a Socket object when a connection is made so that you can send and receive data. In addition, it has methods to set various options and the usual miscellaneous methods such as toString( ).

The basic life cycle of a server is:

1. A new ServerSocket is created on a particular port using a ServerSocket( ) constructor.
2. The ServerSocket listens for incoming connection attempts on that port using its accept( ) method. accept( ) blocks until a client attempts to make a connection, at which point accept( ) returns a Socket object connecting the client and the server.
3. Depending on the type of server, either the Socket's getInputStream( ) method, getOutputStream( ) method, or both are called to get input and output streams that communicate with the client.
4. The server and the client interact according to an agreed-upon protocol until it is time to close the connection.
5. The server, the client, or both close the connection.
6. The server returns to step 2 and waits for the next connection.

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