Socket Programming (Server)

26 November 2007

Socket programming is easy and fun. In this post, I will create a simple Socket Server that will listen on a post for client requests. It will get data from the client and will send the acknowledgment.

First step is to declare a server socket, a client socket and input/output streams. I assume that you are familiar with streams. Then we have to open a connection on a port. Usually it is not a wise decision to use port number that is less than 1023 because ports less than 1023 are reserved of OS use. We will create a SeverSocket object and will specify port on which that ServerSocket will be opened. Once we have the ServerSocket object, we will create a client socket object that will wait for the client’s request.

clientSocket = echoServer.accept();

Once the connection is made, streams will be initialized and data transfer can take place. We are trying to get data from user and then simply send connection date/time to the client. I know it doesn’t make sense, but you can send any thing back to the client. This is just an example.

        ServerSocket echoServer = null;
        String line;
        DataInputStream is;
        PrintStream os;
        Socket clientSocket = null;
 
 
        try {
           echoServer = new ServerSocket(2222);
        }
        catch (IOException e) {
           System.out.println(e);
        }  
 
    try {
        System.out.println("Waiting for the client request ... ");   
    	clientSocket = echoServer.accept();
    	System.out.println("Request received.");
           is = new DataInputStream(clientSocket.getInputStream());
           os = new PrintStream(clientSocket.getOutputStream());
 
           os.println("Hi. You connected to the Server at: " + new Date()); 
 
           while (true) {
             line = is.readLine();
           System.out.println("Client: "+line);
             os.flush();
           }
        }   
    catch (IOException e) {
          e.printStackTrace();
        }

If the port you are using is already in use, you will get following error:

java.net.BindException: Address already in use: JVM_Bind

The Socket Server will keep on waiting for the connections and will send and receive the data. It is very interesting to use such servers on a network.

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