Java Sockets

What are TCP sockets?

A TCP socket is defined as an endpoint for communication. A socket consists of the pair <IP Address,Port>. For our purposes, a port will be defined as an integer number between 1024 and 65535. This is because all port numbers smaller than 1024 are considered well-known -- for example, telnet uses port 23, http uses 80, ftp uses 21, and so on. On Unix machines, the file /etc/services contains a list of services provided by that machine, along with their well-known ports

A TCP connection consists of a pair of sockets. Sockets are distinguished by client and server sockets. A server listens on a port, waiting for incoming requests from clients.

For example, a web server listens at port 80 for incoming request from clients (web browsers). When a client wishes to make a connection with a server socket, the client is assigned a port from the local host. Suppose that client X (at IP address 146.86.3.15) wishes to browse a web page on the server 146.86.5.20. If the port the local host assigned client X is port 12345, the connection between the client and the server is uniquely identified by the socket pair

            <146.86.3.15 : 12345, 146.86.5.20 : 80>

Java Servers

This section walks you through the code that implements a simple echo server program. The server program begins by creating a new ServerSocket object to listen on a specific port. When writing a server, one should choose a port that is not already dedicated to some other service. In the code below, the server listens omn port 1234:

    try {
        serverSocket = new ServerSocket(1234);
    } catch (IOException e) {
        System.out.println("Could not listen on port: 1234");
        System.exit(-1);
    }

    Socket clientSocket = null;
    try {
        clientSocket = serverSocket.accept();
    } catch (IOException e) {
        System.out.println("Accept failed: 1234");
        System.exit(-1);
    }

After the server successfully establishes a connection with a client, it can communicate with the client using code similar to this:

    PrintWriter out = new PrintWriter(
                          clientSocket.getOutputStream(), true);
    BufferedReader in = new BufferedReader(
                        new InputStreamReader(
                            clientSocket.getInputStream()));
    String inputLine, outputLine;

    while ((inputLine = in.readLine()) != null) {	
        outputLine = "Server: " + inputline;
        out.println(outputLine);
        if outputLine.equals("bye"))
            break;
    }

Well-behaved servers should clean up by closing all of the input and output streams, the client socket, and the server socket:

    out.close();
    in.close();
    clientSocket.close();
    serverSocket.close();

Java Clients

This section walks you through the code that implements the client program corresponding to the server above. When you start the client program, the server should already be running and listening to the port, waiting for a client to request a connection. So, the first thing the client program does is to open a socket that is connected to the server running on the hostname and port specified:

    mySocket = new Socket("localhost", 1234);
    out = new PrintWriter(mySocket.getOutputStream(), true);
    in = new BufferedReader(new InputStreamReader(
                                mySocket.getInputStream()));

    while (1) {
        fromUser = System.in.readLine();
        if(fromUser == null)
            break;
        out.println(fromUser);
        fromServer = in.readLine();        
        System.out.println(fromServer);
        if (fromServer.equals("bye"))
            break;
    }

In the interest of good housekeeping, the client should also closes its input and output streams and the socket:

    out.close();
    in.close();
    mySocket.close();

Complete Java Client-Server Echo Program

Testing the client-server echo program