The java.net.ServerSocket Constructors

The java.net.ServerSocket class has three constructors that let you specify the port to bind to, the queue length for incoming connections, and the IP address to bind to:

 public ServerSocket(int port) throws IOException
 public ServerSocket(int port, int backlog) throws IOException
 public ServerSocket(int port, int backlog, InetAddress bindAddr) 
  throws IOException

Normally you only specify the port you want to listen on, like this:

 try {
    ServerSocket ss = new ServerSocket(80);
  }
  catch (IOException ex) {
    System.err.println(ex);
  }

When you create a ServerSocket object, it attempts to bind to the port on the local host given by the port argument. If another server socket is already listening to the port, then a java.net.BindException, a subclass of java.io.IOException, is thrown. No more than one process or thread can listen to a particular port at a time. This includes non-Java processes or threads. For example, if there's already an HTTP server running on port 80, you won't be able to bind to port 80.

On Unix systems (but not Windows or the Mac) your program must be running as root to bind to a port between 1 and 1023.

0 is a special port number. It tells Java to pick an available port. You can then find out what port it's picked with the getLocalPort() method. This is useful if the client and the server have already established a separate channel of communication over which the chosen port number can be communicated.

For example, the ftp protocol uses two sockets. The initial connection is made by the client to the server to send commands. One of the commands sent tells the server the name of the port on which the client is listening. The server then connects to the client on this port to send data.

 try {
    ServerSocket ftpdata = new ServerSocket(0);
    int port = ftpdata.getLocalPort();
  }
  catch (IOException ex) {
    System.err.println(ex);
  }