How to find an available port?

If you don't mind the port used, specify a port of 0 to the ServerSocket constructor and it will listen on any free port.

ServerSocket s = new ServerSocket(0);System.out.println("listening on port: " + s.getLocalPort());

If you want to use a specific set of ports, then the easiest way is probably to iterate through them until one works. Something like this:

public ServerSocket create(int[] ports) throws IOException {    for (int port : ports) {        try {            return new ServerSocket(port);        } catch (IOException ex) {            continue; // try next port        }    }    // if the program gets here, no port in the range was found    throw new IOException("no free port found");}

Could be used like so:

try {    ServerSocket s = create(new int[] { 3843, 4584, 4843 });    System.out.println("listening on port: " + s.getLocalPort());} catch (IOException ex) {    System.err.println("no available ports");}

If you pass 0 as the port number to the constructor of ServerSocket, It will allocate a port for you.

Starting from Java 1.7 you can use try-with-resources like this:

  private Integer findRandomOpenPortOnAllLocalInterfaces() throws IOException {    try (        ServerSocket socket = new ServerSocket(0);    ) {      return socket.getLocalPort();    }  }

If you need to find an open port on a specific interface check ServerSocket documentation for alternative constructors.

Warning: Any code using the port number returned by this method is subject to a race condition - a different process / thread may bind to the same port immediately after we close the ServerSocket instance.