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.

According to Wikipedia, you should use ports 49152 to 65535 if you don't need a 'well known' port.

AFAIK the only way to determine wheter a port is in use is to try to open it.

If you need in range use:

public int nextFreePort(int from, int to) {
    int port = randPort(from, to);
    while (true) {
        if (isLocalPortFree(port)) {
            return port;
        } else {
            port = ThreadLocalRandom.current().nextInt(from, to);
        }
    }
}

private boolean isLocalPortFree(int port) {
    try {
        new ServerSocket(port).close();
        return true;
    } catch (IOException e) {
        return false;
    }
}