How to get the dynamic port number in Java for Socket client

Question

I try to find out how to get the Portnumber for a Socket dynamic.

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

I found this example on StackOverflow. It works but I do not need a ServerSocket. I need a Socket:

Socket s = new Socket("http://www.google.at", 8081);
System.out.println("listening on port: " + s.getLocalPort())

I was searching with google and did not find the Solution. Maybe one can help me. I am sure there is a way to get the Portnumber for Socket Dynamic. But I could not find it on StackOverflow and on google.

I already try this one:

Socket s = new Socket("http://www.google.at", 0);
System.out.println("listening on port: " + s.getLocalPort())

greetings Christoph

Answer 1

You have already found the solution. Socket.getLocalPort() gets you the local port of a Socket . Strangely enough. If you haven't bound that Socket , as here, or bound it to port zero, the local port is allocated dynamically.

new Socket("...", 0) (a) is illegal and (b) would constitute an attempt to connect to a dynamic remote port, if it was legal, which it isn't.

Why you need to know the local port of a Socket is another question.

NB ServerSockets listen. Sockets connect.