How can I know a port being used or not

Question

Hello, I have a trouble, I don't know how to realize whether a port has been used or not.
For example, I have some command:
ServerSocket server1 = new ServerSocket(2001);
ServerSocket server2 = new ServerSocket(2002);
ServerSocket server3 = new ServerSocket(2003);
ServerSocket server4 = new ServerSocket(2004);
I wonder how I can realize the fact that server1, server1, server3 or server4 is avalable?
Please help me. Thank you.

Answer 1

Maybe you misunderstood my question. Not in the slightest. I answered the question you asked.
The fact is that you asked the wrong question. You asked about ports in use. You're now asking how to know if a Socket is active. That's a different question. A port is different from a socket, and they're both different from a connection.
To answer that new question, Java provides Socket.isClosed(), Socket.isConnected(), etc.
But note that all these APIs can do, in any language, is tell you what you have personally done with that Socket instance, in the current application. So actually these APIs are formally redundant: all they do is just maintain a few booleans recording what methods have been called. What they don't do, and can't do, in any language, is tell you about the state of the connection. Only reading or writing can do that.