How would I choose in java that which port is free to use for ServerSocket or Socket?

Question

I'm working on java. How would I check that a port is free to use for a ServerSocket? Moreover when a Socket is returned by the accept() function is it given a port and IP address by default or I would have to specify that too?

Answer 1

You can use the java.net.ServerSocket constructor with port 0 which tells ServerSocket to find a free port.

documentation:

port - the port number, or 0 to use a port number that is automatically allocated.

Example:

int port = -1;
try {
    ServerSocket socket = new ServerSocket(0);
    // here's your free port
    port = socket.getLocalPort();
    socket.close();
} 
catch (IOException ioe) {}

Answer 2

Use Try catch to find a free port

private static int port=9000;
public static int detectPort(int prt)
{
try{
//connect to port
}
catch(Exception e)
{
return detectPort(prt+1);
}
return prt;
}

// Write detectPort(port); inside main