Question

I’ve written the following code to implement a Threaded Server:

ServerSocket passiveSocket = new ServerSocket(port, maxConnections);  
while(true){
  final Socket socket = passiveSocket.accept();                       
  new Thread(new Runnable() {
      public void run() {
          //access socket as needed to communicate. E.g.:
          PrintWriter writer = new PrintWriter(socket.getOutputStream());
          //Finally close socket.
          socket.close();
      }
  }).start();
}

Now this seems to work, but in reflection I do not really understand what’s going on with the final socket variable socket when the next connection arrives. How is each thread associated with the socket instance which was current when starting the thread? – is the final keyword responsible for that?

Answer 1

Think of it this way: socket is secretly passed as an argument to the new Runnable constructor, and it’s kept as a variable in the anonymous Runnable class. (That’s actually how it works, at the bytecode level.)

The created Runnable object contains a reference to the one specific Socket value at its creation time, so it can close that specific socket when it’s done.