Question

Classic example of a simple server:

class ThreadPerTaskSocketServer {
   public static void main(String[] args) throws IOException {
      ServerSocket socket = new ServerSocket(80);
      while (true) {
          final Socket connection = socket.accept();
          Runnable task = new Runnable() {
              public void run() {
                 handleRequest(connection);
              }
          };
          new Thread(task).start();
      }
   }
}

Why should the Socket be declared as final? Is it because the new Thread that handles the request could refer back to the socket variable in the method and cause some sort of ConcurrentModificationException?

Answer 1

In this case, the variable must be final to be used inside the anonymous Runnable implmentation.

This is because that object will exist when the variable has already gone out of scope and has thus disappeared. The object gets a copy of the variable. In order to hide this, the variable must be final so that nobody can expect changes in one copy to be visible to the other.