java unit test ExecutorService

I'm having trouble finding a way to unit test some code that uses multiple threads. Below is the code that is giving me problems:

public class ThreadHandler {
  private ExecutorService executorService;
  private boolean isFinished = false;

  public void start() {
    isFinished = false;
    executorService = Executors.newFixedThreadPool(10);
  }

  public void stop() {
    isFinished = true;
    executorService.shutdown();
  }

  public void listen(final ServerSocket serverSocket) throws IOException {
    Runnable runnable = new Runnable() {
      @Override
      public void run() {
        Socket socket = null;
        try {
          while (!isFinished) {
            socket = serverSocket.accept();
            executorService.submit(new ClassIWantToMockOut(socket));
          }
        } catch (IOException e) {
          ...
        } finally {
          try {
            socket.close();
          } catch(IOException e) {
            ...
          }
        }
      }
    };

    this.createThread(runnable);
  }

  public void createThread(Runnable runnable) {
    Thread serverThread = new Thread(runnable);
    serverThread.start();
  }
}

I would like to test the listen method by mocking out ClassIWantToMockOut(socket). As far as I've read, though, it is not possible to mock a class constructor. Is there anything I could do to make this code more unit test friendly?