Question

I am fairly new to JAVA and especially concurrency, so probably/hopefully this is fairly straight forward problem.

Basically from my main thread I have this:

public void playerTurn(Move move)
{

  // Wait until able to move
  while( !gameRoom.game.getCurrentPlayer().getAllowMove() )
  {
    try {
      Thread.sleep(200);
      trace("waiting for player to be available");
    } catch (InterruptedException e) {
      e.printStackTrace();
    }
  }
  gameRoom.getGame().handle(move);
}

gameRoom.getGame() is on its own thread.
gameRoom.getGame().handle() is synchronized
gameRoom.game.getCurrentPlayer() is on a varible of gameRoom.getGame(), it is in the same thread

allowMoves is set to false as soon as handle(move) is called, and back to true once it has finished processing the move.

I call playerTurn() multiple times. I actually call it from a SmartFoxServer extension, as and when it receives a request, often in quick succession.

My problem is, most times it works. However SOMETIMES it is issuing multiple handle(move) calls even though allowMoves should be false. Its not waiting for it to be true again. I thought its possible that the game thread didn’t have a chance to set allowMoves before another handle(move) was called. I added volatile to allowMoves, and ensured the functions on the game thread were set to synchronized. But the problem is still happening.

These are in my Game class:

synchronized public void handle(Object msg)
{
  lastMessage = msg;
  notify();
} 

synchronized public Move move() throws InterruptedException
{
  while (true)
  {
   allowMoves = true;
   System.out.print(" waiting for move()...");
   wait();
   allowMoves = false;
   if (lastMessage instanceof Move)
   {
    System.out.print(" process move()...");
    Move m = (Move) lastMessage;
    return m;
   }
  }
}

public volatile boolean allowMoves;
synchronized public boolean getAllowMoves()
{
  return allowMoves;
}

As I said, I am new to this and probably a little ahead of myself (as per usual, but its kinda my style to jump into the deep end, great for a quick learning curve anyway).

Cheers for your help.

Answer 1

Not sure if this will help, but what if you will use AtomicBoolean instead of synchronized and volatile? It says that it is lock-free and thread-safe.