Waiting for game server connection

Question

public boolean connectedOnGameServer = false;
public final Object conGameServerMonitor = new Object();

public void connectedToGameServer() {
    synchronized (conGameServerMonitor) {
        if (connectedOnGameServer != false)
            throw new RuntimeException("Player connected twice");
        connectedOnGameServer = true;
        conGameServerMonitor.notifyAll();
    }
}

public void waitForGameServerConnection() {
    synchronized (conGameServerMonitor) {
        try {
            long startTime = System.currentTimeMillis();
            long waited = 0;
            while (!connectedOnGameServer && waited < GAMESERVER_CONNECT_TIMEOUT) {
                conGameServerMonitor.wait(GAMESERVER_CONNECT_TIMEOUT - waited);
                waited = System.currentTimeMillis() - startTime;
            }
            if (waited > GAMESERVER_CONNECT_TIMEOUT && connectedOnGameServer) {
                throw new RuntimeException("Client didn't connect to game server in time (" + GAMESERVER_CONNECT_TIMEOUT + " ms)");
            }
        } catch (InterruptedException e) {
            throw new RuntimeException("Interrupted while waiting for client to connect to game server", e);
        }
    }
}

What I need is:

Thread A calls waitForGameServerConnection
Thread B calls connectedToGameServer
Thread A continues

Michael K · Accepted Answer · 2011-02-08 15:12:08Z

My code examples are excerpts. Don't copy/paste them, they are suggestions you can incorporate into your code.

if (waited > GAMESERVER_CONNECT_TIMEOUT && connectedOnGameServer)
{
   throw new RuntimeException("Client didn't connect to game server in time (" + GAMESERVER_CONNECT_TIMEOUT + " ms)");
}

can be reduced to:

if (!connectedOnServer) {
   throw new RuntimeException("Client didn't connect to game server in time (" + GAMESERVER_CONNECT_TIMEOUT + " ms)");
}

Now, you can handle this without an exception. Connections fail quite regularly, and it's easy to handle that.

public boolean waitForConnection() {
   synchronized (conGameServerMonitor) {
      // loop, guard
      return connectedOnGameServer? true : false;
   }
}

With this you can ask the player if he wants to try again, or wait a bit and try again, etc without having to catch an exception. But what if B calls connectedToGameServer() after the wait completes?

boolean connectionInProgress;

public void connectedToGame() {
   synchronized (conGameServerMonitor) {
      if (connectionInProgress) {
         connectedOnGameServer = true;
         conGameServerMonitor.notifyAll(); 
      }
   }
}

public boolean waitForConnection() {
   synchronized (conGameServerMonitor) {
      connectionInProgress = true;
      // wait
      connectionInProgress = false;
      return connectedOnGameServer? true : false;
   }
}

You handle the wakeups and interuptions correctly. Kudos for the waited part - I thought "Useless!" for a second, but I forgot that a thread could wake anytime, and it's the thread's responsibility to be sure that it's conditions are fulfilled.

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