IllegalMonitorStateException

[AndrewM16921]

I'm working on an EventManager system for my latest project, and I want it to work in its own thread. Naturally, I want it to wait while there are no events to process, then wake up when there is work to be done. But I seem to be getting a java.lang.IllegalMonitorStateException when I implement it, so I'm guessing that I'm not quite doing it right. Help appreciated...

package org.nubcraft.nd.event.handlers;

import java.util.HashMap;
import java.util.LinkedList;
import java.util.Map;

import org.nubcraft.api.threading.GracefulRunnable;
import org.nubcraft.nd.core.ND;
import org.nubcraft.nd.event.Event;

public class EventManager extends GracefulRunnable
{
	private final Map<Event.Type, EventHandler> handlers = new HashMap<Event.Type, EventHandler>();
	private final LinkedList<Event> queue = new LinkedList<Event>();
	
	public EventManager()
	{
		
	}
	
	@Override
	public void _run()
	{
		while(queue.isEmpty())
		{
			try
			{
				queue.wait();
			}
			catch(InterruptedException e)
			{
				ND.logger.warn(e, true);
			}
		}
		
		while(!queue.isEmpty())
		{
			Event event;
			synchronized(queue)
			{
				event = queue.remove(0);
			}
			try
			{
				handlers.get(event.getType()).handle(event);
			}
			catch(Exception e)
			{
				ND.logger.warn(e, true);
			}
		}
	}
	
	public void queue(Event event)
	{
		synchronized(queue)
		{
			queue.add(event);
		}
		queue.notify();
	}

	@Override
	public void onStop()
	{
		ND.logger.info(queue.size() + " events unhandled on shutdown", true);
	}
}

package org.nubcraft.api.threading;

public abstract class GracefulRunnable implements Runnable, _Runnable, Stoppable
{
	private boolean running;
	
	public GracefulRunnable()
	{
		
	}
	
	public Thread start()
	{
		Thread thread = new Thread(this);
		thread.start();
		
		return thread;
	}
	
	public Thread start(String threadName)
	{
		Thread thread = new Thread(this, threadName);
		thread.start();
		
		return thread;
	}
	
	@Override
	public void requestStop()
	{
		running = false;
	}
	
	@Override
	public final void run()
	{
		running = true;
		while(running)
		{
			_run();
		}
		onStop();
	}
}

[Fubarable]

Could you please tell us if the IllegalMonitorStateException text indicates which line is throwing the exception, if it always occurs, if it's always at the same location, and any other information that might help us?

[AndrewM16921]

Exception in thread "Event Manager" java.lang.IllegalMonitorStateException
at java.lang.Object.wait(Native Method)
at java.lang.Object.wait(Object.java:503)
at org.nubcraft.nd.event.handlers.EventManager._run(E ventManager.java:28)
at org.nubcraft.api.threading.GracefulRunnable.run(Gr acefulRunnable.java:40)
at java.lang.Thread.run(Thread.java:722)

...
while(queue.isEmpty())
{
	try
	{
		queue.wait(); // <-- here
	}
	catch(InterruptedException e)
	{
		ND.logger.warn(e, true);
	}
}
...

[Fubarable]

I believe that you need to be in a synchronized block for Object#wait() to work correctly. It looks like your wait method is called in a non-synchronized section of code.

[AndrewM16921]

Oh wow. That fixed it. How simple, lol. Thanks! :)

[Fubarable]

You're welcome! I've not used them, but as of Java 1.5 I believe that there are other tools available for locking, waiting and notifying threads, tools that allow greater flexibility and that are part of the java.util.concurrent library including Locks. You might want to look into them a bit.

[AndrewM16921]

Ok I shall, thanks again.