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Thread: how to avoid this concurrent modification exception?

  1. #1
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    Default how to avoid this concurrent modification exception?

    I have two threads.

    the main thread and the rendering thread.

    the main thread constantly iterates through my entities lists and makes them think and does game logic, etc..

    the rendering thread constantly iterates through the entities list and renders them

    in my main thread, I put in some code that would delete an entity after such and such occurred. however, there is a random chance that this throws a concurrent modification exception with my rendering thread, and I was wondering on what I should do to avoid this?

    I'm using the synchronization keywords, but it's possible that I'm just not using them correctly to fit my situation. My understanding is that two synchronized methods/blocks cannot run concurrently.

            public void RemoveEntity( Entity e )
            {
     
                    synchronized( e )
                    {
     
                            entList.remove( e );
     
                    }
     
            }
     
            public synchronized void Draw( GL10 gl )
            {
     
                    Iterator itr = entList.iterator();
     
                    while( itr.hasNext() )
                    {
     
     
                            Entity e = ( Entity )itr.next();
     
     
     
                                    ( e ).Draw( gl );
     
     
     
                    }


  2. #2
    Super Moderator Norm's Avatar
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    Default Re: how to avoid this concurrent modification exception?

    You are synchronizing on different objects. Change the code so you are using one object.

  3. #3
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    Default Re: how to avoid this concurrent modification exception?

    Quote Originally Posted by Norm View Post
    You are synchronizing on different objects. Change the code so you are using one object.
    Alright, both my render method and my entityremove method are apart of my EntityFactory object which is shared between threads. If I made both functions synchronized, it means that one function cannot run while the other function is occuring, correct?

    So this should fix my problem?

    	public synchronized void RemoveEntity( Entity e )
    	{
     
    		entList.remove( e );
     
     
    	}
     
    	public synchronized void Draw( GL10 gl )
    	{
     
    		Iterator itr = entList.iterator();
     
    		while( itr.hasNext() )
    		{
     
     
    			Entity e = ( Entity )itr.next();
     
     
     
    				( e ).Draw( gl );
     
     
     
    		}
     
     
    	}

  4. #4
    Super Moderator Norm's Avatar
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    Default Re: how to avoid this concurrent modification exception?

    Yes, only one method should execute at a time.

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