Synchronization work with conditionals? How can I

Question

Given the following code:

public class SomeClass {

  private boolean shouldBlock = false;
  private Object resource;

  public void handleDrawRequest(Canvas canvas) {
    if (!shouldBlock && resource == null)
    {
       shouldBlock = true;
       loadTheResource();  //which takes awhile
       shouldBlock = false;
    }
    else if (shouldBlock && resrouce == null)
    {
       return;  //another thread is taking care of the loading of the resource
                //and its not ready yet, so just ignore this request
    }

    drawResourceOn(canvas);
  }
}

How can I make this code thread safe? What I'm trying to accomplish is for one and only one thread to load the resource while any other thread attempting access this code at the same time to be discarded (e.g. follow the 'else if' logic) until the resource is loaded. There could be many threads trying to access this code at the same time and I don't want to synchronize the entire method and have a whole stack of threads pile up.

Answer 1

You're looking for an AtomicBoolean

public class SomeClass {
  // AtomicBolean defaults to the value false.
  private AtomicBoolean loadingResource = new AtomicBoolean();
  private volatile Object resource;

  public void handleDrawRequest(Canvas canvas) {
    if (resource == null) {
      if (loadingResource.compareAndSet(false, true)) {
        loadTheResource();  //which takes awhile
      } else {
        //another thread is taking care of the loading of the resource
        //and its not ready yet, so just ignore this request
        return;  
      }
    } else {
      drawResourceOn(canvas);
    }
  }
}

Answer 2

With double checked non-blocking locking:

public class SomeClass {

    private Lock lock = new Lock();
    private volatile Object resource;

    public void handleDrawRequest(Canvas canvas) {
        if( resource==null ) {
            if( lock.tryLock() ) {
                try {
                    if( resource==null )
                        resource = loadResource();
                }
                finally {
                    lock.unlock();
                }
            }
            else {
                return;
            }
        }
        drawResourceOn(canvas);
    }
}

If you don't make resource volatile, threads are free to cache it and might never read the updated value. In particular, the second null check will always return true, even if the resource has been loaded after the first one.