Using Java's ReferenceQueue

Question

Do SoftReference and WeakReference really only help when created as instance variables? Is there any benefit to using them in method scope?

Answer 1

One common idiom with reference queues is to e.g. subclass WeakReference to attach information that's needed to clean up things, and then to poll a ReferenceQueue to get cleanup tasks.

ReferenceQueue fooQueue = new ReferenceQueue();

class ReferenceWithCleanup extends WeakReference {
  Bar bar;
  ReferenceWithCleanup(Foo foo, Bar bar) {
    super(foo, fooQueue);
    this.bar = bar;
  }
  public void cleanUp() {
    bar.cleanUp();
  }
}

public Thread cleanupThread = new Thread() {
  public void run() {
    while(true) {
      ReferenceWithCleanup ref = (ReferenceWithCleanup)fooQueue.remove();
      ref.cleanUp();
    }
  }
}

public void doStuff() {
  cleanupThread.start();
  Foo foo = new Foo();
  Bar bar = new Bar();
  ReferenceWithCleanup ref = new ReferenceWithCleanup(foo, bar);
  ... // From now on, once you release all non-weak references to foo,
      // then at some indeterminate point in the future, bar.cleanUp() will
      // be run. You can force it by calling ref.enqueue().
}

For example, the internals of Guava's CacheBuilder implementation when weakKeys are selected uses this approach.