引用队列的一种常见用法是，例如，子类
WeakReference附加需要清理的信息，然后轮询
ReferenceQueue来获取清理任务。
ReferenceQueue<Foo> fooQueue = new ReferenceQueue<Foo>();class ReferenceWithCleanup extends WeakReference<Foo> { 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().}例如，选择Guava的内部
CacheBuilder实现时使用的是这种方法。
weakKeys
