仅在必要时使用长弱引用，因为在终止后对象的状态是不可预知的。
Use long weak references only when necessary as the state of the object is unpredictable after finalization.
如果一个指针指向一个不在图中的对象(注意此时图中已包含Finalize可到达的对象)，那么此对象就是一个不可到达的对象，垃圾收集器就将长弱引用表相应的槽置空。
If a pointer pointing to an object not in the graph, then the object is not a reachable object, and then garbage collector sets the slot to null in the long weak reference table.
在短周期对象销毁后，长周期对象对短周期对象的弱引用依然会长久保留下来。
The weak reference from the long-lived object to the short-lived object is kept long after the short-lived object has been destroyed.
在短周期对象销毁后，长周期对象对短周期对象的弱引用依然会长久保留下来。
The weak reference from the long-lived object to the short-lived object is kept long after the short-lived object has been destroyed.
应用推荐