本篇宗旨：simple & stupid
WeakHashMap （弱引用的哈希表）
Hash table based implementation of the Map interface, with weak keys. An entry in a WeakHashMap will automatically be removed when its key is no longer in ordinary use. More precisely, the presence of a mapping for a given key will not prevent the key from being discarded by the garbage collector, that is, made finalizable, finalized, and then reclaimed. When a key has been discarded its entry is effectively removed from the map, so this class behaves somewhat differently from other Map implementations.
基于弱引用的键Key实现的哈希表，当GC发现某个弱引用的键可以回收时，键值对<K,V>也会从表中移除。
用法：
private static final WeakHashMap<Configuration,Object> REGISTRY =
new WeakHashMap<Configuration,Object>();用途：
当需要做缓存Cache时，如果不及时释放一些缓存中的不用的对象所占用的内存，是一种对内存的浪费，因为缓存的生命周期往往和容纳此缓存的组件的生命周期一样长。
如果考虑用哈希表来做<Key,Value>式的缓存，将一个不再使用的对象从哈希表移除的方式就是手工编写代码移除。这很麻烦，也容易忘记。
这种情况，就推荐使用WeakHashMap。
当Key是weak-reachable 弱可达状态时，Key和Value都会从表中移除。
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