JDK官方对WeakReference的定义如下

Weak reference objects, which do not prevent their referents from being made finalizable, finalized, and then reclaimed. Weak references are most often used to implement canonicalizing mappings.
Suppose that the garbage collector determines at a certain point in time that an object is weakly reachable. At that time it will atomically clear all weak references to that object and all weak references to any other weakly-reachable objects from which that object is reachable through a chain of strong and soft references. At the same time it will declare all of the formerly weakly-reachable objects to be finalizable. At the same time or at some later time it will enqueue those newly-cleared weak references that are registered with reference queues.

1. 弱引用对象不会阻止其指向的对象被GC回收
2. 当一个对象是弱可达的，那么GC会原子地清除所有指向该对象的弱引用，并标记该对象为finalizable并在随后将其回收
3. 刚被清除的弱引用会被加入到ReferenceQueue

key被GC回收之后，GC会将弱引用对象本身（不是key对象）加入到queue当中去。

ConcurrentCache的设计简洁而高效，赏心悦目。