weak reference

基本解释

Weakly reachable: An object that is not strongly or softly reachable and can be accessed through a weak reference.
弱可及对象（weakly reachable）：不是强可及对象也不是软可及对象，并且能够通过弱引用访问的对象。
Weak references tell the garbage collector that it should collect an object if there are no references to it other than the weak reference.
弱引用告诉垃圾收集器如果某个对象除了弱引用之外没有其他任何引用，则应该收集该对象。
Using a weak reference here keeps objects that are above the minimum around as long as possible, but makes them available for GC as required.
在此使用弱引用可以使除最低数量对象以外的对象尽可能久地保持活跃状态，但是使它们可供 GC 根据需要使用。