处理软引用之后，弱可及对象的集合被识别——这样的对象上不存在强引用或软引用。
After processing the soft references, the set of weakly reachable objects is identified — objects for which no strong or soft references exist.
而且，内存中的一个对象可以被多个引用(可以是强引用、软引用、弱引用或虚引用)引用。
Furthermore, an object in memory can be referenced by multiple references that could be strong, soft, weak, or phantom. Before proceeding further, let's look at some terminology.
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