All Reference types are cleared before they are enqueued, so the thread handling the post-mortem cleanup never has access to the referent object, only the Reference object.

所有Reference类型在加入队列之前被清除，所以处理事后检查(post -mortem)清除的线程永远不会具有referent对象的访问权，而只具有Reference对象的访问权。