ThreadLocalMap类中的弱引用WeakReference

public abstract class Reference<T> {

看源代码和注释发现。

引用Reference作为抽象类，包含一个私有的T referent --------中文翻译为"指示物"

A Reference instance is in one of four possible internal states：

Active：Subject to special treatment by the garbage collector

Pending：An element of the pending-Reference list, waiting to be enqueued by the Reference-handler thread.  Unregistered instances are never in this state.

Enqueued：An element of the queue with which the instance was registered when it was created.  When an instance is removed from its ReferenceQueue, it is made Inactive.

Inactive：Nothing more to do.  Once an instance becomes Inactive its state will never change again.
可以看到关键字是4种状态Active激活,Pending等待,Enqueued排队,Inactive失效

instance---->registered--->in pending-Reference list(Pending)---->into ReferenceQueue(Enqueued)---->removed from ReferenceQueue(Inactive)
即实例---->注册---->进入等待状态----进入到队列内后排队状态----从队列中移除后失效

   /* When active:   NULL
     *     pending:   this
     *    Enqueued:   next reference in queue (or this if last)
     *    Inactive:   this
     */
    @SuppressWarnings("rawtypes")
    Reference next;

    /* When active:   next element in a discovered reference list maintained by GC (or this if last)
     *     pending:   next element in the pending list (or null if last)
     *   otherwise:   NULL
     */
    transient private Reference<T> discovered;  /* used by VM */


    /* Object used to synchronize with the garbage collector.  The collector
     * must acquire this lock at the beginning of each collection cycle.  It is
     * therefore critical that any code holding this lock complete as quickly
     * as possible, allocate no new objects, and avoid calling user code.
     */
    static private class Lock { }
    private static Lock lock = new Lock();


    /* List of References waiting to be enqueued.  The collector adds
     * References to this list, while the Reference-handler thread removes
     * them.  This list is protected by the above lock object. The
     * list uses the discovered field to link its elements.
     */
    private static Reference<Object> pending = null;