1 /**

  * Reference queue for cleared WeakEntries

  */

 // 所有Entry在构造时都传入该queue

 private final ReferenceQueue<Object> queue = new ReferenceQueue<>();

 /**

  * Expunges stale entries from the table.

  */

 private void expungeStaleEntries() {

     for (Object x; (x = queue.poll()) != null; ) {

         synchronized (queue) {

             // e 为要清理的对象

             @SuppressWarnings("unchecked")

                 Entry<K,V> e = (Entry<K,V>) x;

             int i = indexFor(e.hash, table.length);

             Entry<K,V> prev = table[i];

             Entry<K,V> p = prev;

             // while 循环遍历冲突链

             while (p != null) {

                 Entry<K,V> next = p.next;

                 if (p == e) {

                     if (prev == e)  //表示x是桶链表的第一个元素

                         table[i] = next;

                     else

                         prev.next = next;  //跳过x元素

                     // Must not null out e.next;

                     // stale entries may be in use by a HashIterator

                     // 可以看到这里把value赋值为null，来帮助 GC 回收强引用的value

                     e.value = null; // Help GC

                     size--;

                     break;

                 }

                 prev = p;

                 p = next;

             }

         }

     }

 }