expungeStaleEntries函数解析

 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;
             }
         }
     }
 }