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;
            }
        }
    }
}