expungeStaleEntries函数解析

Posted 2020-09-28 不吃陈皮

 1 /**
 2  * Reference queue for cleared WeakEntries
 3  */
 4 // 所有Entry在构造时都传入该queue
 5 private final ReferenceQueue<Object> queue = new ReferenceQueue<>();
 6 /**
 7  * Expunges stale entries from the table.
 8  */
 9 private void expungeStaleEntries() {
10     for (Object x; (x = queue.poll()) != null; ) {
11         synchronized (queue) {
12             // e 为要清理的对象
13             @SuppressWarnings("unchecked")
14                 Entry<K,V> e = (Entry<K,V>) x;
15             int i = indexFor(e.hash, table.length);
16             Entry<K,V> prev = table[i];
17             Entry<K,V> p = prev;
18             // while 循环遍历冲突链
19             while (p != null) {
20                 Entry<K,V> next = p.next;
21                 if (p == e) {
22                     if (prev == e)  //表示x是桶链表的第一个元素
23                         table[i] = next;
24                     else
25                         prev.next = next;  //跳过x元素
26                     // Must not null out e.next;
27                     // stale entries may be in use by a HashIterator
28                     // 可以看到这里把value赋值为null，来帮助 GC 回收强引用的value
29                     e.value = null; // Help GC
30                     size--;
31                     break;
32                 }
33                 prev = p;
34                 p = next;
35             }
36         }
37     }
38 }