Map.WeakHashMap

#WeakHashMap

• 結構同HashMap，內部數組，數組相同hash鏈表，不一樣HashMap，WeakHashMap內部鏈表節點繼承弱引用，並在get的時候根據弱引用的構造參數隊列，來判斷GC，同時移除Key

private static class Entry<K,V> extends WeakReference<Object> implements Map.Entry<K,V> {
        V value;
        final int hash;
        Entry<K,V> next;
        /**
         * Creates new entry.
         */
        Entry(Object key, V value,
              ReferenceQueue<Object> queue,
              int hash, Entry<K,V> next) {
            //弱引用構造
            super(key, queue);
            this.value = value;
            this.hash  = hash;
            this.next  = next;
        }

public V get(Object key) {
        Object k = maskNull(key);
        int h = hash(k);
        //得到通過移除key的數組table[]
        Entry<K,V>[] tab = getTable();
        int index = indexFor(h, tab.length);
        Entry<K,V> e = tab[index];
        //相同hash鏈表 遍歷查找相同
        while (e != null) {
            if (e.hash == h && eq(k, e.get()))
                return e.value;
            e = e.next;
        }
        return null;
    }

//getTable調用最後到這裏
    //弱引用key回收會加入到隊列，由JVM完成
     private void expungeStaleEntries() {
        //遍歷要回收的key
        for (Object x; (x = queue.poll()) != null; ) {
            //同步塊，考慮回收時，有Put操做改變table[]
            synchronized (queue) {
                @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 (p != null) {
                    //鏈表下一節點
                    Entry<K,V> next = p.next;
                    //發現回收節點
                    if (p == e) {
                        if (prev == e)
                            //刪除e，鏈表開頭指向回收節點的next
                            table[i] = next;
                        else
                            prev.next = next;
                        // Must not null out e.next;
                        // stale entries may be in use by a HashIterator
                        e.value = null; // Help GC
                        size--;
                        break;
                    }
                    prev = p;
                    p = next;
                }
            }
        }
    }