Java ConcurrentHashMap实现线程安全的代码示例

final V put(K key, int hash, V value, boolean onlyIfAbsent) {
    // 在往该 Segment 写入前，先确保获取到锁
    HashEntry<K,V> node = tryLock() ? null : scanAndLockForPut(key, hash, value); 
    V oldValue;
    try {
        // Segment 内部数组
        HashEntry<K,V>[] tab = table;
        int index = (tab.length - 1) & hash;
        HashEntry<K,V> first = entryAt(tab, index);
        for (HashEntry<K,V> e = first;;) {
            if (e != null) {
                K k;
                // 更新已有值...
            }
            else {
                // 放置 HashEntry 到特定位置，如果超过阈值则进行 rehash
                // 忽略其他代码...
            }
        }
    } finally {
        // 释放锁
        unlock();
    }
    return oldValue;
}

final V putVal(K key, V value, boolean onlyIfAbsent) { if (key == null || value == null) throw new NullPointerException();
    int hash = spread(key.hashCode());
    int binCount = 0;
    for (Node<K,V>[] tab = table;;) {
        Node<K,V> f; int n, i, fh; K fk; V fv;
        if (tab == null || (n = tab.length) == 0)
            tab = initTable();
        else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) { // 节点为空
            // 利用 CAS 去进行无锁线程安全操作，如果 bin 是空的
            if (casTabAt(tab, i, null, new Node<K,V>(hash, key, value)))
                break; 
        }
        else if ((fh = f.hash) == MOVED)
            tab = helpTransfer(tab, f);
        else if (onlyIfAbsent
                 && fh == hash
                 && ((fk = f.key) == key || (fk != null && key.equals(fk)))
                 && (fv = f.val) != null)
            return fv;
        else {
            V oldVal = null;
            synchronized (f) {
                   // 细粒度的同步修改操作... 
                }
            }
            // 如果超过阈值，升级为红黑树
            if (binCount != 0) {
                if (binCount >= TREEIFY_THRESHOLD)
                    treeifyBin(tab, i);
                if (oldVal != null)
                    return oldVal;
                break;
            }
        }
    }
    addCount(1L, binCount);
    return null;
}