AbstractQueuedSynchronizer 源码理解

UnSafe类（原子操作）：

/** 

* 比较obj的offset处内存位置中的值和期望的值，如果相同则更新。此更新是不可中断的。 

*  

* @param obj 需要更新的对象 

* @param offset obj中整型field的偏移量 

* @param expect 希望field中存在的值 

* @param update 如果期望值expect与field的当前值相同，设置filed的值为这个新值 

* @return 如果field的值被更改返回true 

*/  

public native boolean compareAndSwapInt(Object obj, long offset, int expect, int update);  

AbstractQueuedSynchronizer类：

     * <p>To enqueue into a CLH lock, you atomically splice it in as new

     * tail. To dequeue, you just set the head field.

     * <pre>

     *      + ------+  prev +----- +       +-----+

     * head |      | <---- |     | <---- |     |  tail

     *      + ------+       +----- +       +-----+

     * </pre>

 /**

     * Inserts node into queue, initializing if necessary. See picture above.

     * @param node the node to insert

     * @return node's predecessor

     */

    private Node enq( final Node node) {

        for (;;) {

            Node t = tail;

            if (t == null ) { // Must initialize

                //如果头部为空，则更新，并赋值给tail

                if (compareAndSetHead(new Node()))

                    tail = head;

            } else {

                //让tail指向新插入的结点

                node. prev = t;

                if (compareAndSetTail(t, node)) {

                    t. next = node;

                    return t;

                }

            }

        }

    }

 /**

     * Creates and enqueues node for current thread and given mode.

     *

     * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared 标明是否共享： addWaiter(Node. EXCLUSIVE) null 

addWaiter(Node. SHARED); new Node();

     * @return the new node

     */

    private Node addWaiter(Node mode) {

        Node node = new Node(Thread.currentThread(), mode);

        // Try the fast path of enq; backup to full enq on failure

        Node pred = tail;

        if (pred != null) {

            node. prev = pred;

            if (compareAndSetTail(pred, node)) {

                pred. next = node;

                return node;

            }

        }

        enq(node);

        return node;

    }