AQS源码解析

//默认实现为非公平锁，这个构造方法相当于new  ReentrantLock(false) false 使用非公平锁
public ReentrantLock() {
        sync = new NonfairSync();
    }


  public final boolean tryAcquireNanos(int arg, long nanosTimeout)
            throws InterruptedException {
            //判断当前线程是否被中断
        if (Thread.interrupted())
            throw new InterruptedException();
            //2种方式获取锁方法
        return tryAcquire(arg) ||
            doAcquireNanos(arg, nanosTimeout);
    }

//try Acquire(arg) 最终会调用这个方法
final boolean nonfairTryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
                    //获取当前锁状态
            int c = getState();
                    //如果锁是空闲时
            if (c == 0) {
              //cas 更改state值
                if (compareAndSetState(0, acquires)) {
                    //成功的话设置当前锁的独占线程为当前线程
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
                    //锁已经被持有且持有者为当前线程
            else if (current == getExclusiveOwnerThread()) {
                //state增加
                int nextc = c + acquires;
                //如果增加后小于0.则报错
                if (nextc < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                //设置新state值
                setState(nextc);
                return true;
            }
            return false;
        }

        //以独占定时模式获取锁 ，在指定时间nanosTimeout内不断获取锁，并且始终都是当前节点的上一个节点先获取到锁
        //应该是公平锁的获取方式
    private boolean doAcquireNanos(int arg, long nanosTimeout)
            throws InterruptedException {
        if (nanosTimeout <= 0L)
            return false;
      //获取到超时的时间点
        final long deadline = System.nanoTime() + nanosTimeout;
        //加入队列末尾
        final Node node = addWaiter(Node.EXCLUSIVE);
        boolean failed = true;
        try {
            for (;;) {
              //获取当前节点的上一个节点
                final Node p = node.predecessor();
              //如果是头节点且获取锁成功 tryAcquire（非公平锁方法）
                if (p == head && tryAcquire(arg)) {
                  //将头节点变为当前节点
                    setHead(node);
                  //头节点认为设置null，方便垃圾回收
                    p.next = null; // help GC
                    failed = false;
                    return true;
                }
                nanosTimeout = deadline - System.nanoTime();
                //如果超时，则直接返回
                if (nanosTimeout <= 0L)
                    return false;
                
                if (shouldParkAfterFailedAcquire(p, node) &&
                    nanosTimeout > spinForTimeoutThreshold)
                  //虚假中断
                    LockSupport.parkNanos(this, nanosTimeout);
                if (Thread.interrupted())
                    throw new InterruptedException();
            }
        } finally {
          //失败的话，取消获取
            if (failed)
                cancelAcquire(node);
        }
    }

//加入等待队列
private Node addWaiter(Node mode) {
        Node node = new Node(Thread.currentThread(), mode);
        // Try the fast path of enq; backup to full enq on failure
            //最新的对尾元素
        Node pred = tail;
            //如果队列存在
        if (pred != null) {
            node.prev = pred;
            //设置当前队尾元素为当前任务
            if (compareAndSetTail(pred, node)) {
                pred.next = node;
                return node;
            }
        }
        enq(node);
        return node;
    }


//初始化队列
 private Node enq(final Node node) {
        for (;;) {
            Node t = tail;
            if (t == null) { // Must initialize
              //设置当前头元素
                if (compareAndSetHead(new Node()))
                    tail = head;
            } else {
              //加入队为 
                node.prev = t;
              //设置当前队尾元素
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }


    private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
        int ws = pred.waitStatus;
        if (ws == Node.SIGNAL)
            /*
             * This node has already set status asking a release
             * to signal it, so it can safely park.
             */
            //说明需要通知后续节点
            return true;
        if (ws > 0) {
            /*
             * Predecessor was cancelled. Skip over predecessors and
             * indicate retry.
             */
          //大于0说明该抢锁任务被取消了
            do {
                node.prev = pred = pred.prev;
            } while (pred.waitStatus > 0);
            pred.next = node;
        } else {
            /*
             * waitStatus must be 0 or PROPAGATE.  Indicate that we
             * need a signal, but don't park yet.  Caller will need to
             * retry to make sure it cannot acquire before parking.
             */
          //waitStatus 为0 或者-3时 说明需要重试 设置waitStatus的值
          //设置一个信号量，为后续唤醒
            compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
        }
        return false;
    }

//公平锁的加锁方式
   protected final boolean tryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
              //如果当前线程前面有一个排队线程，则等待，如果当前线程位于队列的头部或队列为空，则直接获取
                if (!hasQueuedPredecessors() &&
                    compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
                    //如果当前线程是持有锁线程，则直接获取
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0)
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }