ReentrantLock默认使用非公平锁

    public ReentrantLock() {
        sync = new NonfairSync();
    }

所谓公平锁就是先阻塞的程序先获得锁。非公平锁则有操作系统的调度系统来调度。

NonfairSync就是一个同步器

    final static class NonfairSync extends Sync {
        private static final long serialVersionUID = 7316153563782823691L;

        /**
         * Performs lock.  Try immediate barge, backing up to normal
         * acquire on failure.
         */
        final void lock() {
            if (compareAndSetState(0, 1))
                setExclusiveOwnerThread(Thread.currentThread());
            else
                acquire(1);
        }

        protected final boolean tryAcquire(int acquires) {
            return nonfairTryAcquire(acquires);
        }
    }

而Sync则是这样实现的

    static abstract class Sync extends AbstractQueuedSynchronizer {

现在来关注下ReentrantLock的lock方法

    public void lock() {
        sync.lock();
    }

        final void lock() {
            if (compareAndSetState(0, 1))
                setExclusiveOwnerThread(Thread.currentThread());
            else
                acquire(1);
        }

而这个用的AbstractQueueSynchronizer中的方法来实现的

    protected final boolean compareAndSetState(int expect, int update) {
        // See below for intrinsics setup to support this
        return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
    }

同时执行

    protected final void setExclusiveOwnerThread(Thread t) {
        exclusiveOwnerThread = t;
    }

那执行compareAndSetState失败的函数则执行acquire这个函数。看一下这个函数都干嘛了呢

<pre name="code" class="java">    public final void acquire(int arg) {
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }

        protected final boolean tryAcquire(int acquires) {
            return nonfairTryAcquire(acquires);
        }

        final boolean nonfairTryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                if (compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }

    public final void acquire(int arg) {
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }

    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;
    }

否则初始化一个队列

    public final void acquire(int arg) {
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }

    final boolean acquireQueued(final Node node, int arg) {
        try {
            boolean interrupted = false;
            for (;;) {
                final Node p = node.predecessor();
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    return interrupted;
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } catch (RuntimeException ex) {
            cancelAcquire(node);
            throw ex;
        }
    }

若失败，则执行这个方法shouldParkAfterFailedAcquire，parkAndCheckInterrupt

意思就是如果获取失败，就挂起当前线程，进去这个方法里面，看一看

    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
             */
            return true;
        if (ws > 0) {
            /*
             * Predecessor was cancelled. Skip over predecessors and
             * indicate retry.
             */
	    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. 
             */
            compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
        } 
        return false;
    }

    /**
     * Convenience method to park and then check if interrupted
     *
     * @return {@code true} if interrupted
     */
    private final boolean parkAndCheckInterrupt() {
        LockSupport.park(this);
        return Thread.interrupted();
    }

1. //清除中断标记，并返回上一次中断标记的值  
2.     public static boolean interrupted() { ... }     

若当成的中断是true的话，下面就将执行这句话selfInterrupt方法

  public final void acquire(int arg) {
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }

    /**
     * Convenience method to interrupt current thread.
     */
    private static void selfInterrupt() {
        Thread.currentThread().interrupt();
    }

设置当前线程的中断标记，这里只是在线程上设置了一个中断标记，具体怎么处理这个中断标记，由应用程序来确定。

JDK之ReentrantLock,AbstractQueuedSynchronizer源码分析