总览

• ReentrantLock implements Lodk
• ReentrantLock 只有一个成员变量：private final Sync sync
• Sync extends AbstractQueuedSynchronizer
• Sync 有两个实现类NonfairSync，FairSync 且只有lock()，tryAcquire(int arg)两个函数有不同实现

对比NonfairSync，FairSync

lock()

NonfairSync

    final void lock() {
        if (compareAndSetState(0, 1))
            //第一个区别，非公平锁会先尝试获取锁
            setExclusiveOwnerThread(Thread.currentThread());
        else
            acquire(1);
    }

FairSync

    final void lock() {
        acquire(1);
    }

acquire(int arg)

    public final void acquire(int arg) {
        //这里是第二个区别，tryAcquire函数 公平锁和非公平锁都有自己的实现
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }

tryAcquire(int acquires)

NonfairSync

        protected final boolean tryAcquire(int acquires) {
            //调用父类Sync的nonfairTryAcquire(int acquires)
            return nonfairTryAcquire(acquires);
        }

Sync

final boolean nonfairTryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                if (compareAndSetState(0, acquires)) {
                    //第二个不同点，不管队列有没有等待的都先尝试获取锁
                    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;
        }

FairSync

        protected final boolean tryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                if (!hasQueuedPredecessors() &&
                    compareAndSetState(0, acquires)) {
                    //队列里没有其他优于自己的等待线程，尝试获取锁
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {
                //如果持有锁的线程是自己state+1
                //可重入性
                int nextc = c + acquires;
                if (nextc < 0)
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }

AbstractQueuedSynchronizer

    public final boolean hasQueuedPredecessors() {
        // The correctness of this depends on head being initialized
        // before tail and on head.next being accurate if the current
        // thread is first in queue.
        Node t = tail; // Read fields in reverse initialization order
        Node h = head;
        Node s;
        //h==t || (h.next != null && h.next.thread = currentThread)
        //翻译过来就是队列为空 或者 队列等待的第一个线程就是自己
        return h != t &&
            ((s = h.next) == null || s.thread != Thread.currentThread());
    }

acquireQueued(final Node node, int arg)

AbstractQueuedSynchronizer

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

AbstractQueuedSynchronizer

    private final boolean parkAndCheckInterrupt() {
        LockSupport.park(this);
        return Thread.interrupted();
    }

释放锁

unlock()

Sync

    public void unlock() {
        sync.release(1);
    }

AbstractQueuedSynchronizer

    public final boolean release(int arg) {
        if (tryRelease(arg)) {
            Node h = head;
            if (h != null && h.waitStatus != 0)
                unparkSuccessor(h);
            return true;
        }
        return false;
    }

ReentrantLock

        //逻辑很简单
        protected final boolean tryRelease(int releases) {
            int c = getState() - releases;
            if (Thread.currentThread() != getExclusiveOwnerThread())
                throw new IllegalMonitorStateException();
            boolean free = false;
            if (c == 0) {
                free = true;
                setExclusiveOwnerThread(null);
            }
            setState(c);
            return free;
        }

AbstractQueuedSynchronizer

private void unparkSuccessor(Node node) {
        /*
         * If status is negative (i.e., possibly needing signal) try
         * to clear in anticipation of signalling.  It is OK if this
         * fails or if status is changed by waiting thread.
         */
        int ws = node.waitStatus;
        if (ws < 0)
            compareAndSetWaitStatus(node, ws, 0);

        /*
         * Thread to unpark is held in successor, which is normally
         * just the next node.  But if cancelled or apparently null,
         * traverse backwards from tail to find the actual
         * non-cancelled successor.
         */
        Node s = node.next;
        if (s == null || s.waitStatus > 0) {
            s = null;
            for (Node t = tail; t != null && t != node; t = t.prev)
                if (t.waitStatus <= 0)
                    s = t;
        }
        if (s != null)
            LockSupport.unpark(s.thread);
    }

LockSupport

    public static void unpark(Thread thread) {
        if (thread != null)
            UNSAFE.unpark(thread);
    }