Java ReentrantLock源码分析

ReentrantLock 是java.util.concurrent.locks下的一个类，主要实现了Synchronized关键字的功能，同时扩展了一些功能，例如 锁投票，定时锁等候和中断锁等候。利用volatile关键字和CAS操作实现。

一、 Lock 接口方法

// 请求锁，直到获取到锁
void lock();
// 获取锁，获取不到就跑异常
void lockInterruptibly() throws InterruptedException;
// 尝试获取一次锁，获取到返回true，否则false
boolean tryLock();
// 在指定时间内获取锁
boolean tryLock(long time, TimeUnit unit) throws InterruptedException;
// 解锁
void unlock();
// 条件锁，里面主要有await(); void signal();方法
Condition newCondition();

二、ReentrantLock 源码

ReentrantLock的具体实现是在其内部抽象类Sync，Sync又分成两个实现类FairSync（公平锁）和UnfairSync（非公平锁）。Sync继承了抽象类AbstractQueuedSynchronizer 。
下面贴出ReentrantLock和AbstractQueuedSynchronizer部分代码，完整代码请到jar包中查看源码。

1. RenntrantLock

public class ReentrantLock implements Lock, java.io.Serializable {
    private static final long serialVersionUID = 7373984872572414699L;
    private final Sync sync;
    
    abstract static class Sync extends AbstractQueuedSynchronizer {
        private static final long serialVersionUID = -5179523762034025860L;

        abstract void lock();

       // 非公平锁获取锁
        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;
        }

		// 释放锁
        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;
        }

    }


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

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

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

    static final class FairSync extends Sync {
        private static final long serialVersionUID = -3000897897090466540L;

        final void lock() {
            acquire(1);
        }
	
		// 公平锁获取锁
        protected final boolean tryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                if (!hasQueuedPredecessors() &&
                    compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0)
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }
    }

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

 
    public ReentrantLock(boolean fair) {
        sync = fair ? new FairSync() : new NonfairSync();
    }


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

    public void lockInterruptibly() throws InterruptedException {
        sync.acquireInterruptibly(1);
    }

    public boolean tryLock() {
        return sync.nonfairTryAcquire(1);
    }


    public boolean tryLock(long timeout, TimeUnit unit)
            throws InterruptedException {
        return sync.tryAcquireNanos(1, unit.toNanos(timeout));
    }

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

    public Condition newCondition() {
        return sync.newCondition();
    }
}

2. AbstractQueuedSynchronizer

public abstract class AbstractQueuedSynchronizer
    extends AbstractOwnableSynchronizer
    implements java.io.Serializable {
    static final class Node {
        /** 线程取消 */
        static final int CANCELLED =  1;
        /** 线程等待锁唤醒 unparking */
        static final int SIGNAL    = -1;
        /** 线程等待条件锁唤醒 */
        static final int CONDITION = -2;

        /**
         * 当前节点状态
         */
        volatile int waitStatus;

        /**
         * 前一个节点
         */
        volatile Node prev;

        /**
         * 后一个节点
         */
        volatile Node next;

        /**
         * 当前节点线程
         */
        volatile Thread thread;

        /**
         * 下一个条件锁等待的节点
         */
        Node nextWaiter;

        /**
         * 返回前一个节点
         */
        final Node predecessor() throws NullPointerException {
            Node p = prev;
            if (p == null)
                throw new NullPointerException();
            else
                return p;
        }

    }

    /**
     * 头结点
     */
    private transient volatile Node head;

    /**
     * 尾节点
     */
    private transient volatile Node tail;

    /**
     * 锁状态
     */
    private volatile int state;

    /**
     * 把节点加入等待锁的队列中
     */
    private Node enq(final Node node) {
        for (;;) {
            Node t = tail;
            if (t == null) { // Must initialize
                if (compareAndSetHead(new Node()))
                    tail = head;
            } else {
                node.prev = t;
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }

    /**
     * 把当前线程加入等待锁的队列中
     */
    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;
    }

    /**
     * 唤醒挂起的线程
     * @param node the node
     */
    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);
    }

    /**
     * 获取锁异常后的，把当前节点移出队列
     * @param node the node
     */
    private void cancelAcquire(Node node) {
        // Ignore if node doesn't exist
        if (node == null)
            return;

        node.thread = null;

        Node pred = node.prev;
        while (pred.waitStatus > 0)
            node.prev = pred = pred.prev;

        Node predNext = pred.next;

        node.waitStatus = Node.CANCELLED;
        if (node == tail && compareAndSetTail(node, pred)) {
            compareAndSetNext(pred, predNext, null);
        } else {
            // If successor needs signal, try to set pred's next-link
            // so it will get one. Otherwise wake it up to propagate.
            int ws;
            if (pred != head &&
                ((ws = pred.waitStatus) == Node.SIGNAL ||
                 (ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) &&
                pred.thread != null) {
                Node next = node.next;
                if (next != null && next.waitStatus <= 0)
                    compareAndSetNext(pred, predNext, next);
            } else {
                unparkSuccessor(node);
            }

            node.next = node; // help GC
        }
    }

    /**
     * 尝试获取锁失败，判断线程是否要挂起，同时把当前节点的状态设置成SIGNAL
     */
    private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
        int ws = pred.waitStatus;
        if (ws == Node.SIGNAL)
            return true;
        if (ws > 0) {
            do {
                node.prev = pred = pred.prev;
            } while (pred.waitStatus > 0);
            pred.next = node;
        } else {
            compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
        }
        return false;
    }

    /**
     * 线程挂起
     */
    private final boolean parkAndCheckInterrupt() {
        LockSupport.park(this);
        return Thread.interrupted();
    }


    /**
     * 获取锁，获取不到则挂起当前线程，线程如果被打断，要把当前线程从等待锁的队列中移除
     */
    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);
        }
    }

    /**
     * 尝试获取锁，不成功则把线程加入到等待锁的队列中，并挂起线程
     */
    public final void acquire(int arg) {
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }

    /**
     * 释放锁，同时唤醒一个等待锁的线程
     */
    public final boolean release(int arg) {
        if (tryRelease(arg)) {
            Node h = head;
            if (h != null && h.waitStatus != 0)
                unparkSuccessor(h);
            return true;
        }
        return false;
    }

    /**
     * 判断当前节点是条件锁（false）还是非条件锁(true)，
     */
    final boolean isOnSyncQueue(Node node) {
        if (node.waitStatus == Node.CONDITION || node.prev == null)
            return false;
        if (node.next != null) // If has successor, it must be on queue
            return true;
        return findNodeFromTail(node);
    }

    /**
     * Returns true if node is on sync queue by searching backwards from tail.
     * Called only when needed by isOnSyncQueue.
     * @return true if present
     */
    private boolean findNodeFromTail(Node node) {
        Node t = tail;
        for (;;) {
            if (t == node)
                return true;
            if (t == null)
                return false;
            t = t.prev;
        }
    }

    /**
     * 把一个节点从条件等待的队列移到非条件等待锁的队列中
     */
    final boolean transferForSignal(Node node) {
        if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
            return false;
        Node p = enq(node);
        int ws = p.waitStatus;
        if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
            LockSupport.unpark(node.thread);
        return true;
    }

    /**
     * 当前线程完全释放锁，不管加了几层锁
     */
    final int fullyRelease(Node node) {
        boolean failed = true;
        try {
            int savedState = getState();
            if (release(savedState)) {
                failed = false;
                return savedState;
            } else {
                throw new IllegalMonitorStateException();
            }
        } finally {
            if (failed)
                node.waitStatus = Node.CANCELLED;
        }
    }

    /**
     * 等待锁条件对象
     */
    public class ConditionObject implements Condition, java.io.Serializable {
        private static final long serialVersionUID = 1173984872572414699L;
        /** 首节点 */
        private transient Node firstWaiter;
        /** 尾节点 */
        private transient Node lastWaiter;

        /**
         * 把当前节点加入到条件锁队列中
         */
        private Node addConditionWaiter() {
            Node t = lastWaiter;
            // If lastWaiter is cancelled, clean out.
            if (t != null && t.waitStatus != Node.CONDITION) {
                unlinkCancelledWaiters();
                t = lastWaiter;
            }
            Node node = new Node(Thread.currentThread(), Node.CONDITION);
            if (t == null)
                firstWaiter = node;
            else
                t.nextWaiter = node;
            lastWaiter = node;
            return node;
        }

        /**
         * 唤醒当前条件等待锁队列中的一个节点
         */
        private void doSignal(Node first) {
            do {
                if ( (firstWaiter = first.nextWaiter) == null)
                    lastWaiter = null;
                first.nextWaiter = null;
            } while (!transferForSignal(first) &&
                     (first = firstWaiter) != null);
        }

        /**
         * 唤醒当前条件等待锁队列中的所有节点
         */
        private void doSignalAll(Node first) {
            lastWaiter = firstWaiter = null;
            do {
                Node next = first.nextWaiter;
                first.nextWaiter = null;
                transferForSignal(first);
                first = next;
            } while (first != null);
        }

        /**
         * 取消获取锁的线程移出队列
         */
        private void unlinkCancelledWaiters() {
            Node t = firstWaiter;
            Node trail = null;
            while (t != null) {
                Node next = t.nextWaiter;
                if (t.waitStatus != Node.CONDITION) {
                    t.nextWaiter = null;
                    if (trail == null)
                        firstWaiter = next;
                    else
                        trail.nextWaiter = next;
                    if (next == null)
                        lastWaiter = trail;
                }
                else
                    trail = t;
                t = next;
            }
        }

        // public methods

        /**
         * 唤醒一个线程
         */
        public final void signal() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            Node first = firstWaiter;
            if (first != null)
                doSignal(first);
        }

        /**
         * 唤醒所有等待的线程
         */
        public final void signalAll() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            Node first = firstWaiter;
            if (first != null)
                doSignalAll(first);
        }


        /**
         * 将当前线程加入条件锁等待队列中，同时释放当前线程占用的锁，然后把当前线程挂起等待唤醒。
         * 唤醒后重新获取锁（这里节点state值会变成释放前的状态）。
         */
        public final void await() throws InterruptedException {
            if (Thread.interrupted())
                throw new InterruptedException();
            Node node = addConditionWaiter();
            int savedState = fullyRelease(node);
            int interruptMode = 0;
            while (!isOnSyncQueue(node)) {
                LockSupport.park(this);
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
            }
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            if (node.nextWaiter != null) // clean up if cancelled
                unlinkCancelledWaiters();
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
        }

    }
}

三 lock 与unlock 的整体流程

我们从非公平锁来看加锁和解锁的流程

lock 流程

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

1. 首先通过CAS操作设置当前锁状态，如果成功，把锁的占有线程设置成当前线程获取锁成功。否则进入第二步

2. 执行nonfairTryAcquire 方法，判断锁状态是否是占有状态，如果不是，则尝试获取锁获取；如果是占有状态则判断当前锁是否是当前线程占有，如果是，则给锁的state值加一个权重默认都是1。两个条件都不符合则走第三步。方法：boolean nonfairTryAcquire(int acquires)

3. addWaiter(Node.EXCLUSIVE)把当前线程以独占锁的模式加入等待队列中。方法：addWaiter(Node mode)

4. 如果当前节点在头节点尝试获取锁， 成功则把节点从队列中移除；如果非当前节点或者获取锁失败则将当前节点状态设置成等待唤醒状态，然后把当前线程挂起等待唤醒。唤醒后判断当前线程是否已经中断，然后重复步骤4直到锁获取成功。方法：acquireQueued(final Node node, int arg)

unlock流程

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

1. tryRelease(int releases) 判断是否是当前线程，不是会抛异常。把state 减去 release的值，如果不是0则当前线程还有锁，返回false；则将当前锁的占有线程设置为空 执行步骤2

2. 获取锁等待队列的头结点，如果不为空，则唤醒头结点的线程unparkSuccessor(Node node)

四、await 与 signal 流程

await() 流程

public final void await() throws InterruptedException {
            if (Thread.interrupted())
                throw new InterruptedException();
            Node node = addConditionWaiter();
            long savedState = fullyRelease(node);
            int interruptMode = 0;
            while (!isOnSyncQueue(node)) {
                LockSupport.park(this);
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
            }
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            if (node.nextWaiter != null) // clean up if cancelled
                unlinkCancelledWaiters();
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
        }

1. 把当前线程加入到条件锁的等待队列中。方法：ConditionObject.addConditionWaiter()
2. 把当前线程的锁释放。方法： fullRelease(Node node)
3. 线程挂起。
4. 唤醒线程后重新获取锁并设置锁状态。方法：acquireQueued(node, savedState)
5. 清理条件等待锁队列中状态不对的节点，unlinkCancelledWaiters()

signal() 流程

private void doSignal(Node first) {
    do {
        if ( (firstWaiter = first.nextWaiter) == null)
            lastWaiter = null;
        first.nextWaiter = null;
    } while (!transferForSignal(first) &&
             (first = firstWaiter) != null);
}

final boolean transferForSignal(Node node) {
       
        if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
            return false;

        Node p = enq(node);
        int ws = p.waitStatus;
        if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
        	// 唤醒线程
            LockSupport.unpark(node.thread);
        return true;
    }

1. 找到当前条件等待队列的第一个节点。
2. 把节点的状态从Node.CONDITION改成0，
3. 把节点加入到锁队列中
4. 唤醒线程

其它方法

ReentrantLock 中的其它方法都是在此基础上加上了一些条件。比如规定在一定时间内获取锁、获取不到锁抛异常等待。这些方法的实现都类似。