【Java多线程】JUC锁 06. ReentrantReadWriteLock

ReentrantReadWriteLock

1. 前言

• 读写锁，它维护了一对相关的锁 ：“读取锁”和“写入锁”，一个用于读取操作，另一个用于写入操作
• 默认不公平模式，支持公平模式，不会强加对锁的读取或写入优先访问顺序
• 写入锁可以降级为读取锁，读取锁不能升级为写入锁

2. 源码解析

2.1 数据结构

• ReentrantReadWriteLock包含一对读锁ReadLock和写锁WriteLock，Sync对象；读锁ReadLock和写锁WriteLock继承自Lock接口，内部的Sync对象和ReentrantReadWriteLock的Sync对象是一样的
• ReentrantReadWriteLock的公平锁和非公平锁由Sync对象实例决定

2.2 内部类

• ReadLock

public static class ReadLock implements Lock, java.io.Serializable {
    private static final long serialVersionUID = -5992448646407690164L;
    private final Sync sync;

    protected ReadLock(ReentrantReadWriteLock lock) {
        sync = lock.sync;	// 同ReentrantReadWriteLock Sync一样
    }

    //获取锁
    public void lock() {
        sync.acquireShared(1);
    }

    //获取锁，除非当前线程被中断
    public void lockInterruptibly() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);
    }

    //尝试获取锁，立即返回
    public boolean tryLock() {
        return sync.tryReadLock();
    }

    //获取读锁，除非在指定时间内写锁被占用或者当前线程中断
    public boolean tryLock(long timeout, TimeUnit unit)
            throws InterruptedException {
        return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
    }

    //释放锁
    public void unlock() {
        sync.releaseShared(1);
    }
    
    //读锁不支持Condition条件
    public Condition newCondition() {
            throw new UnsupportedOperationException();
    }

    public String toString() {
        int r = sync.getReadLockCount();
        return super.toString() +
            "[Read locks = " + r + "]";
    }
}

• WriteLock

public static class WriteLock implements Lock, java.io.Serializable {
    private static final long serialVersionUID = -4992448646407690164L;
    private final Sync sync;

    protected WriteLock(ReentrantReadWriteLock lock) {
        sync = lock.sync;	//sync 同ReentrantReadWriteLock.sync
    }

    //获取锁
    public void lock() {
        sync.acquire(1);
    }

    //获取锁除非当前线程中断
    public void lockInterruptibly() throws InterruptedException {
        sync.acquireInterruptibly(1);
    }

    //尝试获取锁
    public boolean tryLock( ) {
        return sync.tryWriteLock();
    }

    //指定时间尝试获取锁，除非线程中断
    public boolean tryLock(long timeout, TimeUnit unit)
            throws InterruptedException {
        return sync.tryAcquireNanos(1, unit.toNanos(timeout));
    }

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

    public Condition newCondition() {
        return sync.newCondition();
    }
    
    public String toString() {
        Thread o = sync.getOwner();
        return super.toString() + ((o == null) ?
                                   "[Unlocked]" :
                                   "[Locked by thread " + o.getName() + "]");
    }

    //查询锁是否被当前线程占据
    public boolean isHeldByCurrentThread() {
        return sync.isHeldExclusively();
    }

    //当前线程重入锁数
    public int getHoldCount() {
        return sync.getWriteHoldCount();
    }
}

2.3 共享锁

2.3.1 获取锁

获取共享锁ReadLock思想：首先通过tryAcquireShared()尝试获取；失败则通过doAcquireShared()不断的循环并尝试获取锁，若有需要，则阻塞等待。

• lock()

在ReadLock中获取锁

public void lock() {
    sync.acquireShared(1);
}

//AQS中实现，获取共享锁
public final void acquireShared(int arg) {
    if (tryAcquireShared(arg) < 0)
        doAcquireShared(arg);
}

• tryAcquireShared

在ReentrantReadWriteLock内部类Sync实现tryAcquireShared()

//尝试获取共享锁
protected final int tryAcquireShared(int unused) {
    /*
     * Walkthrough:
     * 1. If write lock held by another thread, fail.
     * 2. Otherwise, this thread is eligible for
     *    lock wrt state, so ask if it should block
     *    because of queue policy. If not, try
     *    to grant by CASing state and updating count.
     *    Note that step does not check for reentrant
     *    acquires, which is postponed to full version
     *    to avoid having to check hold count in
     *    the more typical non-reentrant case.
     * 3. If step 2 fails either because thread
     *    apparently not eligible or CAS fails or count
     *    saturated, chain to version with full retry loop.
     */
    Thread current = Thread.currentThread();
    int c = getState();	//锁状态	
    // 如果“锁”是“互斥锁”，并且获取锁的线程不是current线程；则返回-1。
    if (exclusiveCount(c) != 0 &&
        getExclusiveOwnerThread() != current)
        return -1;
    // 获取“读取锁”的共享计数
    int r = sharedCount(c);
    // 如果“不需要阻塞等待”，并且“读取锁”的共享计数小于MAX_COUNT；
    // 则通过CAS函数更新“锁的状态”，将“读取锁”的共享计数+1。
    if (!readerShouldBlock() &&
        r < MAX_COUNT &&
        compareAndSetState(c, c + SHARED_UNIT)) {
        if (r == 0) {
            firstReader = current;
            firstReaderHoldCount = 1;
        } else if (firstReader == current) {
            firstReaderHoldCount++;
        } else {
            //HoldCounter是用来统计该线程获取“读取锁”的次数。
            HoldCounter rh = cachedHoldCounter;
            if (rh == null || rh.tid != getThreadId(current))
                cachedHoldCounter = rh = readHolds.get();
            else if (rh.count == 0)
                readHolds.set(rh);
            rh.count++;
        }
        return 1;
    }
    return fullTryAcquireShared(current);
}

其中判断是否需要阻塞readerShouldBlock与锁模式有关：

//FairSync:
final boolean readerShouldBlock() {
    return hasQueuedPredecessors();
}
//判断当前线程前是否有线程在等待
public final boolean hasQueuedPredecessors() {
    Node t = tail; // Read fields in reverse initialization order
    Node h = head;
    Node s;
    return h != t &&
        ((s = h.next) == null || s.thread != Thread.currentThread());
}

//NonfairSync:
final boolean readerShouldBlock() {
    return apparentlyFirstQueuedIsExclusive();
}
//AQS中实现，等待队列首线程不是独占锁
final boolean apparentlyFirstQueuedIsExclusive() {
    Node h, s;
    return (h = head) != null &&
        (s = h.next)  != null &&
        !s.isShared()         &&
        s.thread != null;
}

• fullTryAcquireShared

final int fullTryAcquireShared(Thread current) {
    HoldCounter rh = null;
    for (;;) {
        // 获取“锁”的状态
        int c = getState();
        // 如果“锁”是“互斥锁”，并且获取锁的线程不是current线程；则返回-1。
        if (exclusiveCount(c) != 0) {
            if (getExclusiveOwnerThread() != current)
                return -1;
        // 如果“需要阻塞等待”。
        // (01) 当“需要阻塞等待”的线程是第1个获取锁的线程的话，则继续往下执行。
        // (02) 当“需要阻塞等待”的线程获取锁的次数=0时，则返回-1。
        } else if (readerShouldBlock()) {
            // 如果想要获取锁的线程(current)是第1个获取锁(firstReader)的线程
            if (firstReader == current) {
            } else {
                if (rh == null) {
                    rh = cachedHoldCounter;
                    if (rh == null || rh.tid != current.getId()) {
                        rh = readHolds.get();
                        if (rh.count == 0)
                            readHolds.remove();
                    }
                }
                // 如果当前线程获取锁的计数=0,则返回-1。
                if (rh.count == 0)
                    return -1;
            }
        }
        // 如果“不需要阻塞等待”，则获取“读取锁”的共享统计数；
        // 如果共享统计数超过MAX_COUNT，则抛出异常。
        if (sharedCount(c) == MAX_COUNT)
            throw new Error("Maximum lock count exceeded");
        // 将线程获取“读取锁”的次数+1。
        if (compareAndSetState(c, c + SHARED_UNIT)) {
            // 如果是第1次获取“读取锁”，则更新firstReader和firstReaderHoldCount。
            if (sharedCount(c) == 0) {
                firstReader = current;
                firstReaderHoldCount = 1;
            // 如果想要获取锁的线程(current)是第1个获取锁(firstReader)的线程，
            // 则将firstReaderHoldCount+1。
            } else if (firstReader == current) {
                firstReaderHoldCount++;
            } else {
                if (rh == null)
                    rh = cachedHoldCounter;
                if (rh == null || rh.tid != current.getId())
                    rh = readHolds.get();
                else if (rh.count == 0)
                    readHolds.set(rh);
                // 更新线程的获取“读取锁”的共享计数
                rh.count++;
                cachedHoldCounter = rh; // cache for release
            }
            return 1;
        }
    }
}

• doAcquireShared

doAcquireShared()的作用是在CLH队列中自旋获取共享锁。doAcquireShared()在每一次尝试获取锁时，是通过tryAcquireShared()来执行的！

private void doAcquireShared(int arg) {
    // 创建“当前线程”对应的Shared节点，并将该线程添加到CLH队列中。
    final Node node = addWaiter(Node.SHARED);
    boolean failed = true;
    try {
        boolean interrupted = false;
        for (;;) {
            // 获取“node”的前一节点
            final Node p = node.predecessor();
            // 如果“当前线程”是CLH队列的表头，则尝试获取共享锁。
            if (p == head) {
                int r = tryAcquireShared(arg);
                if (r >= 0) {
                    setHeadAndPropagate(node, r);
                    p.next = null; // help GC
                    if (interrupted)
                        selfInterrupt();
                    failed = false;
                    return;
                }
            }
            // 如果“当前线程”不是CLH队列的表头，则通过shouldParkAfterFailedAcquire()判断是否需要等待，
            // 需要的话，则通过parkAndCheckInterrupt()进行阻塞等待。若阻塞等待过程中，线程被中断过，则设置interrupted为true。
            if (shouldParkAfterFailedAcquire(p, node) &&
                parkAndCheckInterrupt())
                interrupted = true;
        }
    } finally {
        if (failed)
            cancelAcquire(node);
    }
}

2.3.2 释放锁

释放共享锁的思想，是先通过tryReleaseShared()尝试释放共享锁。尝试成功的话，则通过doReleaseShared()唤醒“其他等待获取共享锁的线程”，并返回true；否则的话，返回flase。

• unlock()

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

• releaseShared()

//AQS中实现
public final boolean releaseShared(int arg) {
    if (tryReleaseShared(arg)) {
        doReleaseShared();
        return true;
    }
    return false;
}

• tryReleaseShared()

protected final boolean tryReleaseShared(int unused) {
    // 获取当前线程，即释放共享锁的线程。
    Thread current = Thread.currentThread();
    // 如果想要释放锁的线程(current)是第1个获取锁(firstReader)的线程，
    // 并且“第1个获取锁的线程获取锁的次数”=1，则设置firstReader为null；
    // 否则，将“第1个获取锁的线程的获取次数”-1。
    if (firstReader == current) {
        // assert firstReaderHoldCount > 0;
        if (firstReaderHoldCount == 1)
            firstReader = null;
        else
            firstReaderHoldCount--;
    // 获取rh对象，并更新“当前线程获取锁的信息”。
    } else {
 
        HoldCounter rh = cachedHoldCounter;
        if (rh == null || rh.tid != current.getId())
            rh = readHolds.get();
        int count = rh.count;
        if (count <= 1) {
            readHolds.remove();
            if (count <= 0)
                throw unmatchedUnlockException();
        }
        --rh.count;
    }
    for (;;) {
        // 获取锁的状态
        int c = getState();
        // 将锁的获取次数-1。
        int nextc = c - SHARED_UNIT;
        // 通过CAS更新锁的状态。
        if (compareAndSetState(c, nextc))
            return nextc == 0;
    }
}

• doReleaseShared()

//AQS中实现，会释放“共享锁”。从前往后的遍历CLH队列，依次“唤醒”然后“执行”队列中每个节点对应的线程；最终的目的是让这些线程释放它们所持有的锁。
private void doReleaseShared() {
    for (;;) {
        // 获取CLH队列的头节点
        Node h = head;
        // 如果头节点不为null，并且头节点不等于tail节点。
        if (h != null && h != tail) {
            // 获取头节点对应的线程的状态
            int ws = h.waitStatus;
            // 如果头节点对应的线程是SIGNAL状态，则意味着“头节点的下一个节点所对应的线程”需要被unpark唤醒。
            if (ws == Node.SIGNAL) {
                // 设置“头节点对应的线程状态”为空状态。失败的话，则继续循环。
                if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
                    continue;
                // 唤醒“头节点的下一个节点所对应的线程”。
                unparkSuccessor(h);
            }
            // 如果头节点对应的线程是空状态，则设置“文件点对应的线程所拥有的共享锁”为其它线程获取锁的空状态。
            else if (ws == 0 &&
                     !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
                continue;                // loop on failed CAS
        }
        // 如果头节点发生变化，则继续循环。否则，退出循环。
        if (h == head)                   // loop if head changed
            break;
    }
}

3. 参考

http://www.cnblogs.com/skywang12345/p/3505809.html