Java多线程之ReentrantReadWriteLock源码解析

一、介绍

1.1 ReentrantReadWriteLock

ReentrantReadWriteLock 是一个读写锁，允许多个读或者一个写线程在执行。

内部的 Sync 继承自 AQS，这个 Sync 包含一个共享读锁 ReadLock 和一个独占写锁 WriteLock。

该锁可以设置公平和非公平，默认非公平。

一个持有写锁的线程可以获取读锁。如果该线程先持有写锁，再持有读锁并释放写锁，称为锁降级。

WriteLock支持Condition并且与ReentrantLock语义一致，而ReadLock则不能使用Condition，否则抛出UnsupportedOperationException异常。

public class ReentrantReadWriteLock implements ReadWriteLock {
    /** 读锁 */
    private final ReentrantReadWriteLock.ReadLock readerLock;
    /** 写锁 */
    private final ReentrantReadWriteLock.WriteLock writerLock;
    /** 持有的AQS子类对象 */
    final Sync sync;

    abstract static class Sync extends AbstractQueuedSynchronizer {}

    static final class NonfairSync extends Sync {}

    static final class FairSync extends Sync {}

    public static class ReadLock implements Lock {}

    public static class WriteLock implements Lock {}

    //默认非公平
    public ReentrantReadWriteLock() {
        this(false);
    }

    public ReentrantReadWriteLock(boolean fair) {
        sync = fair ? new FairSync() : new NonfairSync();
        readerLock = new ReadLock(this);
        writerLock = new WriteLock(this);
    }

    public static class ReadLock implements Lock {
    	private final Sync sync;
        protected ReadLock(ReentrantReadWriteLock lock) {
            sync = lock.sync;
        }
    }

    public static class WriteLock implements Lock {
    	private final Sync sync;
        protected WriteLock(ReentrantReadWriteLock lock) {
            sync = lock.sync;
        }
    }

}

1.2 state

Sync 继承了 AQS，其中有一个 int 的成员变量 state，int 共32位，这里将其视为两部分，高16位表示读的数量，低16位表示写的数量，这里的数量表示线程重入后的总数量。

abstract static class Sync extends AbstractQueuedSynchronizer {
  	//继承的一个int的成员变量，将其拆分为高16位和低16位
    //private volatile int state;
    static final int SHARED_SHIFT   = 16;
  	//读一次，锁增加的值
    static final int SHARED_UNIT    = (1 << SHARED_SHIFT);
    static final int MAX_COUNT      = (1 << SHARED_SHIFT) - 1;
    static final int EXCLUSIVE_MASK = (1 << SHARED_SHIFT) - 1;

    //读的数量
    static int sharedCount(int c)    { return c >>> SHARED_SHIFT; }
    //写的数量
    static int exclusiveCount(int c) { return c & EXCLUSIVE_MASK; }
}

1.3 HoldCounter

读锁使用了一个 ThreadLocal<HoldCounter> 让每个线程有一个线程私有的 HoldCounter，HoldCounter包含一个线程 id 以及读重入的次数。

查找对应线程的HoldCounter 其实只用一个 ThreadLocalHoldCounter 也足够了。这里为了加快查询，用了两个额外的缓存，即 cachedHoldCounter、firstReader 和 firstReaderHoldCount（后两个组合起来相当于一个 HoldCounter）。

在读锁的相关操作中，先检查 firstReader 是否为当前线程，否则检查 cachedHoldCounter 内部的线程是否为当前线程，如果失败最后会通过 readHolds 来获取当前线程的 HoldCounter。

static final class HoldCounter {
    int count = 0;
    // 使用线程id，而不是线程的引用。这样可以防止垃圾不被回收
    final long tid = getThreadId(Thread.currentThread());
}

static final class ThreadLocalHoldCounter
    extends ThreadLocal<HoldCounter> {
    public HoldCounter initialValue() {
        return new HoldCounter();
    }
}
//使用的ThreadLocal
private transient ThreadLocalHoldCounter readHolds;
//一个缓存
private transient HoldCounter cachedHoldCounter;
//组合起来相当于一个缓存
private transient Thread firstReader = null;
private transient int firstReaderHoldCount;

二、读锁

2.1 读锁的获取

下面讲解 tryAcquireShared 和 tryReadLock，tryReadLock 是一种直接抢占的非公平获取，和 tryAcquireShared 中的非公平获取有所不同。

2.1.1 tryAcquireShared

根据注释

1.检查是否存在其他线程持有的写锁，是的话失败，返回 -1；

2.判断在当前公平状态下能否读，以及是否超过读的最大数量，满足条件则尝试 CAS 修改状态，让 state 加一个单位的读 SHARED_UNIT；修改成功后会根据三种情况，即首次读、firstReader 是当前线程，以及其他情况分别进行处理，成功，返回1；

3.前面未返回结果，会执行 fullTryAcquireShared。

可以将该方法视为 fullTryAcquireShared 的一次快速尝试，如果尝试失败，会在 fullTryAcquireShared 的自旋中一直执行，直到返回成功或者失败。

//ReadLock
public void lock() {
    sync.acquireShared(1);
}
//AQS
public final void acquireShared(int arg) {
    if (tryAcquireShared(arg) < 0)
        doAcquireShared(arg);
}
//Sync
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();
  	// 如果写的数量不是0，且写线程不是当前线程，失败
    if (exclusiveCount(c) != 0 &&
        getExclusiveOwnerThread() != current)
        return -1;
  	// 获取读的个数
    int r = sharedCount(c);
  	// 如果当前线程想要读，没有被堵塞
  	// 当前读的数量未超过最大允许的读的个数
  	// CAS执行成功
    if (!readerShouldBlock() &&
        r < MAX_COUNT &&
        compareAndSetState(c, c + SHARED_UNIT)) {
      	// 第一次读，修改firstReader和firstReaderHoldCount
        if (r == 0) {
            firstReader = current;
            firstReaderHoldCount = 1;
          // 如果当前线程正好是firstReader
        } else if (firstReader == current) {
            firstReaderHoldCount++;
          // 其他情况，经过一系列处理后，使得rh为当前线程的HoldCounter
          // 对rh的记数加一
        } else {
            HoldCounter rh = cachedHoldCounter;
          	// 如果cached为null或者不是当前线程
            if (rh == null || rh.tid != getThreadId(current))
              	// 从readHolds中get，并修改cached
                cachedHoldCounter = rh = readHolds.get();
          	// 如果cached不是null，但记数为null
          	// 这种情况表示当前线程的HoldCounter已经被删除，即为null，
          	// 但cached仍然保留着null之前的那个HoldCounter，
          	// 为了方便，直接将cached设置给ThreadLocal即可
            else if (rh.count == 0)
                readHolds.set(rh);
          	//执行到这里，rh表示当前线程的HoldCounter，记数加1
            rh.count++;
        }
        return 1;
    }
  	// 前面未返回结果，执行第三步
    return fullTryAcquireShared(current);
}

2.1.2 fullTryAcquireShared

在上述的简单尝试 tryAcquireShared 未能确定结果后，执行第三步 fullTryAcquireShared 自旋来不断尝试获取读锁，直到成功获取锁返回1，或者满足相应条件认定失败返回-1。

1.其他线程持有写锁，失败

2.当前线程读的尝试满足堵塞条件表示当前线程排在其他线程后面，且当前线程没有持有锁即非重入的情况，失败

3.其他情况则不断自旋CAS，达到最大读的数量会抛出异常，其他情况在成功后返回1。

final int fullTryAcquireShared(Thread current) {
    /*
     * This code is in part redundant with that in
     * tryAcquireShared but is simpler overall by not
     * complicating tryAcquireShared with interactions between
     * retries and lazily reading hold counts.
     */
    HoldCounter rh = null;
    for (;;) {
        int c = getState();
        if (exclusiveCount(c) != 0) {
          	// 存在其他线程持有写锁，返回-1
            if (getExclusiveOwnerThread() != current)
                return -1;
            // else we hold the exclusive lock; blocking here
            // would cause deadlock.
          //没有写锁，且该线程排在其他线程后面，应该被堵塞
          //如果已经持有读锁，此次获取是重入，可以执行else if 之后的操作；
          //否则，会被堵塞，返回-1。
        } else if (readerShouldBlock()) {
            // Make sure we're not acquiring read lock reentrantly
          	//检查firstReader
            if (firstReader == current) {
                // assert firstReaderHoldCount > 0;
            } else {
                if (rh == null) {
                    rh = cachedHoldCounter;
                    if (rh == null || rh.tid != getThreadId(current)) {
                      	//执行到下一步rh是cached或者readHolds.get()，检查rh
                        rh = readHolds.get();
                      	//在get时，如果不存在，会产生一个新的HoldCounter
                      	//记数为0表示不是重入锁，会删除让其重新为null
                        if (rh.count == 0)
                            readHolds.remove();
                    }
                }
              	//返回失败
                if (rh.count == 0)
                    return -1;
            }
        }
      	//达到最大值，不允许继续增加
        if (sharedCount(c) == MAX_COUNT)
            throw new Error("Maximum lock count exceeded");
      	//和2.1.1中相似
        if (compareAndSetState(c, c + SHARED_UNIT)) {
            if (sharedCount(c) == 0) {
                firstReader = current;
                firstReaderHoldCount = 1;
            } else if (firstReader == current) {
                firstReaderHoldCount++;
            } else {
                if (rh == null)
                    rh = cachedHoldCounter;
                if (rh == null || rh.tid != getThreadId(current))
                    rh = readHolds.get();
                else if (rh.count == 0)
                    readHolds.set(rh);
                rh.count++;
                cachedHoldCounter = rh; // cache for release
            }
            return 1;
        }
    }
}

2.1.3 readerShouldBlock

该方法返回当前线程请求获得读锁是否应该被堵塞，在公平锁和非公平锁中的实现不同

在公平锁中，返回在排队的队列中当前线程之前是否存在其他线程，是的话返回 true，当前线程在队列头部或者队列为空返回 false。

// FairSync
final boolean readerShouldBlock() {
    return hasQueuedPredecessors();
}
// AQS
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;
    return h != t &&
        ((s = h.next) == null || s.thread != Thread.currentThread());
}

在非公平锁中，队列中存在两个节点，且第二个节点是独占的写节点，会返回 true，使得新来的读线程堵塞。

这种方式只能在第二个节点是请求写锁的情况下返回 true，避免写锁的无限等待；如果写锁的请求节点在队列的其他位置，返回 false，不影响新来的读线程获取读锁。

如果不按照这种方式处理，而按照队列中的顺序进行处理，则只要存在其他线程在读，每次来一个新的线程请求读锁，总是成功，写锁会一直等待下去。

// NonfairSync
final boolean readerShouldBlock() {
    /* As a heuristic to avoid indefinite writer starvation,
     * block if the thread that momentarily appears to be head
     * of queue, if one exists, is a waiting writer.  This is
     * only a probabilistic effect since a new reader will not
     * block if there is a waiting writer behind other enabled
     * readers that have not yet drained from the queue.
     */
    return apparentlyFirstQueuedIsExclusive();
}
// AQS
final boolean apparentlyFirstQueuedIsExclusive() {
    Node h, s;
    return (h = head) != null &&
        (s = h.next)  != null &&
        !s.isShared()         &&
        s.thread != null;
}

2.1.4 tryReadLock

和 fullTryAcquireShared 有相似之处，该方法总是直接去抢占锁，直到其他线程获取写锁返回失败，或者当前当前线程获取读锁返回成功。

//ReadLock
public boolean tryLock() {
    return sync.tryReadLock();
}
//Sync
final boolean tryReadLock() {
    Thread current = Thread.currentThread();
    for (;;) {
        int c = getState();
        if (exclusiveCount(c) != 0 &&
            getExclusiveOwnerThread() != current)
            return false;
        int r = sharedCount(c);
        if (r == MAX_COUNT)
            throw new Error("Maximum lock count exceeded");
        if (compareAndSetState(c, c + SHARED_UNIT)) {
            if (r == 0) {
                firstReader = current;
                firstReaderHoldCount = 1;
            } else if (firstReader == current) {
                firstReaderHoldCount++;
            } else {
                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 true;
        }
    }
}

2.2 读锁的释放

tryReleaseShared 在 if/else 中实现了通过 first/cached/readHolds 获取相应的 HoldCounter，并修改其中的记数，记数为0则删除；在 for 中，不断自旋实现 CAS 修改状态 c，如果修改后的状态为0，表示读写锁全部释放，返回 true，否则是 false。

// ReadLockpublic void unlock() {    sync.releaseShared(1);}// AQSpublic final boolean releaseShared(int arg) {    if (tryReleaseShared(arg)) {        doReleaseShared();        return true;    }    return false;}// Syncprotected final boolean tryReleaseShared(int unused) {    Thread current = Thread.currentThread();  	// 先检查 firstReader是否是当前线程    if (firstReader == current) {        // assert firstReaderHoldCount > 0;        if (firstReaderHoldCount == 1)            firstReader = null;        else            firstReaderHoldCount--;      //否则，处理 cached/readHolds中的HoldCounter    } else {        HoldCounter rh = cachedHoldCounter;        if (rh == null || rh.tid != getThreadId(current))            rh = readHolds.get();        int count = rh.count;        if (count <= 1) {            readHolds.remove();            if (count <= 0)                throw unmatchedUnlockException();        }        --rh.count;    }  	//自旋修改 state    for (;;) {        int c = getState();        int nextc = c - SHARED_UNIT;        if (compareAndSetState(c, nextc))            // Releasing the read lock has no effect on readers,            // but it may allow waiting writers to proceed if            // both read and write locks are now free.          	//只有读写锁均释放干净，才返回true            return nextc == 0;    }}

三、写锁

3.1 写锁的获取

下面讲解 tryAcquire 和 tryWriteLock，tryWriteLock 是一种非公平的获取。

3.1.1 tryAcquire

根据注释，tryAcquire 分为三步

1.如果读记数非0，或者写记数非0且写线程不是当前线程，失败

2.写锁的获取应该被堵塞或者CAS失败，失败

3.其他情况，写重入和新来的写线程，均成功

//WriteLockpublic void lock() {    sync.acquire(1);}//AQSpublic final void acquire(int arg) {    if (!tryAcquire(arg) &&        acquireQueued(addWaiter(Node.EXCLUSIVE), arg))        selfInterrupt();}//Syncprotected final boolean tryAcquire(int acquires) {    /*     * Walkthrough:     * 1. If read count nonzero or write count nonzero     *    and owner is a different thread, fail.     * 2. If count would saturate, fail. (This can only     *    happen if count is already nonzero.)     * 3. Otherwise, this thread is eligible for lock if     *    it is either a reentrant acquire or     *    queue policy allows it. If so, update state     *    and set owner.     */    Thread current = Thread.currentThread();    int c = getState();    int w = exclusiveCount(c);  	//c分为两部分，写和读    if (c != 0) {        // (Note: if c != 0 and w == 0 then shared count != 0)      	// c非0，w是0，则读记数非0 || 独占的写线程不是当前线程      	// 返回 false        if (w == 0 || current != getExclusiveOwnerThread())            return false;        if (w + exclusiveCount(acquires) > MAX_COUNT)            throw new Error("Maximum lock count exceeded");        // Reentrant acquire      	// 重入的情况        setState(c + acquires);        return true;    }  	// 写应该被堵塞或者CAS失败，返回false    if (writerShouldBlock() ||        !compareAndSetState(c, c + acquires))        return false;  	// 非重入，在CAS成功后，设定独占写线程为当前线程，返回true    setExclusiveOwnerThread(current);    return true;}

3.1.2 writerShouldBlock

在公平锁中，检查队列前面是否有其他线程在排队，在非公平锁中，总是返回false，即总是不堵塞。

//FairSyncfinal boolean writerShouldBlock() {    return hasQueuedPredecessors();}//NonfairSyncfinal boolean writerShouldBlock() {    return false; // writers can always barge}

3.1.3 tryWriteLock

和 tryAcquire 在非公平锁的写法基本一样。

final boolean tryWriteLock() {    Thread current = Thread.currentThread();    int c = getState();    if (c != 0) {        int w = exclusiveCount(c);        if (w == 0 || current != getExclusiveOwnerThread())            return false;        if (w == MAX_COUNT)            throw new Error("Maximum lock count exceeded");    }    if (!compareAndSetState(c, c + 1))        return false;    setExclusiveOwnerThread(current);    return true;}

3.2 写锁的释放

在 tryRelease 中，修改相应的状态，如果修改后写锁记数为0，则返回 true。

//WriteLockpublic void unlock() {    sync.release(1);}//AQSpublic final boolean release(int arg) {    if (tryRelease(arg)) {        Node h = head;        if (h != null && h.waitStatus != 0)            unparkSuccessor(h);        return true;    }    return false;}//Syncprotected final boolean tryRelease(int releases) {  	// 首先检查当前线程是否持有写锁    if (!isHeldExclusively())        throw new IllegalMonitorStateException();    int nextc = getState() - releases;  	// 根据修改后的写记数来确定free    boolean free = exclusiveCount(nextc) == 0;  	// 此时，写锁完全释放，设定写独占线程为null    if (free)        setExclusiveOwnerThread(null);    setState(nextc);  	// 返回 free    return free;}

四、锁降级

如果一个线程已经持有写锁，再去获取读锁并释放写锁，这个过程称为锁降级。

持有写锁的时候去获取读锁，只有该持有写锁的线程能够成功获取读锁，然后再释放写锁，保证此时当前线程是有读锁的；如果有写锁，先释放写锁，再获取读锁，可能暂时不能获取读锁，会在队列中排队等待。

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