ReentrantReadWriteLock源码解析

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

public final void acquireShared(int arg) {
        if (tryAcquireShared(arg) < 0)
            doAcquireShared(arg);
 

        static final int SHARED_SHIFT   = 16;
        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;

        /** Returns the number of shared holds represented in count  */
        static int sharedCount(int c)    { return c >>> SHARED_SHIFT; }  
        /** Returns the number of exclusive holds represented in count  */
        static int exclusiveCount(int c) { return c & EXCLUSIVE_MASK; }

c的低16位表示写锁重入的次数，高16位表示读线程的个数

sharedCount表示高16位，exclusiveCounThread current = Thread.currentThread();

int c = getState();
            if (exclusiveCount(c) != 0 &&
                getExclusiveOwnerThread() != current) //如果有写线程且线程不是当前线程就返回-1
                return -1;
            int r = sharedCount(c); 
            if (!readerShouldBlock() &&   //readershouldblock 如果阻塞队列里有写线程，读线程就不能获得锁
                r < MAX_COUNT &&
                compareAndSetState(c, c + SHARED_UNIT)) {  //通过cas操作将读线程的数量加1
                if (r == 0) {  //r等于0说明没有读锁进入，
                    firstReader = current; 
                    firstReaderHoldCount = 1;
                } else if (firstReader == current) {
                    firstReaderHoldCount++;
                } else {
                    HoldCounter rh = cachedHoldCounter;
                    if (rh == null || rh.tid != getThreadId(current))
                        cachedHoldCounter = rh = readHolds.get(); //通过ThreadLocal给每个线程绑定了一个HoldCount对象，每个HoldCount都有一个count属性用来 
　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　//计数，还有一个ThreadId
                    else if (rh.count == 0)
                        readHolds.set(rh);
                    rh.count++;
                }
                return 1;
            }
            return fullTryAcquireShared(current);
        }

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 (;;) {                                                        //cas操作可能失败，失败的话，在循环获取读锁。
                int c = getState();
                if (exclusiveCount(c) != 0) {               //如果写线程不为0且线程不为当前线程就返回-1
                    if (getExclusiveOwnerThread() != current)
                        return -1;
                    // else we hold the exclusive lock; blocking here
                    // would cause deadlock.
                } else if (readerShouldBlock()) {                           //即使readerShouldblock为true的情况下也可以获取读锁，因为可能这个线程之前
                    // Make sure we're not acquiring read lock reentrantly  //就获得过一次读锁，此次为重入
                    if (firstReader == current) {           //如果当前线程为第一个进入的读线程的话，count必然不为0
                        // assert firstReaderHoldCount > 0;
                    } else {                            
                        if (rh == null) {
                            rh = cachedHoldCounter;      
                            if (rh == null || rh.tid != getThreadId(current)) {
                                rh = readHolds.get();
                                if (rh.count == 0)      //判断当前线程获得过读锁，没获得过得话，count就为0，并remove掉HoldCount对象
                                    readHolds.remove();
                            }
                        }
                        if (rh.count == 0)
                            return -1;
                    }
                }
                if (sharedCount(c) == MAX_COUNT)
                    throw new Error("Maximum lock count exceeded");
                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;
                }
            }
        }

如果tryAcquire返回-1的话就会进入到阻塞队列中去

private void doAcquireShared(int arg) {
        final Node node = addWaiter(Node.SHARED); 
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        if (interrupted)
                            selfInterrupt();
                        failed = false;
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

private void setHeadAndPropagate(Node node, int propagate) {
        Node h = head; // Record old head for check below
        setHead(node);
        /*
         * Try to signal next queued node if:
         *   Propagation was indicated by caller,
         *     or was recorded (as h.waitStatus either before
         *     or after setHead) by a previous operation
         *     (note: this uses sign-check of waitStatus because
         *      PROPAGATE status may transition to SIGNAL.)
         * and
         *   The next node is waiting in shared mode,
         *     or we don't know, because it appears null
         *
         * The conservatism in both of these checks may cause
         * unnecessary wake-ups, but only when there are multiple
         * racing acquires/releases, so most need signals now or soon
         * anyway.
         */
        if (propagate > 0 || h == null || h.waitStatus < 0 ||
            (h = head) == null || h.waitStatus < 0) {
            Node s = node.next;
            if (s == null || s.isShared())
                doReleaseShared();
        }
    }

读锁的unlock方法

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

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

 protected final boolean tryReleaseShared(int unused) {
            Thread current = Thread.currentThread(); 
            if (firstReader == current) {      //首先判断当前线程是否为第一个读线程
                // assert firstReaderHoldCount > 0;
                if (firstReaderHoldCount == 1) //在判断count数是否为0，为0的话，就说明读锁也完全释放
                    firstReader = null;  
                else
                    firstReaderHoldCount--; //说明可重入锁还没有完全释放
            } else {
                HoldCounter rh = cachedHoldCounter; //如果不是第一个线程的话，就通过ThreadLocal来获得HoldCount对象，
                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;
            }
            for (;;) {     //只释放一个读锁并不会唤醒阻塞队列里的线程，必须读锁和写锁都为0，即state为0才会唤醒.
                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.
                    return nextc == 0;
            }
        }

private void doReleaseShared() {
        /*
         * Ensure that a release propagates, even if there are other
         * in-progress acquires/releases.  This proceeds in the usual
         * way of trying to unparkSuccessor of head if it needs
         * signal. But if it does not, status is set to PROPAGATE to
         * ensure that upon release, propagation continues.
         * Additionally, we must loop in case a new node is added
         * while we are doing this. Also, unlike other uses of
         * unparkSuccessor, we need to know if CAS to reset status
         * fails, if so rechecking.
         */
        for (;;) {
            Node h = head;
            if (h != null && h != tail) {
                int ws = h.waitStatus;
                if (ws == Node.SIGNAL) {
                    if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
                        continue;            // loop to recheck cases
                    unparkSuccessor(h);
                }
                else if (ws == 0 &&
                         !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
                    continue;                // loop on failed CAS
            }
            if (h == head)                   // loop if head changed
                break;
        }
    }

写锁的lock方法

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

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

protected 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);
            if (c != 0) {          //说明有锁，但不清楚是读锁还是写锁
                // (Note: if c != 0 and w == 0 then shared count != 0)
                if (w == 0 || current != getExclusiveOwnerThread()) //如果没有写锁或者有写锁但不是当前写锁都返回false。
                    return false;
                if (w + exclusiveCount(acquires) > MAX_COUNT)
                    throw new Error("Maximum lock count exceeded");
                // Reentrant acquire
                setState(c + acquires); //走到这一步说明只有一个它本身的写锁，可以重入
                return true;
            }
            if (writerShouldBlock() || //到了这一步就说明当前不含任何线程，cas操作state+1，并把线程设为系统线程
                !compareAndSetState(c, c + acquires))
                return false;
            setExclusiveOwnerThread(current);
            return true;
        }

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);
        }
    }

和ReentrantLock的方法一样.

写锁的unlock方法

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

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

protected final boolean tryRelease(int releases) {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            int nextc = getState() - releases;
            boolean free = exclusiveCount(nextc) == 0;
            if (free)
                setExclusiveOwnerThread(null);
            setState(nextc);
            return free;
        }

private void doReleaseShared() {