concurrent-5-AQS-ReentrantReadWriteLock

ReentrantReadWriteLock#WriteLock#lock

public void lock() {
            sync.acquire(1);   //尝试获取资源
        }

ReentrantReadWriteLock#tryAcquire

 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();    //获取线程状态
            int w = exclusiveCount(c);   //计算低16位值
            if (c != 0) {
                // (Note: if c != 0 and w == 0 then shared count != 0)
                if (w == 0 || current != getExclusiveOwnerThread())   //read 共享数量不为0
                    return false;
                if (w + exclusiveCount(acquires) > MAX_COUNT)  //最大为2的16次方-1 65535
                    throw new Error("Maximum lock count exceeded");
                // Reentrant acquire
                setState(c + acquires);  //设置状态 +1
                return true;
            }
            //c = 0 则表示没有读锁与写锁，cas尝试获取写锁
            //writerShouldBlock : 公平锁判断是否为head的下一个继承者,如果有排队则直接返回
            if (writerShouldBlock() ||
                !compareAndSetState(c, c + acquires))
                return false;
            setExclusiveOwnerThread(current);  //标明独占线程
            return true;
        }

ReentrantReadWriteLock#WriteLock#unlock

 public void unlock() {
            sync.release(1);   //尝试释放资源
        }

ReentrantReadWriteLock#tryRelease

  protected final boolean tryRelease(int releases) {
            if (!isHeldExclusively())  //判断是否为当前线程持有锁
                throw new IllegalMonitorStateException();
            int nextc = getState() - releases;   //获取释放后的状态值
            boolean free = exclusiveCount(nextc) == 0;   //如果状态值为0 则表示锁已经完全释放
            if (free)
                setExclusiveOwnerThread(null);
            setState(nextc);
            return free;   //释放return true  则唤醒一个节点操作
        }

ReentrantReadWriteLock#ReadLock#lock

 public void lock() {
            sync.acquireShared(1);  //尝试获取共享锁资源
        }

AQS#acquireShared

public final void acquireShared(int arg) {
        if (tryAcquireShared(arg) < 0)  //获取资源失败
            doAcquireShared(arg);    //加入同步队列后，再循环cas获取资源
    }

ReentrantReadWriteLock#tryAcquireShared

 protected final int tryAcquireShared(int unused) {
            /*
             * Walkthrough:
             * 1. If write lock held by another thread, fail.  //如果写锁被其它线程获取了，则失败
             * 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();   //获取线程状态
            if (exclusiveCount(c) != 0 &&   //排他锁状态信息 //已经含有写锁，且不是当前线程
                getExclusiveOwnerThread() != current)
                return -1;
            int r = sharedCount(c);  //共享锁状态信息
            //获取读锁成功
            if (!readerShouldBlock() &&
                    //非公平锁：apparentlyFirstQueuedIsExclusive 判断下一个队列是否为独占节点
                    //公平锁：判断是否下一个等待的节点是否为当前线程
                r < MAX_COUNT &&    //数量为65535 2的16次方-1
                compareAndSetState(c, c + SHARED_UNIT)) {   //尝试获取共享资源
                if (r == 0) {   //写锁数量为0，则将当前线程设为队列头
                    firstReader = current;
                    firstReaderHoldCount = 1;
                } else if (firstReader == current) {   //重入数量+1
                    firstReaderHoldCount++;
                } else {
                    HoldCounter rh = cachedHoldCounter;
                    if (rh == null || rh.tid != getThreadId(current))
                        cachedHoldCounter = rh = readHolds.get();  // theadLocal 子类 初始化 cachedHoldCounter
                    else if (rh.count == 0)
                        readHolds.set(rh);
                    rh.count++;   //用threadlocal 管理每个线程的计数器
                }
                return 1;
            }
            //获取读锁失败，循环 cas 操作
            return fullTryAcquireShared(current);
        }

ReentrantReadWriteLock#fullTryAcquireShared

  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) {   //重复判断是否有写锁
                    if (getExclusiveOwnerThread() != current)
                        return -1;
                    // else we hold the exclusive lock; blocking here
                    // would cause deadlock.
                } else if (readerShouldBlock()) {     //需要blocking 直到能获取资源
                    // Make sure we're not acquiring read lock reentrantly
                    if (firstReader == current) {
                        // assert firstReaderHoldCount > 0;
                    } else {
                        if (rh == null) {
                            rh = cachedHoldCounter;
                            if (rh == null || rh.tid != getThreadId(current)) {
                                rh = readHolds.get();
                                if (rh.count == 0)
                                    readHolds.remove();
                            }
                        }
                        if (rh.count == 0)
                            return -1;
                    }
                }
                //无写锁  获取读锁资源
                if (sharedCount(c) == MAX_COUNT)
                    throw new Error("Maximum lock count exceeded");
                if (compareAndSetState(c, c + SHARED_UNIT)) {   //重复获取操作
                    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;
                }
            }
        }

AQS#doAcquireShared

private void doAcquireShared(int arg) {
        final Node node = addWaiter(Node.SHARED);  //将share节点入队列
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {  //队列中只有头节点能获取资源
                    int r = tryAcquireShared(arg);  //尝试获取读锁资源
                    if (r >= 0) {  //资源获取成功
                        setHeadAndPropagate(node, r);  //设置为头节点，并尝试唤醒下一个共享节点
                        p.next = null; // help GC
                        if (interrupted)   //判断中断
                            selfInterrupt();
                        failed = false;
                        return;
                    }
                }
                //清除cancel节点，后park等待signal
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

AQS#setHeadAndPropagate

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())  //如果下一个节点也是共享锁，则唤醒下一个节点
                doReleaseShared();
        }
    }

ReentrantReadWriteLock#readlock#unlock

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

AQS#releaseShared

public final boolean releaseShared(int arg) {
        if (tryReleaseShared(arg)) {  //尝试释放资源 读锁全部释放后为true
            doReleaseShared();   //唤醒后继节点
            return true;
        }
        return false;
    }

ReentrantReadWriteLock#tryReleaseShared

 protected final boolean tryReleaseShared(int unused) {
            Thread current = Thread.currentThread();
            if (firstReader == current) {  // 判断当前线程时否为第一个reader
                // assert firstReaderHoldCount > 0;
                if (firstReaderHoldCount == 1)
                    firstReader = null;
                else
                    firstReaderHoldCount--;
            } else {
                HoldCounter rh = cachedHoldCounter;
                if (rh == null || rh.tid != getThreadId(current))
                    rh = readHolds.get();   //获取当前线程的计数器
                int count = rh.count;
                if (count <= 1) {
                    readHolds.remove();   //< 1 则释放当前计数器
                    if (count <= 0)
                        throw unmatchedUnlockException();
                }
                --rh.count;
            }
            for (;;) {
                int c = getState();
                int nextc = c - SHARED_UNIT;
                if (compareAndSetState(c, nextc))   //循环cas 操作释放资源
                    // 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;  //读锁free
            }
        }

AQS#doReleaseShared

//释放共享锁，唤醒下一个节点
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;
            //最终状态 signal = 》0 = 》 PROPAGATE  经过一步0的操作时因为unparkSuccessor 中会有cas设置为0状态
            if (h != null && h != tail) {
                int ws = h.waitStatus;  //获取头节点的状态
                if (ws == Node.SIGNAL) {  //如果节点的状态为唤醒，则置为0
                    if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0)) //失败则跳出循环重新开始
                        continue;            // loop to recheck cases
                    unparkSuccessor(h);   //唤醒下一个节点
                } else if (ws == 0 &&
                         !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))  //将节点设置为可传播
                    continue;                // loop on failed CAS
            }
            //如果头节点被更换，即下一个节点被唤醒
            if (h == head)                   // loop if head changed
                break;
        }
    }