ReentrantReadWriteLock acquireShared源码分析

    public final void acquireShared(int arg) {
        if (tryAcquireShared(arg) < 0)
            doAcquireShared(arg);
    }
      
       
        //该方法不会阻塞，该方法只是尝试获取而已，获取失败后就会调用doAcquireShared，
        //doAcquireShared方法如果获取不到锁则对应线程会被阻塞
        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();
            if (exclusiveCount(c) != 0 &&
                getExclusiveOwnerThread() != current)
                return -1;
            int r = sharedCount(c);
            //readerShouldBlock公平锁、非公平锁处理方式不同
            //公平锁会考虑前面是不是有已经等待的线程（不管是获取读锁还是写锁）
            //非公平锁则只关注第一个等待节点是不是获取写锁
            //如果有几个线程同时竞争，可能导致compareAndSetState失败进入fullTryAcquireShared
            //自旋获取到锁
            if (!readerShouldBlock() &&
                r < MAX_COUNT &&
                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();
                    //执行下面分支条件：
                   //1.当前线程曾经获取到读锁并释放了，再次获取读锁，且期间没有任何其他线程获取 
                   //读锁！！！
                   //2.该线程不是第一个获取到读锁的线程，即在该线程获取到读取之前有其他线程已经
                   //获取到读锁，而且仍没有退出！！
                    else if (rh.count == 0)
                        readHolds.set(rh);
                    rh.count++;
                }
                return 1;
            }
            return fullTryAcquireShared(current);
        }

    static final class FairSync extends Sync {
        private static final long serialVersionUID = -2274990926593161451L;
        final boolean writerShouldBlock() {
            return hasQueuedPredecessors();
        }
        final boolean readerShouldBlock() {
            //公平锁则会按照FIFO的规则来办事
            return hasQueuedPredecessors();
        }
    }

    static final class NonfairSync extends Sync {
        private static final long serialVersionUID = -8159625535654395037L;
        final boolean writerShouldBlock() {
            return false; // writers can always barge
        }
        final boolean readerShouldBlock() {
            //非公平锁则是需要判断同步队列第一个节点是不是写锁，这样做的目的是为了
            //避免写锁被饿死的现象，因为写锁跟读写锁都是互斥，但是读锁则只跟写锁互斥
            return apparentlyFirstQueuedIsExclusive();
        }
    }

HoldCounter rh = cachedHoldCounter;
if (rh == null || rh.tid != getThreadId(current))
    cachedHoldCounter = rh = readHolds.get();
else if (rh.count == 0)
    readHolds.set(rh);
rh.count++;

cachedHoldCounter = rh = readHolds.get();

private transient ThreadLocalHoldCounter readHolds;

        static final class ThreadLocalHoldCounter
            extends ThreadLocal<HoldCounter> {
            public HoldCounter initialValue() {
                return new HoldCounter();
            }
        }

        static final class HoldCounter {
            int count = 0;
            // Use id, not reference, to avoid garbage retention
            final long tid = getThreadId(Thread.currentThread());
        }

else if (rh.count == 0)
     readHolds.set(rh);

        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();
                //存在写互斥锁且互斥锁的拥有者不是当前线程则放弃尝试自旋
                if (exclusiveCount(c) != 0) {
                    //如果没有下面的判断，试想这么一种情况，一个线程先获取到写锁，然后
                    //未释放写锁之前该线程又试图获取读锁，如果没有
                    //getExclusiveOwnerThread() != current这个条件判断直接返回-1，那么
                    //获取到写锁的线程将进入睡眠无法唤醒，也就是传说中的死锁状态
                    if (getExclusiveOwnerThread() != current)
                        return -1;
                } else if (readerShouldBlock()) {
                    // 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;
                    }
                }
                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;
                }
            }
        }

    private void doAcquireShared(int arg) {
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                final Node p = node.predecessor();
                //如果自身节点不是第一个等待节点则直接竞争失败，但这不代表自身不能获取到锁
                //这个要归功于setHeadAndPropagate，如果第一个节点不是自身节点，但是第一个节点
                //是其他读节点，那么当第一个读节点获取到读锁以后会通过setHeadAndPropagate唤醒 
                //后面其他读节点
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        //成功获取到读共享锁，那么将唤醒所有其他读锁，不唤醒写互斥锁线程！！！
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        if (interrupted)
                            selfInterrupt();
                        failed = false;
                        return;
                    }
                }
                //获取读锁失败，进入阻塞等待被唤醒
                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;
            //当前节点下一节点是获取共享锁则唤醒，不抢写锁的饭碗，否则可能会导致获取写锁的线程饿 
            //死的情况，因为该方法是获取到读锁后调用的，考虑到读写锁互斥所以如果下一个等待节点是
            //准备获取写锁，则不唤醒后续节点，反之则唤醒
            if (s == null || s.isShared())
                doReleaseShared();
        }
    }