独占锁和共享锁

多线程访问共享变量存在数据同时修改导致不一致问题，就需要锁来对共享数据的访问进行管理。

多个线程A B C 去竞争同一个锁L，存在线程获取锁的同步状态管理，排队获取锁，竞争获取锁，等待获取锁，释放锁唤醒其他等待中的线程问题。

Java中的synchronized关键字是一种锁的实现，隐式的管理多线程与锁的问题，由JVM实现。

JUC包下提供了显示锁来管理多线程和锁问题，Lock接口，队列同步器(AQS)AbstractQueuedSynchronizer分别定义了锁的操作接口和锁的实现基础模板。
AQS同步器的模板方法基本分为3类，独占式的获取与释放同步状态，共享式的获取与释放同步状态，查询等待队列中的等待线程情况。

独占锁获取了同步状态之后，其他独占和共享访问都会阻塞。 共享锁获取了同步状态之后，共享访问不会被阻塞，独占访问会被阻塞。

独占锁的获取过程

public final void acquire(int arg) {
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }
// FairSync尝试获取同步状态 
protected final boolean tryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            // 当前状态为0 去尝试
            if (c == 0) {
            //  没有前驱节点并且获取成功  将当前线程设为持有锁的线程
                if (!hasQueuedPredecessors() &&
                    compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            // 当前状态不为0  判断持有锁的线程是不是当前线程 是的话重新设置状态值 说明ReentrantLock.FairSync是可重入锁
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0)
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }
    }

// 尝试获取同步状态
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;
                }
                // 获取失败 修改状态为等待装填 
                if (shouldParkAfterFailedAcquire(p, node) &&
	                // 阻塞线程并检查中断状态
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }
// 阻塞线程
private final boolean parkAndCheckInterrupt() {
		// 阻塞线程
        LockSupport.park(this);
        return Thread.interrupted();
    }

acquire();同步器的获取独占锁的方法，先尝试获取同步状态即获取锁tryAcquire(arg) ，获取失败去创建得带队列节点然后加入到等待队列addWaiter(Node.EXCLUSIVE)，等待队列节点自旋尝试去获取同步状态 acquireQueued()。获取同步状态失败之后阻塞该线程 LockSupport.park(this);。

private void doAcquireInterruptibly(int arg)
        throws InterruptedException {
        final Node node = addWaiter(Node.EXCLUSIVE);
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return;
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    // 响应中断 抛出异常 和普通独占获取不一样的点
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

独占式获取，中断时仍然处于阻塞等待状态。如果是独占时中断响应式获取锁，中断后抛出异常InterruptedException();

private boolean doAcquireNanos(int arg, long nanosTimeout)
        throws InterruptedException {
        long lastTime = System.nanoTime();
        final Node node = addWaiter(Node.EXCLUSIVE);
        boolean failed = true;
        try {
        // 自旋尝试获取锁 
            for (;;) {
                final Node p = node.predecessor();
                // 前一节点是头结点尝试获取 都一样 独占式获取  独占响应中断式获取 独占超时获取 
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return true;
                }
                // 到达超时时间 返回失败
                if (nanosTimeout <= 0)
                    return false;
                if (shouldParkAfterFailedAcquire(p, node) &&
                    nanosTimeout > spinForTimeoutThreshold)
                    // 超时时间未到 阻塞超时时间
                    LockSupport.parkNanos(this, nanosTimeout);
                    // 重新计算超时时间
                long now = System.nanoTime();
                nanosTimeout -= now - lastTime;
                lastTime = now;
                // 如果中断 抛出异常
                if (Thread.interrupted())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

如果是超时获取锁，会在到达超时时间后直接返回false，如果该过程中中断仍然抛出异常InterruptedException();；

释放独占锁

 public final boolean release(int arg) {
// 尝试释放锁
        if (tryRelease(arg)) {
            Node h = head;
            if (h != null && h.waitStatus != 0)
            // 唤醒后续节点的线程
                unparkSuccessor(h);
            return true;
        }
        return false;
    }
// 释放锁 重入锁需要多次释放
protected final boolean tryRelease(int releases) {
            int c = getState() - releases;
            if (Thread.currentThread() != getExclusiveOwnerThread())
                throw new IllegalMonitorStateException();
            boolean free = false;

            if (c == 0) {
                free = true;
               // 完全释放之后 持有锁的线程设为null 
                setExclusiveOwnerThread(null);
            }
            setState(c);
            return free;
        }

// 唤醒后续节点
private void unparkSuccessor(Node node) {
        /*
         * If status is negative (i.e., possibly needing signal) try
         * to clear in anticipation of signalling.  It is OK if this
         * fails or if status is changed by waiting thread.
         */
        int ws = node.waitStatus;
        if (ws < 0)
            compareAndSetWaitStatus(node, ws, 0);

        /*
         * Thread to unpark is held in successor, which is normally
         * just the next node.  But if cancelled or apparently null,
         * traverse backwards from tail to find the actual
         * non-cancelled successor.
         */
        Node s = node.next;
        if (s == null || s.waitStatus > 0) {
            s = null;
            for (Node t = tail; t != null && t != node; t = t.prev)
                if (t.waitStatus <= 0)
                    s = t;
        }
        if (s != null)
        // 唤醒后续节点
            LockSupport.unpark(s.thread);
    }

release()同步器的释放锁方法。释放锁的时候 先去尝试释放锁成功之后去唤醒后续节点的线程 LockSupport.unpark(s.thread);

获取共享锁

public final void acquireShared(int arg) {
// 先尝试获取共享状态  
        if (tryAcquireShared(arg) < 0)
        // 尝试获取共享状态失败 自旋去尝试获取
            doAcquireShared(arg);
    }

private void doAcquireShared(int arg) {
// 加入到等待队列
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {
                // 前驱节点为头结点  尝试获取同步状态
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {、
                        // 获取成功 将当前节点设为头结点
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        if (interrupted)
                            selfInterrupt();
                        failed = false;
                        return;
                    }
                }
                // 获取失败 设置状态为等待并阻塞线程
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

释放共享锁

public final boolean releaseShared(int arg) {
// 尝试释放 完全释放才会进行自旋尝试 唤醒后续线程
        if (tryReleaseShared(arg)) {
        // 共享锁尝试释放失败完全释放之后 
        doReleaseShared();
            return true;
        }
        return false;
    }

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;
        }
    }