ReentrantLock与AQS

类结构

ReentrantLock实现了Lock, java.io.Serializable这两个接口，里面有三个静态内部类：Sync、NonfairSync和FairSync。其中Sync继承了AbstractQueuedSynchronizer，也就是AQS。另外两个内部类是对抽象类Sync的实现。

加锁

在JDK8中，加锁的流程和JDK11有些区别，JDK8参考美团的代码：

// java.util.concurrent.locks.ReentrantLock#NonfairSync

// 非公平锁
static final class NonfairSync extends Sync {
	...
	final void lock() {
		if (compareAndSetState(0, 1))
			setExclusiveOwnerThread(Thread.currentThread());
		else
			acquire(1);
		}
  ...
}

// java.util.concurrent.locks.ReentrantLock#FairSync

static final class FairSync extends Sync {
  ...
	final void lock() {
		acquire(1);
	}
  ...
}

而在JDK11中，，不管是构建ReentrantLock时，指定的是公平锁还是非公平锁，加锁后都会进入下面的流程：

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


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

其中，公平锁和非公平锁对tryAcquire()的实现有一些区别：

static final class NonfairSync extends Sync {
        private static final long serialVersionUID = 7316153563782823691L;
        protected final boolean tryAcquire(int acquires) {
            return nonfairTryAcquire(acquires);
        }
    }


@ReservedStackAccess
        final boolean nonfairTryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                if (compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }




    /**
     * Sync object for fair locks
     */
    static final class FairSync extends Sync {
        private static final long serialVersionUID = -3000897897090466540L;
        /**
         * Fair version of tryAcquire.  Don't grant access unless
         * recursive call or no waiters or is first.
         */
        @ReservedStackAccess
        protected final boolean tryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                if (!hasQueuedPredecessors() &&
                    compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0)
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }
    }

可以看出，无论是公平锁还是非公平锁，实现流程都是类似的，只是公平锁需要通过hasQueuedPredecessors()判断当前线程有没有前驱节点，如果有的话就不去抢锁，而是加入等待队列后等待。

排队

再看acquire():

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

当前线程会通过addWaiter()方法变为等待队列上的一个节点，然后这个节点会传入acquireQueued()去执行相应的流程：

private Node addWaiter(Node mode) {
        Node node = new Node(mode);

        for (;;) {
            Node oldTail = tail;
            if (oldTail != null) {
                node.setPrevRelaxed(oldTail);
                if (compareAndSetTail(oldTail, node)) {
                    oldTail.next = node;
                    return node;
                }
            } else {
                initializeSyncQueue();
            }
        }
    }

首先新建一个节点，然后采用自旋(for循环)的方式将节点加入等待队列之中。此时会先获取一下队列的尾节点，但是由于此时可能有多个线程在操作这个队列，所以仅仅能把当前节点指向这个获取的尾节点，只有通过CAS(compareAndSetTail)判断并修改尾节点为当前节点后，才能将刚才的尾节点指向当当前节点，否则就一直自旋，直到成功将节点加入队列。如果当前没有尾节点，就要将队列初始化。最终将当前节点返回：

final boolean acquireQueued(final Node node, int arg) {
        boolean interrupted = false;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    return interrupted;
                }
                if (shouldParkAfterFailedAcquire(p, node))
                    interrupted |= parkAndCheckInterrupt();
            }
        } catch (Throwable t) {
            cancelAcquire(node);
            if (interrupted)
                selfInterrupt();
            throw t;
        }
    }

自旋判断当前节点的前驱节点是不是头节点，如果是，就去尝试获取锁。如果获取失败或者不是头节点，就判断当前节点是否需要阻塞，也就是当前线程是否需要挂起以避免空转浪费CPU资源。这里注意第7行的setHead(node)方法，实现是这样：

private void setHead(Node node) {
        head = node;
        node.thread = null;
        node.prev = null;
    }

因为当前节点的线程已经获取了锁，所以就把当前节点设置为虚节点，把不必要的属性置为null。

private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
        int ws = pred.waitStatus;
        if (ws == Node.SIGNAL)
            /*
             * This node has already set status asking a release
             * to signal it, so it can safely park.
             */
            return true;
        if (ws > 0) {
            /*
             * Predecessor was cancelled. Skip over predecessors and
             * indicate retry.
             */
            do {
                node.prev = pred = pred.prev;
            } while (pred.waitStatus > 0);
            pred.next = node;
        } else {
            /*
             * waitStatus must be 0 or PROPAGATE.  Indicate that we
             * need a signal, but don't park yet.  Caller will need to
             * retry to make sure it cannot acquire before parking.
             */
            pred.compareAndSetWaitStatus(ws, Node.SIGNAL);
        }
        return false;
    }

/** Marker to indicate a node is waiting in shared mode */
        static final Node SHARED = new Node();
        /** Marker to indicate a node is waiting in exclusive mode */
        static final Node EXCLUSIVE = null;

        /** waitStatus value to indicate thread has cancelled. */
        static final int CANCELLED =  1;
        /** waitStatus value to indicate successor's thread needs unparking. */
        static final int SIGNAL    = -1;
        /** waitStatus value to indicate thread is waiting on condition. */
        static final int CONDITION = -2;
        /**
         * waitStatus value to indicate the next acquireShared should
         * unconditionally propagate.
         */
        static final int PROPAGATE = -3;

如果前置节点的状态是SIGNAL，就需要将当前线程挂起，其他情况都不挂起。但是由于自旋，这个方法会逐渐将所有空闲线程都挂起。具体流程由parkAndCheckInterrupt()执行：

private final boolean parkAndCheckInterrupt() {
        LockSupport.park(this);
        return Thread.interrupted();
    }

当线程被唤醒后，线程会执行上面代码块的第3行，return这个线程当前的中断状态。

唤醒

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;
    }

先去尝试释放资源，如果释放成功，就把头节点的后继节点唤醒。因为刚才头节点获取锁以后，就变成了虚节点，所以这里唤醒的是头节点的后继节点：

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)
            node.compareAndSetWaitStatus(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 p = tail; p != node && p != null; p = p.prev)
                if (p.waitStatus <= 0)
                    s = p;
        }
        if (s != null)
            LockSupport.unpark(s.thread);
    }

选择去唤醒一个waitStatus状态小于0的节点。为什么倒着遍历呢？因为之前构建等待队列的时候，将当前节点挂在获取的尾节点之后，可能存在多个线程并发的问题，更新尾节点要通过CAS。这时候如果正着遍历就有断链的危险。虽然说倒着遍历，但是最终选择的还是最靠近队列头的节点。

被唤醒的节点重新获取锁：

final boolean acquireQueued(final Node node, int arg) {
        boolean interrupted = false;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    return interrupted;
                }
                if (shouldParkAfterFailedAcquire(p, node))
                    interrupted |= parkAndCheckInterrupt();
            }
        } catch (Throwable t) {
            cancelAcquire(node);
            if (interrupted)
                selfInterrupt();
            throw t;
        }
    }