ReentrantLock源码分析（待补充）

摘要：先分析下同步器，这个是用于锁实现的类，就用到了它的方法就是调用去做事情的。方法简单来说是构造节点，然后用的方式把这个节点加入同步器中节点链表的尾巴。通过调用同步器的方法，等待队列中的头节点线程安全地移动到同步队列。

先分析下同步器AbstractQueuedSynchronizer，这个是用于锁实现的类，ReentrantLock就用到了它

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

ReentrantLock的lock()方法就是调用acquire(int arg)去做事情的。
其中protected boolean tryAcquire(int arg) 是留给子类去实现的，所以这里是采用了模板设计模式。这个方法简单来说就是去获取锁。

private Node addWaiter(Node mode) {
        Node node = new Node(Thread.currentThread(), mode);
        // Try the fast path of enq; backup to full enq on failure
        Node pred = tail;
        if (pred != null) {
            node.prev = pred;
            if (compareAndSetTail(pred, node)) {
                pred.next = node;
                return node;
            }
        }
        enq(node);
        return node;
    }

addWaiter方法简单来说是构造节点，然后用CAS的方式把这个节点加入同步器中节点链表的尾巴。

final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                final Node p = node.predecessor();
                //如果前驱结点是头节点的话，那么尝试获取锁（也就是同步状态）
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                //如果失败的话
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
}

acquireQueued()方法中，当前线程在“死循环”中尝试获取同步状态，并且只有前驱节点是头节点才能够尝试获取同步状态。

/*
* 查询是否有线程比当前线程更早地请求获取锁
*/
public final boolean hasQueuedPredecessors() {
        // The correctness of this depends on head being initialized
        // before tail and on head.next being accurate if the current
        // thread is first in queue.
        Node t = tail; // Read fields in reverse initialization order
        Node h = head;
        Node s;
        //如果h==t，返回false，表示没有。h==t的时候要么没有等待者，要么只有一个。当只有一个的时候，
        //那么这一个节点肯定是首节点，而首节点中的线程必定是获取了锁的。
        // h!=t&&((s = h.next) == null || s.thread != Thread.currentThread())
        //如果h!=t，则进入后面的判断
        //当头节点的后继节点为null，但是这个时候tail为null（head和tail节点都是懒初始
        //化），或者头节点的后继节点不为null，但是头节点的后继节点中的线程不是当前线程，则返回true
        return h != t &&
            ((s = h.next) == null || s.thread != Thread.currentThread());
}

如果等待队列为空，或者只有首节点，或者首节点的后继节点中的线程是当前线程，那么当前线程就可以去获取同步状态

 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.
             */
            /*
             *如果这个节点的waitStatus已经被标记为SIGNAL（-1）了的话，那么
             *这个节点就需要park，所以方法返回true
             */
            return true;
        if (ws > 0) {
            /*
             * Predecessor was cancelled. Skip over predecessors and
             * indicate retry.
             */
             /*
             * 该节点的前驱节点如果被取消了（waitStatus为1时表示取消状态，目前只有这个状态的值大于
             * 0），那么跳过前驱节点（通过死循环的方式把前驱结点的前驱结点设置为自己的前驱结点）。然
             * 后退出if条件语句，调到末尾，返回false，表示自己还可以抢救一下，可以进行获取的锁的尝
             * 试，而不是park。
             * 
             */
            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.
             */
             /*
             * 如果以上情况都不成立的话，那么就把自己的waitStatus标记为SIGNAL，返回true，表示自己要
             * park，战略性投降。
             */
            compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
        }
        return false;
    }

public class ReentrantLock implements Lock, java.io.Serializable {
    .....
    abstract static class Sync extends AbstractQueuedSynchronizer
    /**
    public ReentrantLock() {
        sync = new NonfairSync();
    }
    public ReentrantLock(boolean fair) {
        sync = fair ? new FairSync() : new NonfairSync();
    }
    .....
}

    /**
     * Implements interruptible condition wait.
     * 

     * 
 If current thread is interrupted, throw InterruptedException.
     * 
 Save lock state returned by {@link #getState}.
     * 
 Invoke {@link #release} with saved state as argument,
     *      throwing IllegalMonitorStateException if it fails.
     * 
 Block until signalled or interrupted.
     * 
 Reacquire by invoking specialized version of
     *      {@link #acquire} with saved state as argument.
     * 
 If interrupted while blocked in step 4, throw InterruptedException.
     * 
     */
    public final void await() throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        Node node = addConditionWaiter();
        long savedState = fullyRelease(node);
        int interruptMode = 0;
        while (!isOnSyncQueue(node)) {
            //线程将会在调用park方法后阻塞，直到被重新唤醒，从condition队列加入同步队列，从await()方
            //法中的while循环中退出（isOnSyncQueue(Node node)方法返回true，节点已经在同步队列中），
            //进而调用同步器的acquireQueued()方法加入到获取同步状态的竞争中。
            LockSupport.park(this);
            if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                break;
        }
        if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
            interruptMode = REINTERRUPT;
        if (node.nextWaiter != null) // clean up if cancelled
            unlinkCancelledWaiters();
        if (interruptMode != 0)
            reportInterruptAfterWait(interruptMode);
        //成功获取同步状态（或者说锁）之后，被唤醒的线程将从先前调用的await()方法返回，此时该线程已
        //经成功地获取了锁。
    }
    /**
     * Invokes release with current state value; returns saved state.
     * Cancels node and throws exception on failure.
     * @param node the condition node for this wait
     * @return previous sync state
     */
    final long fullyRelease(Node node) {
        boolean failed = true;
        try {
            long savedState = getState();
            if (release(savedState)) {
                failed = false;
                return savedState;
            } else {
                //如果释放同步状态失败，就throws exception，在finally那里还会Cancels node
                throw new IllegalMonitorStateException();
            }
        } finally {
            //如果释放同步状态失败，就Cancels node
            if (failed)
                node.waitStatus = Node.CANCELLED;
        }
    }

public final void signal() {
    if (!isHeldExclusively())
        throw new IllegalMonitorStateException();
    Node first = firstWaiter;
    if (first != null)
        doSignal(first);
}

调用Condition的signal()方法，将会唤醒在等待队列中等待时间最长的节点（首节点），在唤醒节点之前，会将节点移到同步队列中。

调用该方法的前置条件是当前线程必须获取了锁，可以看到signal()方法进行了isHeldExclusively()检查，也就是当前线程必须是获取了锁的线程。接着获取等待队列的首节点，将其移动到同步队列并使用LockSupport唤醒节点中的线程。

    /**
     * Removes and transfers nodes until hit non-cancelled one or
     * null. Split out from signal in part to encourage compilers
     * to inline the case of no waiters.
     * @param first (non-null) the first node on condition queue
     */
    private void doSignal(Node first) {
        do {
            //当没有waiters时进行的一些优化，以便编译器进行方法内联，transferForSignal方法才是重点
            if ( (firstWaiter = first.nextWaiter) == null)
                lastWaiter = null;
            first.nextWaiter = null;
        } while (!transferForSignal(first) &&
                 (first = firstWaiter) != null);
    }
    /**
     * Transfers a node from a condition queue onto sync queue.
     * Returns true if successful.
     * @param node the node
     * @return true if successfully transferred (else the node was
     * cancelled before signal)
       把节点从condition队列“传输”到同步队列去。”传输“成功或者在唤醒前节点已经取消，就返回true。
     */
    final boolean transferForSignal(Node node) {
        /*
         * If cannot change waitStatus, the node has been cancelled.
         */
        if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
            return false;

        /*
         * Splice onto queue and try to set waitStatus of predecessor to
         * indicate that thread is (probably) waiting. If cancelled or
         * attempt to set waitStatus fails, wake up to resync (in which
         * case the waitStatus can be transiently and harmlessly wrong).
         * 通过调用同步器的enq(Node node)方法，等待队列中的头节点线程安全地移动到同步队列。
         */
        Node p = enq(node);
        int ws = p.waitStatus;
        if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
            //当节点移动到同步队列后，当前线程再使用LockSupport唤醒该节点的线程。
            LockSupport.unpark(node.thread);
        return true;
    }