Condition源码解析

Posted 2020-12-14 lzh66

可以通过ReentrantLock.newCondition()来获得Condition对象

public Condition newCondition() {
        return sync.newCondition(); //实际上是调用AQS的方法
    }

final ConditionObject newCondition() {
            return new ConditionObject(); //ConditionObject实现了Condition方法
        }

Condition的await()方法

public final void await() throws InterruptedException {
            if (Thread.interrupted())
                throw new InterruptedException();
            Node node = addConditionWaiter(); //添加到等待队列里
            int savedState = fullyRelease(node); //释放锁并唤醒线程
            int interruptMode = 0;
            while (!isOnSyncQueue(node)) {
                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);
        }

 

 private Node addConditionWaiter() {
            Node t = lastWaiter;
            // If lastWaiter is cancelled, clean out.
            if (t != null && t.waitStatus != Node.CONDITION) {
                unlinkCancelledWaiters();
                t = lastWaiter;
            }
            Node node = new Node(Thread.currentThread(), Node.CONDITION); //新建一个Node对象，WaitStatus属性设置为CONDITION
            if (t == null)
                firstWaiter = node;        //设为首节点
            else 
                t.nextWaiter = node;      //添加到链表尾
            lastWaiter = node;
            return node;
        }

 

 final int fullyRelease(Node node) {
        boolean failed = true;
        try {
            int savedState = getState();
            if (release(savedState)) {   //释放锁，并唤醒队列里的第一个线程
                failed = false;
                return savedState;
            } else {
                throw new IllegalMonitorStateException();
            }
        } finally {
            if (failed)
                node.waitStatus = Node.CANCELLED;
        }
    }

singal()方法

 public final void signal() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            Node first = firstWaiter;
            if (first != null)
                doSignal(first); //doSignal主要就是把等待队列的第一个节点移动同步队列的尾节点中去
        }

 private void doSignal(Node first) {
            do {
                if ( (firstWaiter = first.nextWaiter) == null)//把下一个节点设为首节点，如果首节点的下一个节点为空，说明队列只有一个节点，
                    lastWaiter = null;         //把尾节点置为null；
                first.nextWaiter = null;       //把首节点从队列中清除
            } while (!transferForSignal(first) &&
                     (first = firstWaiter) != null);
        }

 

final boolean transferForSignal(Node node) {
        /*
         * If cannot change waitStatus, the node has been cancelled.
         */
        if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))  //把节点的waitStatuss设为CONDITION
            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).
         */
        Node p = enq(node);  //把节点添加到同步队列的尾部
        int ws = p.waitStatus;
        if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))  //通过cas把节点的waitStatus设为SIGNAL
            LockSupport.unpark(node.thread);
        return true;
    }

所以Condition的await方法和singal方法的作用：

await就是把当前获得锁的线程添加到等待队列中，并阻塞队列，同时释放锁并唤醒同步队列中的线程

singal就是把等待队列的头结点添加到同步队列中的尾部，等待唤醒.

public final void await() throws InterruptedException {
    if (Thread.interrupted())
        throw new InterruptedException();
    Node node = addConditionWaiter();
    int savedState = fullyRelease(node);
    int interruptMode = 0;
    while (!isOnSyncQueue(node)) {
        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);
}