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java并发编程学习之Condition-分析(二)

ConditionObject

Condition在ReentrantLock中，实际上是创建AQS的ConditionObject对象，主要的成员变量有Node类型的firstWaiter和lastWaiter，作为头节点和尾节点，是单向链表。当调用await时，加入队列，signal时，加入到AQS的阻塞队列。

await方法

把节点移到Condition队列后挂起

public final void await() throws InterruptedException {
    if (Thread.interrupted())
        throw new InterruptedException();
    Node node = addConditionWaiter();//添加一个节点成为尾节点
    int savedState = fullyRelease(node);//释放所有持有的锁
    int interruptMode = 0;
    while (!isOnSyncQueue(node)) {//要么中断，要么进入阻塞队列，退出while循环
        LockSupport.park(this);//挂起
        if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)//中断过，就跳出循环
            break;
    }
    if (acquireQueued(node, savedState) && interruptMode != THROW_IE)//是否被中断。acquireQueued之前讲过
        interruptMode = REINTERRUPT;
    if (node.nextWaiter != null) // clean up if cancelled如果尾节点不为空
        unlinkCancelledWaiters();//将不是CONDITION状态的移除出去
    if (interruptMode != 0)
        reportInterruptAfterWait(interruptMode);//重新中断
}

addConditionWaiter，如果尾节点不在队列里，先移除已取消的节点，添加一个节点成为尾节点

private Node addConditionWaiter() {
    Node t = lastWaiter;
    // If lastWaiter is cancelled, clean out.如果尾节点不为空，但是状态不是CONDITION，说明已取消，不想在Condition的队列里，就移除
    if (t != null && t.waitStatus != Node.CONDITION) {
        unlinkCancelledWaiters();//将不是CONDITION状态的移除出去
        t = lastWaiter;
    }
    Node node = new Node(Thread.currentThread(), Node.CONDITION);//创建状态是CONDITION的节点
    if (t == null)
        firstWaiter = node;
    else
        t.nextWaiter = node;
    lastWaiter = node;//加入到尾节点
    return node;
}

fullyRelease方法

final int fullyRelease(Node node) {
    boolean failed = true;
    try {
        int savedState = getState();
        if (release(savedState)) {
            failed = false;
            return savedState;
        } else {
            throw new IllegalMonitorStateException();
        }
    } finally {
        if (failed)//如果释放失败，状态就变成CANCELLED，而不是CONDITION，所以会在addConditionWaiter方法中被移除
            node.waitStatus = Node.CANCELLED;
    }
}

isOnSyncQueue方法

final boolean isOnSyncQueue(Node node) {
    if (node.waitStatus == Node.CONDITION || node.prev == null)//如果状态是CONDITION或者前置节点为空，说明还在Condition队列里
        return false;
    if (node.next != null) // If has successor, it must be on queue如果有后续节点了，肯定是在阻塞队列里
        return true;
    /*
     * node.prev can be non-null, but not yet on queue because
     * the CAS to place it on queue can fail. So we have to
     * traverse from tail to make sure it actually made it.  It
     * will always be near the tail in calls to this method, and
     * unless the CAS failed (which is unlikely), it will be
     * there, so we hardly ever traverse much.
     */
    return findNodeFromTail(node);//从阻塞队列的尾节点遍历，如果找到当前node，说明在阻塞队列里返回true
}

checkInterruptWhileWaiting方法，唤醒前已经中断，返回THROW_IE，唤醒后中断，返回REINTERRUPT，没有中断，返回0

private int checkInterruptWhileWaiting(Node node) {
    return Thread.interrupted() ?
        (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) :
        0;
}
final boolean transferAfterCancelledWait(Node node) {
    //如果cas操作成功，且预期值是CONDITION状态，说明在唤醒前就中断了，并加入阻塞队列
    if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) {
        enq(node);
        return true;
    }
    
    while (!isOnSyncQueue(node))//一直到阻塞队列里，在唤醒后才中断
        Thread.yield();
    return false;
}

signal方法

唤醒Condition队列的节点

public final void signal() {
    if (!isHeldExclusively())//非独占抛异常
        throw new IllegalMonitorStateException();
    Node first = firstWaiter;//获取头结点
    if (first != null)
        doSignal(first);//唤醒头节点
}
//如果头结点已经取消，就继续往下个节点寻找
private void doSignal(Node first) {
    do {
        if ( (firstWaiter = first.nextWaiter) == null)//把下一个节点移到头结点
            lastWaiter = null;
        first.nextWaiter = null;//help gc
    } while (!transferForSignal(first) &&
             (first = firstWaiter) != null);
}
final boolean transferForSignal(Node node) {
    /*
     * 如果cas操作没成功，说明已经取消了，就继续下一个节点
     */
    if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
        return false;

    /*
     * 没有取消，就加入阻塞队列，这个方法在aqs讲过了
     * 
     */
    Node p = enq(node);
    int ws = p.waitStatus;//前置节点的状态
    //大于0，说明前置节点已取消，就轮到当前节点，可以唤醒
    //小于等于哦，把前置节点的状态设置为-1，如果失败了，唤醒
    if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
        LockSupport.unpark(node.thread);//唤醒
    return true;
}

原文 https://segmentfault.com/a/1190000019883589

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