Condition的await-signal流程详解

// AbstractQueuedSynchronizer.ConditionObject.class

final boolean isOnSyncQueue(Node node) {
    // 当进入Condition队列时，waitStatus肯定为CONDITION，如果同时别的线程调用signal，Node会从Condition队列中移除，并且移除时会清除CONDITION状态。
    // 从移除到进入release队列，中间这段时间prev必然为null，所以还是返回false，即被park
    if (node.waitStatus == Node.CONDITION || node.prev == null)
        return false;
    // 当别的线程进入release队列时，会和前一个Node建立前后关系，所以如果next存在，说明一定在release队列中
    if (node.next != null) // If has successor, it must be on queue
        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.
     */
    // 可能该Node刚刚最后一个进入release队列，所以是tail，其next必然是null，所以需要从队尾向前查找
    return findNodeFromTail(node);
}

// AbstractQueuedSynchronizer.ConditionObject.class

final boolean transferForSignal(Node node) {
    /*
     * If cannot change waitStatus, the node has been cancelled.
     */
    // 如果改变waitStatus失败，说明已经被取消，没必要再进入release队列了。外部再循环找到一个Condition Node
    // 如果改变waitStatus成功，但是之后又被取消会怎么样？没关系，虽然已经进入release队列了，但是release方法里的unpark操作会跳过已取消的Node。这里的检查只是为了减少unpark时不必要的工作
    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).
     */
    // p是该Node的前驱
    Node p = enq(node);
    int ws = p.waitStatus;
    // 这里影响设置waitStatus只可能发生于线程被取消，那时会调用cancelAcquire方法将waitStatus设置为CANCEL，但它不是CAS的
    if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
        LockSupport.unpark(node.thread);
    return true;
}