signal()源码

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

doSignal

        private void doSignal(Node first) {
            do {
                if ( (firstWaiter = first.nextWaiter) == null)
                    lastWaiter = null;
                first.nextWaiter = null;
            } while (!transferForSignal(first) &&
                     (first = firstWaiter) != null);
        }

transferForSignal

    final boolean transferForSignal(Node node) {
        //-2 表示需要Signal 的节点
        if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
            //一般不会发生
            return false;
        // 从条件队列转移到阻塞队列
        Node p = enq(node); // p为node的前一个节点
        int ws = p.waitStatus; //如果为p的状态为-1，则它有责任直接唤醒当前node
        if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
            LockSupport.unpark(node.thread);
        return true;
    }

enq 转移到阻塞队列

    private Node enq(final Node node) {
        for (;;) {
            Node t = tail;
            if (t == null) { // Must initialize
                if (compareAndSetHead(new Node()))
                    tail = head;
            } else {
                node.prev = t;
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }