一. 简介
使用 Condition 可以让线程等待不同条件,典型的例子就是 ArrayBlockingQueue。
二. 源码解析
Condition 接口的具体实现是在 AbstractQueuedSynchronized 类中一个叫做 ConditionObject 的内部类。
核心字段:
/** condition 队列的第一个节点 */
private transient Node firstWaiter;
/** condition 队列的最后一个节点 */
private transient Node lastWaiter;
- await() 方法:实现了可中断的条件等待
(1)如果当前线程中断,抛出中断异常
(2)保存锁状态通过 getStat 返回
(3)调用 release,参数是已保存的状态
(4)线程阻塞直到 signal 或者中断
(5)调用特定的 acquire 重新获取
(6)如果在(4)中断的话,抛出中断异常
- public final void await() throws InterruptedException {
- if (Thread.interrupted())
- throw new InterruptedException();
- Node node = addConditionWaiter(); // 当前线程加入等待队列,此时Node状态为等待状态
- int savedState = fullyRelease(node); // 释放当前线程占有的锁(await 是在 lock 后才能调用)
- int interruptMode = 0;
- while (!isOnSyncQueue(node)) { // 不在AQS的同步等待队列里,说明还没达到获取锁的条件,这时线程还在Condition的队列里,得//signal了才能进入AQS同步队列。所以现在只能卡在这
- LockSupport.park(this);
- if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)// 如果线程中断,则退出死循环
- break;
- }
- if (acquireQueued(node, savedState) && interruptMode != THROW_IE) // 因为上述线程释放锁,所以AQS等待队列节点可以获取锁
- interruptMode = REINTERRUPT;
- if (node.nextWaiter != null) // clean up if cancelled
- unlinkCancelledWaiters();
- if (interruptMode != 0)
- reportInterruptAfterWait(interruptMode);
- }
- public final void signal() {
- if (!isHeldExclusively())
- throw new IllegalMonitorStateException();
- Node first = firstWaiter;
- if (first != null)
- doSignal(first);
- }
- private void doSignal(Node first) {
- do {
- if ( (firstWaiter = first.nextWaiter) == null)
- lastWaiter = null;
- first.nextWaiter = null;
- // 将节点从 Condition 队列移动到 AQS 的同步队列,移出一个节点循环就会结束
- } while (!transferForSignal(first) &&
- (first = firstWaiter) != null);
- }
- public final void signalAll() {
- if (!isHeldExclusively())
- throw new IllegalMonitorStateException();
- Node first = firstWaiter;
- if (first != null)
- doSignalAll(first);
- }
- private void doSignalAll(Node first) {
- lastWaiter = firstWaiter = null;
- do {
- Node next = first.nextWaiter;
- first.nextWaiter = null;
- transferForSignal(first);
- first = next;
- } while (first != null);// 全部移出 Condition 等待队列
- }
转载自:https://blog.csdn.net/wenniuwuren/article/details/51447767