LockSupport park/unpark

ゞ浴缸里的玫瑰 2022-05-30 12:44 267阅读 0赞

# 介绍 #

LockSupport是JDK中比较底层的类，用来创建锁和其他同步工具类的基本线程阻塞原语。java锁和同步器框架的核心AQS:AbstractQueuedSynchronizer，就是通过调用LockSupport.park()和LockSupport.unpark()实现线程的阻塞和唤醒的。LockSupport很类似于二元信号量(只有1个许可证可供使用)，如果这个许可还没有被占用，当前线程获取许可并继续执行；如果许可已经被占用，当前线程阻塞，等待获取许可。

**park/unpark模型真正解耦了线程之间的同步**，线程之间不再需要一个Object或者其它变量来存储状态，不再需要关心对方的状态。

# API #

/** * Disables the current thread for thread scheduling purposes unless the permit is available. * 阻塞当前线程，直到许可获得 * * <p>If the permit is available then it is consumed and the call returns immediately; otherwise * the current thread becomes disabled for thread scheduling * purposes and lies dormant until one of three things happens: * 以下三种情形将结束线程阻塞： * 1）其它线程unpark当前线程 * 2）其它线程interrupt中断 * 3） * * <ul> * <li>Some other thread invokes {@link #unpark unpark} with the * current thread as the target; or * * <li>Some other thread {@linkplain Thread#interrupt interrupts} * the current thread; or * * <li>The call spuriously (that is, for no reason) returns. * </ul> * * <p>This method does <em>not</em> report which of these caused the * method to return. Callers should re-check the conditions which caused * the thread to park in the first place. Callers may also determine, * for example, the interrupt status of the thread upon return. * * @param blocker the synchronization object responsible for this * thread parking * * @since 1.6 */
        public static void park(Object blocker) {
            Thread t = Thread.currentThread();
            setBlocker(t, blocker);
            UNSAFE.park(false, 0L);
            setBlocker(t, null);
        }

/** * Makes available the permit for the given thread, if it * was not already available. If the thread was blocked on * {@code park} then it will unblock. Otherwise, its next call * to {@code park} is guaranteed not to block. This operation * is not guaranteed to have any effect at all if the given * thread has not been started. * 为指定线程制造许可， * * @param thread the thread to unpark, or {@code null}, in which case * this operation has no effect */
        public static void unpark(Thread thread) {
            if (thread != null)
                UNSAFE.unpark(thread);
        }

# 参考资料 #

[LockSupport详细讲解][LockSupport]

[LockSupport]: http://www.360doc.com/content/14/0812/22/1073512_401408738.shtml