LockSupport park/unpark

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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