LockSupport使用方式

LockSupport类在jdk源码中基本定义就是创建锁和其他同步类的 基本线程阻塞原语,直接与UnSafe类打交道，Unsafe类中众多的native方法实现都是基于C++语言的，而且与系统平台相关，这里基本不做阐述。这里主要说明LockSupport的使用方式。

常见使用方式

LockSupport.park()
LockSupport.unpark(Thread t)

park()方法会阻塞当前线程(线程进入Waiting状态)，除非它获取了"许可证"。
unpark(Thread t)方法会给线程t颁发一个"许可证"。

从park()方法的官方注释中，我们发现还有其他的方式可以使park()方法返回。

    /**
     * Disables the current thread for thread scheduling purposes unless the
     * permit is available.
     *
     * 
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:
     *
     * 

     *
     * 
Some other thread invokes {@link #unpark unpark} with the
     * current thread as the target; or
     *
     * 
Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread; or
     *
     * 
The call spuriously (that is, for no reason) returns.
     * 
     *
     * 
This method does not 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.
     */
    public static void park() {
        UNSAFE.park(false, 0L);
    }

三种原因中的第二种是其他线程中断线程t时，线程t会从park()方法返回。测试代码如下:

public class LockSupportUse {
    public static void main(String[] args) {
        testParkThenInterrupt();
    }

    public static void testParkThenInterrupt(){
        Thread thread = new Thread(new LockSupportThread(), "LockSupportThread");
        thread.start();

        SleepUtil.sleep(2);
        //充分运行线程LockSupportThread两秒钟后，中断该线程，
        //该线程能从park()方法返回
        thread.interrupt();
        //LockSupport.unpark(thread);
    }

    static class LockSupportThread implements Runnable{
        @Override
        public void run() {
            LockSupport.park(); //阻塞自己
            System.out.println(Thread.currentThread().getName() + "从park()中返回");
        }
    }
}

它的应用在同步器AbstractQueueSynchronizer中有所体现，一个节点线程获取不到锁时，会阻塞自己，调用的就是LockSupport的park()方法，返回只有两种方式，前一个节点线程释放锁时unpark()了当前节点；或者当前节点线程被中断返回。

参考:

• 1. Java的LockSupport.park()实现分析