LockSupport的park和unpark的基本使用,以及对线程中断的响应性

时间:2021-08-04 12:24:48

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

 

 

/**
* 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.
*
* @param thread the thread to unpark, or {@code null}, in which case
* this operation has no effect
*/
public static void unpark(Thread thread)

 

 

 

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。

public static void main(String[] args)
{
LockSupport.park();
System.out.println("block.");
}

运行该代码,可以发现主线程一直处于阻塞状态。因为 许可默认是被占用的 ,调用park()时获取不到许可,所以进入阻塞状态。

如下代码:先释放许可,再获取许可,主线程能够正常终止。LockSupport许可的获取和释放,一般来说是对应的,如果多次unpark,只有一次park也不会出现什么问题,结果是许可处于可用状态。

public static void main(String[] args)
{
Thread thread = Thread.currentThread();
LockSupport.unpark(thread);//释放许可
LockSupport.park();// 获取许可
System.out.println("b");
}

LockSupport是可不重入 的,如果一个线程连续2次调用 LockSupport .park(),那么该线程一定会一直阻塞下去。

public static void main(String[] args) throws Exception
{
Thread thread = Thread.currentThread();

LockSupport.unpark(thread);

System.out.println("a");
LockSupport.park();
System.out.println("b");
LockSupport.park();
System.out.println("c");
}

这段代码打印出a和b,不会打印c,因为第二次调用park的时候,线程无法获取许可出现死锁。

下面我们来看下LockSupport对应中断的响应性

public static void t2() throws Exception
{
Thread t = new Thread(new Runnable()
{
private int count = 0;

@Override
public void run()
{
long start = System.currentTimeMillis();
long end = 0;

while ((end - start) <= 1000)
{
count++;
end = System.currentTimeMillis();
}

System.out.println("after 1 second.count=" + count);

//等待或许许可
LockSupport.park();
System.out.println("thread over." + Thread.currentThread().isInterrupted());

}
});

t.start();

Thread.sleep(2000);

// 中断线程
t.interrupt();


System.out.println("main over");
}

最终线程会打印出thread over.true。这说明 线程如果因为调用park而阻塞的话,能够响应中断请求(中断状态被设置成true),但是不会抛出InterruptedException 。