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

Posted 2020-09-23 冰花ぃ雪魄

/**
* 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 。