【高并发系列】15、线程阻塞工具类 LockSupport

1、定时阻塞

java.util.concurrent.locks.LockSupport可以在线程内任意位置让线程阻塞；

与Thread.suspend()方法比，弥补了由于resume()方法在suspend()方法之前发生而导致线程无法继续执行的情况；

与Object.wait()方法比，不需要首先获得某个对象的锁，也不会抛出InterruptedException异常；

使用LockSupport重写suspend()方法永久卡死线程的例子，请见 ==>> 

【高并发系列】3、线程那些事儿 - 5、挂起suspend()和继续执行resume()线程

现修改如下

public class LockSupportDemo {
	public static Object lock = new Object();
	private static ChangeObjectThread t1 = new ChangeObjectThread("t1");
	private static ChangeObjectThread t2 = new ChangeObjectThread("t2");
	public static class ChangeObjectThread extends Thread {
		public ChangeObjectThread(String name) {
			super.setName(name);
		}

		@Override
		public void run() {
			synchronized (lock) {
				System.out.println("In " + getName());
				LockSupport.park();
			}
		}
	}
	public static void main(String[] args) throws InterruptedException {
		t1.start();
		Thread.sleep(2000);
		t2.start();
		LockSupport.unpark(t1);
		LockSupport.unpark(t2);
		t1.join();
		t2.join();
	}
}

把suspend()方法和resume()方法用park()和unpark()方法替换；

无法保证unpark()方法发生在park()方法之后，但是执行发现可以正常结束，不会出现suspend()方法线程永远挂起的情况；

LockSupport类使用类似信号量Semaphore的机制；为每一个线程准备了一个许可；如果许可可用，park()方法立即返回，并且消费这个许可（将许可变为不可用）；如果许可不可用，就会阻塞，而unpark()方法使一个许可变为可用；

与信号量区别在于，这个许可不能累加，每个线程只有一个许可；

2、中断响应

LockSupport不会抛出InterruptedException异常，但可以从Thread.interrupted()等方法中获得中断标记；

t1获得lock锁，park()阻塞t1，t1.interrupt()被中断，t1马上响应并返回，释放lock锁；t2获得lock锁，park()方法阻塞后，unpark()继续执行，退出程序；

public class LockSupportIntDemo {
	public static Object lock = new Object();
	private static ChangeObjectThread t1 = new ChangeObjectThread("t1");
	private static ChangeObjectThread t2 = new ChangeObjectThread("t2");
	public static class ChangeObjectThread extends Thread {
		public ChangeObjectThread(String name) {
			super.setName(name);
		}
		@Override
		public void run() {
			synchronized (lock) {
				System.out.println("In " + getName());
				LockSupport.park();
				if (Thread.interrupted()) {
					System.out.println(getName() + " is interrupted.");
				}
			}
			System.out.println(getName() + " is finished.");
		}
	}
	public static void main(String[] args) throws InterruptedException {
		t1.start();
		TimeUnit.MILLISECONDS.sleep(1000);
		t2.start();
		t1.interrupt();
		TimeUnit.MILLISECONDS.sleep(1000);
		LockSupport.unpark(t2);
	}
}

console

In t1
t1 is interrupted.
t1 is finished.
In t2
t2 is finished.