JAVA多线程两个实用的辅助类(CountDownLatch和AtomicBoolean)

AtomicBoolean它允许一个线程等待一个线程完成任务，然后运行:

A boolean value that may be updated atomically. See the java.util.concurrent.atomic package specification for description of the properties of atomic variables. An AtomicBoolean is used in applications such as atomically updated flags, and cannot be used as a replacement for a Boolean.

public static void main(String[] args) {
		Thread t2 = new Thread(new BarWorker("bb"));
		Thread t1 = new Thread(new BarWorker("aa"));
		t2.run();
		t1.run();
	}

	private static class BarWorker implements Runnable {

		private static AtomicBoolean exists = new AtomicBoolean(false);

		private String name;

		public BarWorker(String name) {
			this.name = name;
		}

		public void run() {
			if (exists.compareAndSet(false, true)) {  //当第一个线程设置为true后，另外的线程是进不来的

				System.out.println(name + " enter"+"currentvalue="+exists.get());
				try {
					System.out.println(name + " working");
					Thread.sleep(2000);
				} catch (InterruptedException e) {
					// do nothing
				}
				System.out.println(name + " leave");
				exists.set(false);
			} else {
				System.out.println(name + " give up");
			}
		}

	}

打印的结果:

bb entercurrentvalue=true
bb working
bb leave
aa entercurrentvalue=true
aa working
aa leave

CountDownLatch

一个同步辅助类。在完毕一组正在其它线程中运行的操作之前，它同意一个或多个线程一直等待。

假设设置  final CountDownLatch end = new CountDownLatch(10);  end.countDown();能够降低计数

假设在某个地方写  end.await();  假设计数不为0，全部线程会一直等待,计数不会被重置

private static  CountDownLatch mLatch = new CountDownLatch(5);

	public static void main(String[] args) throws InterruptedException {

        final ExecutorService exec = Executors.newFixedThreadPool(10);  

        for (int index = 0; index < 5; index++) {
            final int NO = index + 1;
            Runnable run = new Runnable() {
                public void run() {
                    try {
                    	System.out.println(NO + " working");
        				Thread.sleep(2000);
                    } catch (InterruptedException e) {
                    } finally {
                    	mLatch.countDown();
                    }
                }
            };
            exec.submit(run);

        }
        mLatch.await();
        System.out.println("finish");
        exec.shutdown();
    }

结果:

1 working
3 working
2 working
4 working
5 working
finish

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