AtomicBoolean可以让一个线程等待另一个线程完成任务后再执行:

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=http://www.mamicode.com/"+exists.get());>
打印的结果:
bb entercurrentvalue=http://www.mamicode.com/true>
CountDownLatch
一个同步辅助类，在完成一组正在其他线程中执行的操作之前，它允许一个或多个线程一直等待。
如果设置  final CountDownLatch end = new CountDownLatch(10);  end.countDown();可以减少计数
如果在某个地方写  end.await();  如果计数不为0，所有线程会一直等待,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