学步园

    考虑这么一个情景：当多个线程运行到某处的时候，需要将所有的线程的计算结果汇总。那如何控制这些线程计算子问题结束后，在汇总的代码处等待其它线程尚未计算完的线程呢？可以考虑使用CountDownLatch和CyclicBarrier。CountDownLatch只能使用一次，CyclicBarrier作为线程集结点 ，却可以循环使用。“cyclic”循环的意思。

    当跑的快的线程运行到CyclicBarrier实例的await()方法处时，会停下来等待其它尚未到达的线程，直到所有的线程都到达这个代码点，然后开闸泄水！

    写了一个练手的程序，这个程序用到了CyclicBarrier可以循环使用的特点：

package thread;


import java.util.concurrent.CyclicBarrier;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;


public class CyclicBarrierTest {
	public static void main(String[] args) {
		ExecutorService es = Executors.newCachedThreadPool();

		for (int i = 0; i < 5; ++i) {
			CBRunnable cb = new CBRunnable(i);
			es.submit(cb);
		}
		
		es.shutdown();
	}
	
}


class CBRunnable implements Runnable {
	private static CyclicBarrier cb = new CyclicBarrier(5);
	private int idx;
	
	public CBRunnable(int idx) {
		this.idx = idx;
	}
	
	@Override
	public void run() {
		System.out.println("Thread " + idx + " starts.");
		try {
			Thread.sleep(idx * 1000);
			System.out.println("step1 : Thread " + idx + " wake up.");
			cb.await();	//第一次被使用
			System.out.println("Thread " + idx + " step1 ends. ");
			Thread.sleep(idx * 1000);
			System.out.println("step2 : Thread " + idx + " wake up.");
			cb.await();	//第二次被使用
			System.out.println("Thread " + idx + " step2 ends.");
		} catch (Exception e) {
			e.printStackTrace();
		}
		//System.out.println("Thread " + idx + " ends.");
	}
	
}

    程序运行结果：

Thread 0 starts.
Thread 2 starts.
step1 : Thread 0 wake up.
Thread 4 starts.
Thread 1 starts.
Thread 3 starts.
step1 : Thread 1 wake up.
step1 : Thread 2 wake up.
step1 : Thread 3 wake up.
step1 : Thread 4 wake up.
Thread 4 step1 ends. 
Thread 0 step1 ends. 
Thread 2 step1 ends. 
Thread 3 step1 ends. 
Thread 1 step1 ends. 
step2 : Thread 0 wake up.
step2 : Thread 1 wake up.
step2 : Thread 2 wake up.
step2 : Thread 3 wake up.
step2 : Thread 4 wake up.
Thread 0 step2 ends.
Thread 4 step2 ends.
Thread 1 step2 ends.
Thread 2 step2 ends.
Thread 3 step2 ends.