java多线程，CountDownLatch和ExecutorService的简单应用

CountDownLatch可以保证程序在子线程都执行完毕的情况下再执行主线程

使用方法：对CountDownLatch对象设置初值，每个线程执行完后调用countDown方法，计数器减1，所有线程都执行完毕后，计数器为0，继续执行主线程。代码如下：

import java.util.concurrent.CountDownLatch;

public class CountDownLatchTest {
	public static void main(String[] args) throws InterruptedException {
		CountDownLatch startSignal = new CountDownLatch(1);
		CountDownLatch doneSignal = new CountDownLatch(4);
		
		new Thread(new Worker(startSignal, doneSignal)).start();
		new Thread(new Worker(startSignal, doneSignal)).start();
		new Thread(new Worker(startSignal, doneSignal)).start();
		new Thread(new Worker(startSignal, doneSignal)).start();

		System.out.println("主程序执行...");//由于startSignal是1，上边的4个线程还不能执行
		startSignal.countDown(); // startSignal=0，线程可以开始执行了
		doneSignal.await(); // 等待子线程执行完，然后继续执行下面的代码

		System.out.println("开始执行后续任务...");
	}
}

class Worker implements Runnable {
	private final CountDownLatch startSignal;
	private final CountDownLatch doneSignal;

	Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
		this.startSignal = startSignal;
		this.doneSignal = doneSignal;
	}

	@Override
	public void run() {
		try {
			startSignal.await();  //startSignal的计数到0了吗？到0了，才能执行
			doWork();
			doneSignal.countDown(); //计数减1
		} catch (InterruptedException ex) {
		} 
	}

	void doWork() {
		System.out.println("子线程执行...");
	}
}

ExecutorService用来帮忙管理线程池的，线程任务调用submit提交，执行完关闭

ExecutorService executorService = Executors.newCachedThreadPool();
        for(int i = 0; i < threadNum; i++){
            executorService.submit(new Worker(startSignal, doneSignal));
        }
        try {
            doneSignal.await();  //主线程就能通过await()方法，恢复执行自己的任务
        } catch (InterruptedException e){
            e.printStackTrace();
        }
        doOtherThing()
        executorService.shutdown();