java多线程执行时主线程的等待

1、通过thread.join()方式，注意：如果有多个子线程，需要将全部的线程先start，然后再join。代码示例如下：

public class Main 
{ 
    public static void main(String[] args) 
    { 
        long start = System.currentTimeMillis(); 
         
        List<Thread> list = new ArrayList<Thread>(); 
        for(int i = 0; i < 5; i++) 
        { 
            Thread thread = new TestThread(); 
            thread.start(); 
            list.add(thread); 
        } 
        try 
        { 
            for(Thread thread : list) 
            { 
                thread.join(); 
            } 
        } 
        catch (InterruptedException e) 
        { 
            e.printStackTrace(); 
        } 
         
        long end = System.currentTimeMillis(); 
        System.out.println("子线程执行时长：" + (end - start)); 
    } 
}

2、主线程等待多个子线程（CountDownLatch实现）

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。

主要方法

public CountDownLatch(int count);

public void countDown();

public void await() throws InterruptedException

构造方法参数指定了计数的次数

countDown方法，当前线程调用此方法，则计数减一

await方法，调用此方法会一直阻塞当前线程，直到计时器的值为0

示例代码如下：

public class CountDownLatchDemo {
final static SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
public static void main(String[] args) throws InterruptedException {
CountDownLatch latch=new CountDownLatch(2);//两个工人的协作
Worker worker1=new Worker("zhang san", 5000, latch);
Worker worker2=new Worker("li si", 8000, latch);
worker1.start();//
worker2.start();//
latch.await();//等待所有工人完成工作
System.out.println("all work done at "+sdf.format(new Date()));
}

static class Worker extends Thread{
String workerName;
int workTime;
CountDownLatch latch;
public Worker(String workerName ,int workTime ,CountDownLatch latch){
this.workerName=workerName;
this.workTime=workTime;
this.latch=latch;
}
public void run(){
System.out.println("Worker "+workerName+" do work begin at "+sdf.format(new Date()));
doWork();//工作了
System.out.println("Worker "+workerName+" do work complete at "+sdf.format(new Date()));
latch.countDown();//工人完成工作，计数器减一

}

private void doWork(){
try {
Thread.sleep(workTime);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}