java多線程執行時主線程的等待

一、經過thread.join()方式，注意：若是有多個子線程，須要將所有的線程先start，而後再join。代碼示例以下：

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("子線程執行時長：" + (end - start)); 
    } 
} 

二、主線程等待多個子線程（CountDownLatch實現）

CountDownLatch，一個同步輔助類，在完成一組正在其餘線程中執行的操做以前，它容許一個或多個線程一直等待。

主要方法

 public CountDownLatch(int count);

 public void countDown();

 public void await() throws InterruptedException
 

構造方法參數指定了計數的次數

countDown方法，當前線程調用此方法，則計數減一

await方法，調用此方法會一直阻塞當前線程，直到計時器的值爲0

示例代碼以下：

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);//兩個工人的協做
Worker worker1=new Worker("zhang san", 5000, latch);
Worker worker2=new Worker("li si", 8000, latch);
worker1.start();//
worker2.start();//
latch.await();//等待全部工人完成工做
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();//工做了
System.out.println("Worker "+workerName+" do work complete at "+sdf.format(new Date()));
latch.countDown();//工人完成工做，計數器減一

}

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