Java等待多个线程执行完毕

1、使用join

public class TestCountDownLatch2
{

public static void main(String[] args) throws InterruptedException
{
Thread[] threadArr = new Thread[10];
for(int i = 0; i < 10; i++){
threadArr[i] = new Thread(new WorkThread());
}

for(int i=0;i<10;i++){
threadArr[i].start();
}

for(int i=0;i<10;i++){
threadArr[i].join();
}

System.out.println("main done!");
}
}

class WorkThread implements Runnable {

@Override
public void run()
{
System.out.println(Thread.currentThread().getName() + "  worked");
}
}

2、使用ExecutorService.isTerminated死循环判断

public class TestCountDownLatch
{

public static void main(String[] args) throws InterruptedException
{
ExecutorService executorService = Executors.newCachedThreadPool();
for(int i = 0; i < 10; i++){
executorService.execute(new Thread(new WorkThread()));
}

System.out.println("submit all task");
executorService.shutdown();  // stop accept new task

while(!executorService.isTerminated()){  // must call shutdown first, when before tasks finished, then the flag is set true
; // do nothing
}

System.out.println("main thread done!");
}
}

class WorkThread implements Runnable {

@Override
public void run()
{
System.out.println(Thread.currentThread().getName() + "  worked");
try
{
Thread.sleep(2000);
}
catch (InterruptedException e)
{
e.printStackTrace();
}
}
}

3、使用CountDownLatch

特点：CountDown方法可以用在任何地方，可以是N个线程，也可以是1个线程里的N个执行步骤

public class TestCountDownLatch2
{

public static void main(String[] args) throws InterruptedException
{
CountDownLatch countDownLatch = new CountDownLatch(10);

ExecutorService executorService = Executors.newCachedThreadPool();
for(int i = 0; i < 10; i++){
executorService.execute(new Thread(new WorkThread(countDownLatch)));
}

System.out.println("submit all task");
executorService.shutdown();  // stop accept new task

countDownLatch.await();  // wait until the latch has count down to zero

System.out.println("main done!");
}
}

class WorkThread implements Runnable {

private CountDownLatch countDownLatch;

public WorkThread(CountDownLatch countDownLatch){
this.countDownLatch = countDownLatch;
}

@Override
public void run()
{
System.out.println(Thread.currentThread().getName() + "  worked");
countDownLatch.countDown();
}
}

比较总结：

join用于让当前执行线程等待join线程执行结束。其实现原理是不停检查join线程是否存活，如果join线程存活则让当前线程永远wait,直到join线程中止后，线程的this.notifyAll会被调用

JDK不推荐在线程实例上使用wait，notify和notifyAll方法

推荐使用CountDownLatch