主线程等待所有线程结束之后再执行

如何让主线程等待所有线程结束之后再执行

1、Future的机制，使用Future.get()阻塞等待结果（Future，FutureTask）

2、CountDownLatch同步工具类，此类的作用就是一个线程等待所有线程结束之后再执行

3、CompletableFuture 与Future机制类似，同样是使用get阻塞等待结果，不过此类可以使任务并行合并以及串行，相当于是Future的增强版

4、线程池的isTerminated方法，当调用shutdown()方法后，并且所有提交的任务完成后才会返回为true

5、Thread的join方法，等待主线程结束

Future机制

FutureTask

package com.alibaba.fescar.core.protocol.test;import java.util.concurrent.ExecutionException;
import java.util.concurrent.FutureTask;public class TestFuture {public static void main(String[] args) {FutureTask<Integer> futureTask = new FutureTask<Integer>(()-> {System.out.println("当前线程执行");return 1;});new Thread(futureTask).start();try {// 阻塞获取值Integer integer = futureTask.get();} catch (InterruptedException e) {e.printStackTrace();} catch (ExecutionException e) {e.printStackTrace();}System.out.println("主线程执行");}}

执行结果

Future

package com.alibaba.fescar.core.protocol.test;import java.util.concurrent.*;public class TestFuture {public static void main(String[] args) {ExecutorService pool = Executors.newFixedThreadPool(1);Future<Integer> task = pool.submit(() -> {System.out.println("当前线程执行");return 1;});try {// 阻塞获取结果Integer integer= task.get();} catch (InterruptedException e) {e.printStackTrace();} catch (ExecutionException e) {e.printStackTrace();}System.out.println("主线程执行");}}

执行结果

结果分析：这里借用线程池提交任务，程序没有退出，是因为线程池没有shutdown，熟悉下线程池的submit方法

<T> Future<T> submit(Callable<T> task);
<T> Future<T> submit(Runnable task, T result);         
Future<?> submit(Runnable task);

如上所示，线程池接受Callable 和Runnable任务，返回Future接口的上转型对象，和FutureTask同理，

有关于Runnable、Future、Callable、FutureTask 请参考文章：

Runnable、Future、Callable、FutureTask

CountDownLatch

package com.alibaba.fescar.core.protocol.test;import java.util.concurrent.CountDownLatch;public class TestCountDown {public static void main(String[] args) throws InterruptedException {CountDownLatch latch = new CountDownLatch(1);new Thread(() -> {System.out.println("当前线程结束");latch.countDown();}).start();latch.await();System.out.println("主线程结束");}}

执行结果

latch.dountDown()调用，计数器会减一，latch.await()会阻塞主线程直到0放行

CompletableFuture

package com.alibaba.fescar.core.protocol.test;import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;public class TestCompletableFuture {public static void main(String[] args) throws ExecutionException, InterruptedException {/*** supplyAsync与runAsync的区别在于：supplyAsync有返回值，而runAsync没有返回值*/CompletableFuture<Integer> current1 = CompletableFuture.supplyAsync(() -> {System.out.println("当前线程1执行");return 3;});/*** 合并两个任务，两个任务可以同时执行，都执行成功后，执行最后的BiFunction操作。*/CompletableFuture<Integer> current2 = CompletableFuture.supplyAsync(() -> {System.out.println("当前线程2执行");return 2;}).thenCombine(current1,(res1,res2)->res1 * res2);System.out.println(current2.get());System.out.println("主线程执行");}
}

执行结果

CompletableFuture实现了Future接口，当其get的时候同样阻塞主线程执行， CompletableFuture在于可以多任务串行，以及并行执行，操作空间更广

具体用法请参考文章：CompletableFuture用法

线程池的isTerminated方法

package com.alibaba.fescar.core.protocol.test;import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;public class TestThreadPool{public static void main(String[] args) {ExecutorService pool = Executors.newFixedThreadPool(3);pool.execute(() -> {try {Thread.sleep(2000);} catch (InterruptedException e) {e.printStackTrace();}});pool.execute(() -> {try {Thread.sleep(2000);} catch (InterruptedException e) {e.printStackTrace();}});pool.shutdown();while (true) {if (pool.isTerminated()) {System.out.println("线程池中的任务执行结束");break;}}System.out.println("主线程结束");}}

执行结果

isTerminated，当调用shutdown()方法后，并且所有提交的任务完成后才会返回为true

Thread的join方法

package com.alibaba.fescar.core.protocol.test;public class TestThreadJoin {public static void main(String[] args) throws InterruptedException {Thread t1 = new Thread(() -> {try {System.out.println("t1执行");Thread.sleep(3000);} catch (InterruptedException e) {e.printStackTrace();}});t1.start();Thread t2 = new Thread(() -> {try {System.out.println("t2执行");Thread.sleep(3000);} catch (InterruptedException e) {e.printStackTrace();}});t2.start();t1.join();t2.join();System.out.println("主线程结束");}
}