java并发编程学习:如何等待多个线程执行完成后再继续后续处理(synchronized、join、FutureTask、CyclicBarrier)

多线程应用中,经常会遇到这种场景:后面的处理,依赖前面的N个线程的处理结果,必须等前面的线程执行完毕后,后面的代码才允许执行。

在我不知道CyclicBarrier之前,最容易想到的就是放置一个公用的static变量,假如有10个线程,每个线程处理完上去累加下结果,然后后面用一个死循环(或类似线程阻塞的方法),去数这个结果,达到10个,说明大家都爽完了,可以进行后续的事情了,这个想法虽然土鳖,但是基本上跟语言无关,几乎所有主流编程语言都支持。

package yjmyzz.test;


public class ThreadLockTest {

    public static int flag = 0;//公用变量

    public static void main(String[] args) throws Exception {
        ThreadLockTest testObj = new ThreadLockTest();
        final int threadNum = 10;

        for (int i = 0; i < threadNum; i++) {
            new Thread(new MyRunable(i, testObj)).start();
        }

        while (true) {
            if (testObj.flag >= threadNum) {
                System.out.println("-----------\n所有thread执行完成!");
                break;
            }
            Thread.sleep(10);
        }
    }

    static class MyRunable implements Runnable {
        int _i = 0;
        ThreadLockTest _test;

        public MyRunable(int i, ThreadLockTest test) {
            this._i = i;
            this._test = test;
        }

        @Override
        public void run() {
            try {
                Thread.sleep((long) (Math.random() * 10));
                System.out.println("thread " + _i + " done");
                //利用synchronized获得同步锁
                synchronized (_test) {
                    _test.flag += 1;
                }
                System.out.println("thread " + _i + " => " + _test.flag);//测试用
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
}

输出结果:

thread 0 done
thread 0 => 1
thread 9 done
thread 9 => 2
thread 1 done
thread 1 => 3
thread 3 done
thread 3 => 4
thread 7 done
thread 7 => 5
thread 6 done
thread 6 => 6
thread 2 done
thread 2 => 7
thread 4 done
thread 4 => 8
thread 8 done
thread 8 => 9
thread 5 done
thread 5 => 10
-----------
所有thread执行完成!

 

除了这个方法,还可以借助FutureTask,达到类似的效果,其get方法会阻塞线程,等到该异步处理完成。缺点就是,FutureTask调用的是Callable,必须要有返回值,所以就算你不想要返回值,也得返回点啥

package yjmyzz.test;

import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.FutureTask;

public class FutureTaskTest {

    public static void main(String[] args) throws ExecutionException, InterruptedException {

        FutureTask<String>[] tasks = new FutureTask[10];

        for (int i = 0; i < tasks.length; i++) {
            final int j = i;
            tasks[i] = new FutureTask<String>(new Callable<String>() {
                @Override
                public String call() throws Exception {
                    Thread.sleep((long) (Math.random() * 100));
                    return "task" + j + " done";
                }
            });
            new Thread(tasks[i]).start();
        }

        for (int i = 0; i < tasks.length; i++) {
            System.out.println(tasks[i].get());//依次等待所有task执行完毕
        }

        System.out.println("-----------\n所有task执行完成!");

    }
}

执行结果:

task0 done
task1 done
task2 done
task3 done
task4 done
task5 done
task6 done
task7 done
task8 done
task9 done
-----------
所有task执行完成!

  

此外,Thread的Join方法也可以实现类似的效果,主要代码如下:

    public static void main(String[] args) throws Exception {

        final int threadNum = 10;
        Thread[] threads = new Thread[threadNum];

        for (int i = 0; i < threadNum; i++) {
            threads[i] = new Thread(new MyRunable(i));
            threads[i].start();
        }

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

        System.out.println("-----------\n所有thread执行完成!");

    }

 

当然,这个需求最“正统”的解法应该是使用CyclicBarrier,它可以设置一个所谓的“屏障点”(或称集合点),好比在一项团队活动中,每个人都是一个线程,但是规定某一项任务开始前,所有人必须先到达集合点,集合完成后,才能继续后面的任务。  

package yjmyzz.test;

import java.util.concurrent.CyclicBarrier;

public class ThreadTest {

    public static void main(String[] args) throws Exception {

        final int threadNum = 10;
        CyclicBarrier cb = new CyclicBarrier(threadNum + 1);//注意:10个子线程 + 1个主线程

        for (int i = 0; i < threadNum; i++) {
            new Thread(new MyRunable(cb, i)).start();
        }

        cb.await();
        System.out.println("-----------\n所有thread执行完成!");
    }

    static class MyRunable implements Runnable {
        CyclicBarrier _cb;
        int _i = 0;

        public MyRunable(CyclicBarrier cb, int i) {
            this._cb = cb;
            this._i = i;
        }

        @Override
        public void run() {
            try {
                Thread.sleep((long) (Math.random() * 100));
                System.out.println("thread " + _i + " done,正在等候其它线程完成...");
                _cb.await();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
}

thread 9 done,正在等候其它线程完成...
thread 5 done,正在等候其它线程完成...
thread 0 done,正在等候其它线程完成...
thread 6 done,正在等候其它线程完成...
thread 4 done,正在等候其它线程完成...
thread 2 done,正在等候其它线程完成...
thread 3 done,正在等候其它线程完成...
thread 8 done,正在等候其它线程完成...
thread 7 done,正在等候其它线程完成...
thread 1 done,正在等候其它线程完成...
-----------
所有thread执行完成!

 

参考文章:

http://ifeve.com/concurrency-cyclicbarrier/

http://ifeve.com/thread-synchronization-utilities-5/

http://ifeve.com/semaphore-countdownlatch-cyclicbarrier-phaser-exchanger-in-java/  

http://ifeve.com/thread-management-7/

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