排好队,挨着跑

线程的顺序执行，有很多种方式，比如加锁、用join、使用newSingleThreadExecutor等，最近碰到一个场景：主线程A中需要按顺序执行 a1、a2、a3、a4四个函数（即a1执行完才可以开始a2），而 a1、a2、a3、a4每个函数中又开启了多线程处理业务。

因此存在问题：a1执行完的时候如何通知主线程，告诉a2 可以开启了。

既然分析出了问题，那就好办了，解决问题即可，这里使用java关键词volatile

volatile：是一个变量修饰符，被用来修饰会被不同线程访问和修改的变量。

解决思路：

1.定义全局变量并初始化为 flag = 0；

2.a1执行完修改将变量改为1；a2执行完将其改为2；以此类推；

3.主线程判断某一步执行完才开始下一步，否则sleep(300)

4.当前线程是否执行结束，使用原子计数器AtomicInteger

主线程示例

package com.mos.simple;
import com.mos.simple.thread.ThreadFour;import com.mos.simple.thread.ThreadOne;import com.mos.simple.thread.ThreadThree;import com.mos.simple.thread.ThreadTwo;import lombok.extern.slf4j.Slf4j;@Slf4jpublic class TheadDemo {
    public volatile static int FLAG = 0;

    public static void main(String[] args) {
        for (int i = 0; i < 5; i++) {            new TheadDemo().test(i);            System.out.println("--------------------i am dividing line---------------------");        }
    }
    public void test(int num) {        ThreadOne threadOne = new ThreadOne();        ThreadTwo threadTwo = new ThreadTwo();        ThreadThree threadThree = new ThreadThree();        ThreadFour threadFour = new ThreadFour();
        threadOne.test(num);        threadWait(1);        threadTwo.test(num);        threadWait(2);        threadThree.test(num);        threadWait(3);        threadFour.test(num);        threadWait(4);        FLAG = 0;    }
    private void threadWait(int x) {        while (FLAG != x) {            try {                Thread.sleep(300);            } catch (InterruptedException e) {                throw new RuntimeException(e);            }        }    }

}

a1()示例

package com.mos.simple.thread;
import com.mos.simple.TheadDemo;import lombok.extern.slf4j.Slf4j;
import java.util.HashMap;import java.util.Map;import java.util.concurrent.ArrayBlockingQueue;import java.util.concurrent.Executors;import java.util.concurrent.ThreadPoolExecutor;import java.util.concurrent.TimeUnit;import java.util.concurrent.atomic.AtomicInteger;
@Slf4jpublic class ThreadOne {

    public void test(int num) {        System.out.println("ThreadOne num is >> "+num);        //线程池        ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(5, 5, 60L, TimeUnit.MILLISECONDS,                new ArrayBlockingQueue<>(100), Executors.defaultThreadFactory(),                new ThreadPoolExecutor.AbortPolicy());        AtomicInteger count = new AtomicInteger(0);        for (int i = 0; i < 100; i++) {            threadPoolExecutor.execute(() -> {                if (count.incrementAndGet() == 100) {                    TheadDemo.FLAG = 1;                }            });        }
    }}

执行结果

‍ThreadOne num is >> 0ThreadTwo num is >> 0ThreadThree num is >> 0ThreadFour num is >> 0--------------------i am dividing line---------------------ThreadOne num is >> 1ThreadTwo num is >> 1ThreadThree num is >> 1ThreadFour num is >> 1--------------------i am dividing line---------------------ThreadOne num is >> 2ThreadTwo num is >> 2ThreadThree num is >> 2ThreadFour num is >> 2--------------------i am dividing line---------------------ThreadOne num is >> 3ThreadTwo num is >> 3ThreadThree num is >> 3ThreadFour num is >> 3--------------------i am dividing line---------------------ThreadOne num is >> 4ThreadTwo num is >> 4ThreadThree num is >> 4ThreadFour num is >> 4--------------------i am dividing line---------------------