控制多线程执行顺序

比眉伴天荒 2022-10-18 15:00 223阅读 0赞

目标：创建4个线程分别为thread1，thread2，thread3，thread4让这三个线程依次执行。

# 方法一：join #

public class ThreadSequence { 
        //线程1 ----------------------》
        static Thread thread1 =  new Thread(()->{ 
            System.out.println(Thread.currentThread().getName()+"启动了------》");
        },"线程t1");
        //线程2 ----------------------》
        static Thread thread2 = new Thread(()->{ 
            System.out.println(Thread.currentThread().getName()+"启动了------》");
        },"线程t2");
    
        //线程3 ----------------------》
        static Thread thread3 = new Thread(()->{ 
            System.out.println(Thread.currentThread().getName()+"启动了------》");
        },"线程t3");
    
        //线程4 ----------------------》
        static Thread thread4 = new Thread(()->{ 
            System.out.println(Thread.currentThread().getName()+"启动了------》");
        },"线程t4");
    
    
        public static void main(String[] args) throws Exception{ 
            /*代码解释： * 程序main线程中调用thread1线程的join方法，则main线程会放弃CPU，返回给thread1，直到thread1执行完毕。 * 所以是thread1线程执行完后，才到主线程执行，相当于在main线程中同步thread1线程，thread1执行完了，main线程才有执行的机会 * */
            thread1.start();
            thread1.join();//join代表让主线程即main线程，等待子线程运行结束后才继续执行
            thread2.start();
            thread2.join();
            thread3.start();
            thread3.join();
            thread4.start();
            thread4.join();
        }
    }

未添加join方法时，执行结果是乱序的  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2ppa2UxMTIzMQ_size_16_color_FFFFFF_t_70]  
增加join方法后，执行结果是顺序的  
![在这里插入图片描述][20210708150224418.jpg]  
分析join源码

/** * Waits at most {@code millis} milliseconds for this thread to * die. A timeout of {@code 0} means to wait forever. * * <p> This implementation uses a loop of {@code this.wait} calls * conditioned on {@code this.isAlive}. As a thread terminates the * {@code this.notifyAll} method is invoked. It is recommended that * applications not use {@code wait}, {@code notify}, or * {@code notifyAll} on {@code Thread} instances. * * @param millis * the time to wait in milliseconds * * @throws IllegalArgumentException * if the value of {@code millis} is negative * * @throws InterruptedException * if any thread has interrupted the current thread. The * <i>interrupted status</i> of the current thread is * cleared when this exception is thrown. */
        public final synchronized void join(long millis)
        throws InterruptedException { 
            long base = System.currentTimeMillis();
            long now = 0;
    
            if (millis < 0) { 
                throw new IllegalArgumentException("timeout value is negative");
            }
    
            if (millis == 0) { 
                while (isAlive()) { 
                    wait(0);
                }
            } else { 
                while (isAlive()) { 
                    long delay = millis - now;
                    if (delay <= 0) { 
                        break;
                    }
                    wait(delay);
                    now = System.currentTimeMillis() - base;
                }
            }
        }
      /** * Tests if this thread is alive. A thread is alive if it has * been started and has not yet died. * * @return <code>true</code> if this thread is alive; * <code>false</code> otherwise. */
        public final native boolean isAlive();

源码中的参数millis默认值是0，从英文注释翻译后可以找到，0秒意味着永远等待，也就是thread1执行不完，那主线程你就要一直等着，一直wait，而代码中wait方法其实就是属于Object的方法，负责线程的休眠等待，当main主线程调用thread1.join的时候，main主线程会获得线程对象thread1的锁（wait 意味着拿到该对象的锁)，调用该对象的wait(等待时间)，直到该对象唤醒main主线程 ，比如退出后。这就意味着main 线程调用thread1.join时，必须能够拿到线程thread1对象的锁。

这里有一个\*\*isAlive()\*\*方法很重要。**判断当前线程是否处于活动状态**。活动状态就是线程启动且尚未终止，比如正在运行或准备开始运行。

所以从代码上看，如果线程被生成了，但还未被起动，调用它的 join() 方法是没有作用的，将直接继续向下执行。

在Object.java中，**wait()的作用是让当前线程进入等待状态，同时，wait()也会让当前线程释放它所持有的锁。**

所以Join()主要就是通过wait()方法来实现这个目的的。

# 方法二：ExecutorService()的newSingleThreadExecutor() #

import java.util.concurrent.ExecutorService;
    import java.util.concurrent.Executors;
    
    public class ExecutorServiceSequence { 
        static ExecutorService executorService = Executors.newSingleThreadScheduledExecutor();
        //线程1 ----------------------》
        static Thread thread1 =  new Thread(()->{ 
            System.out.println("启动了线程t1------》");
        });
        //线程2 ----------------------》
        static Thread thread2 = new Thread(()->{ 
            System.out.println("启动了线程t2------》");
        });
    
        //线程3 ----------------------》
        static Thread thread3 = new Thread(()->{ 
            System.out.println("启动了线程t3------》");
        });
    
        //线程4 ----------------------》
        static Thread thread4 = new Thread(()->{ 
            System.out.println("启动了线程t4------》");
        });
    
        public static void main(String[] args) { 
            executorService.submit(thread1);
            executorService.submit(thread2);
            executorService.submit(thread3);
            executorService.submit(thread4);
            executorService.shutdown();
        }
    }

执行结果  
![在这里插入图片描述][20210708152822780.png]

这种方式的原理其实就是将线程用排队的方式扔进一个线程池里，**newSingleThreadScheduledExecutor实例化的对象一次只有一个线程执行**，让所有的任务以单线程的模式，按照FIFO先进先出、LIFO后进先出、优先级等特定顺序执行，但是这种方式也是存在缺点的，就是当一个线程被阻塞时，其它的线程都会受到影响被阻塞，不过依然都会按照自身调度来执行，只是会存在阻塞延迟。

参考文章  
https://www.cnblogs.com/panda001/p/12363532.html  
https://www.jianshu.com/p/481c01528bdb  
https://www.cnblogs.com/xieshengdev/p/10082787.html

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2ppa2UxMTIzMQ_size_16_color_FFFFFF_t_70]: /images/20221005/1ffa010db0a5493e99d97b1adc4e2be4.png
[20210708150224418.jpg]: /images/20221005/1b3599c086fe4dae9b019beff3e911e7.png
[20210708152822780.png]: /images/20221005/11daefda8163433fb0eb8d9f341b101f.png