创建三个线程并依次执行

雨点打透心脏的1/2处 2022-02-27 11:34 217阅读 0赞

### 文章目录 ###

*   *  一、join方法
     *  二、线程池

昨天做笔试题的时候遇到一个多线程的问题，需要我们创建三个线程，操作同一个String，并且依次赋值，最后打印。

操作同一个String只需要我们同步或者加锁就可以保证其原子性了，可是如何保证顺序执行呢？

## 一、join方法 ##

我们来看api文档中的说明，等待本线程死亡。  
也就是说，我们在其他线程run方法内部调用本线程的join方法，其他线程必须等待本线程死亡后才能继续执行后续的逻辑代码块。

public final void join()
                    throws InterruptedException
    Waits for this thread to die.
    An invocation of this method behaves in exactly the same way as the invocation
    
    join(0)
    Throws:
    InterruptedException - if any thread has interrupted the current thread. The interrupted status of the current thread is cleared when this exception is thrown.

所以我们可以这么写:

final Thread t1 = new Thread(new Runnable() {
                @Override
                public void run() {
                    try {
                        System.out.print(Thread.currentThread().getName() + " 1 ");
                    }catch (Exception e){
    
                    }
    
                }
            },"_A");
            
    		//lambda
            final Thread t2 = new Thread(() -> {
                try {
                    t1.join();
                    System.out.print(Thread.currentThread().getName() + " 2 ");
                }catch (Exception e){
    
                }
    
            },"_B");
    
    		//lambda
            final Thread t3 = new Thread(() -> {
                try {
                    t2.join();
                    System.out.print(Thread.currentThread().getName() + " 3 ");
                }catch (Exception e){
    
                }
    
            },"_C");
    
            t1.run();
            t2.run();
            t3.run();

![在这里插入图片描述][20190327160343697.png]  
因为是在main方法里执行，所以打印出来的是main.

## 二、线程池 ##

我们可以利用单线程线程池:

ExecutorService es =Executors.newSingleThreadExecutor();
            es.submit(t1);
            es.submit(t2);
            es.submit(t3);
    
            if(!es.isShutdown()){
                es.shutdown();
            }

这段逻辑是如何实现顺序执行的呢？  
我们先来欣赏一下源码:

/**
         * Creates an Executor that uses a single worker thread operating
         * off an unbounded queue. (Note however that if this single
         * thread terminates due to a failure during execution prior to
         * shutdown, a new one will take its place if needed to execute
         * subsequent tasks.)  Tasks are guaranteed to execute
         * sequentially, and no more than one task will be active at any
         * given time. Unlike the otherwise equivalent
         * {@code newFixedThreadPool(1)} the returned executor is
         * guaranteed not to be reconfigurable to use additional threads.
         *
         * @return the newly created single-threaded Executor
         */
        public static ExecutorService newSingleThreadExecutor() {
            return new FinalizableDelegatedExecutorService
                (new ThreadPoolExecutor(1, 1,
                                        0L, TimeUnit.MILLISECONDS,
                                        new LinkedBlockingQueue<Runnable>()));
        }

首先线程池的工作线程是单线程的，线程池维护了一个阻塞队列，队列中线程都出于休眠状态。工作线程会每次只会调出一个线程来执行，当当前线程的run()执行完并顺利结束后，线程池工作线程会继续从队列中取出一个线程并唤醒notify()。当队列为空时，线程池不会结束，而是会会等待队列中出现新的线程。当我们需要结束线程池时需要调用shutdown()方法。

![在这里插入图片描述][20190327221433763.png]

注意，当我们保证顺序执行时，牺牲了并行性。

--------------------

参考资料:

[https://docs.oracle.com/javase/7/docs/api/index.html?index-filesindex-1.html][https_docs.oracle.com_javase_7_docs_api_index.html_index-filesindex-1.html]  
[https://www.cnblogs.com/MOBIN/p/5436482.html][https_www.cnblogs.com_MOBIN_p_5436482.html]  
[https://blog.csdn.net/eene894777/article/details/74942485][https_blog.csdn.net_eene894777_article_details_74942485]

[20190327160343697.png]: /images/20220227/36d69ee1086e4b2994ec2d83410fa9cb.png
[20190327221433763.png]: /images/20220227/1366da6e42104fdd82b820198a351dc5.png
[https_docs.oracle.com_javase_7_docs_api_index.html_index-filesindex-1.html]: https://docs.oracle.com/javase/7/docs/api/index.html?index-filesindex-1.html
[https_www.cnblogs.com_MOBIN_p_5436482.html]: https://www.cnblogs.com/MOBIN/p/5436482.html
[https_blog.csdn.net_eene894777_article_details_74942485]: https://blog.csdn.net/eene894777/article/details/74942485