《阿里巴巴Android开发手册》Executors进程池创建迷之问题探讨

àì夳堔傛蜴生んèń 2024-02-19 20:00 8阅读 0赞

## 《阿里巴巴Android开发手册》Executors进程池创建迷之问题探讨 ##

《阿里巴巴Android开发手册(2018.3.5 ，version 1.0.1)》（以下简称《手册》）第五章《进程、线程与消息通信》第4小节（第33页开始），写道：

>   
> *【强制】线程池不允许使用 Executors 去创建，而是通过 ThreadPoolExecutor 的方式，这样的处理方式让写的同学更加明确线程池的运行规则，规避资源耗尽的风险。*  
>

然后手册中举出例子加以说明：

>   
> *Executors 返回的线程池对象的弊端如下：  
> 1) FixedThreadPool 和 SingleThreadPool ： 允许的请求队列长度为Integer.MAX\_VALUE，可能会堆积大量的请求，从而导致 OOM；*  
>

**以下是对上述问题的探讨：**

**一**，《手册》原文的“SingleThreadPool”意为Java Executors的newSingleThreadExecutor静态方法。作者可能是为了简写或者记忆模糊写的。

Java源代码中的Executors静态方法newSingleThreadExecutor实现为：

/**
         * 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>()));
        }

《手册》不知何故，似乎很钟情于用ThreadPoolExecutor来创建线程池，然而《手册》所反对的newSingleThreadExecutor，恰恰就是通过ThreadPoolExecutor实现的，Java原生Executors的源代码写的很清楚，并且，newSingleThreadExecutor创建的线程队列长度根本不是《手册》危言耸听的Integer.MAX\_VALUE，而是1，队列长度为1的线程池如何OOM？如果仅有1条线程的线程池都OOM，那就不要怪Java原生的newSingleThreadExecutor，多从自身写的代码找找原因吧！

**二**，《手册》中提到的FixedThreadPool （newFixedThreadPool）情况和newSingleThreadExecutor类似。《手册》反对使用newSingleThreadExecutor，理由也是“允许的请求队列长度为Integer.MAX\_VALUE，可能会堆积大量的请求，从而导致 OOM”。  
我们还是先看看Java原生代码是怎么实现newFixedThreadPool吧：

/**
         * Creates a thread pool that reuses a fixed number of threads
         * operating off a shared unbounded queue.  At any point, at most
         * {@code nThreads} threads will be active processing tasks.
         * If additional tasks are submitted when all threads are active,
         * they will wait in the queue until a thread is available.
         * If any thread terminates due to a failure during execution
         * prior to shutdown, a new one will take its place if needed to
         * execute subsequent tasks.  The threads in the pool will exist
         * until it is explicitly {@link ExecutorService#shutdown shutdown}.
         *
         * @param nThreads the number of threads in the pool
         * @return the newly created thread pool
         * @throws IllegalArgumentException if {@code nThreads <= 0}
         */
        public static ExecutorService newFixedThreadPool(int nThreads) {
            return new ThreadPoolExecutor(nThreads, nThreads,
                                          0L, TimeUnit.MILLISECONDS,
                                          new LinkedBlockingQueue<Runnable>());
        }

（1）《手册》提倡ThreadPoolExecutor反对newFixedThreadPool，但是代码面前一览无余，Java原生代码newFixedThreadPool正是用ThreadPoolExecutor实现的，不知道《手册》在线程池创建问题上到底反对什么提倡什么。  
（2）Java原生newFixedThreadPool有两个不同参数类型的构造函数，都必须传入一个整型值指定请求队列的长度，Java原生的newFixedThreadPool根本不是《手册》说的那样会创建Integer.MAX\_VALUE长度的请求队列。除非你自己故意给newFixedThreadPool传入Integer.MAX\_VALUE！

附：

《阿里巴巴Android开发手册(2018.3.5 ，version 1.0.1)》阿里巴巴官方链接地址：[https://edu.aliyun.com/article/76][https_edu.aliyun.com_article_76]

[https_edu.aliyun.com_article_76]: https://edu.aliyun.com/article/76