线程池ThreadPoolExecutor拒绝策略RejectedExecutionHandler

逃离我推掉我的手 2023-09-29 14:36 122阅读 0赞

### 引子问题 ###

问题1：什么时候触发拒绝策略？

问题2：拒绝策略有哪些？

问题3：默认的拒绝策略是什么？

问题4：你实际开发经历中的拒绝策略是什么？

###  ###

### 创建线程池 ###

核心线程数、最大线程数、空闲存活时间根据业务决定

任务队列**避免使用无界队列**，以免产生OOM

int coreSize = 4;
            int maximumPoolSize = 8;
            long keepAliveTime = 5;
            TimeUnit unit = TimeUnit.SECONDS;
            BlockingQueue<Runnable> workQueue = new ArrayBlockingQueue<>(20);
            RejectedExecutionHandler handler = new ThreadPoolExecutor.AbortPolicy();
    
            ThreadPoolExecutor pool = new ThreadPoolExecutor(coreSize,maximumPoolSize,keepAliveTime,unit,workQueue,handler);

### 拒绝策略有哪些 ###

严格来说，只要实现了RejectedExecutionHandler接口的，都可以作为拒绝策略，这里讨论的是JUC自带的拒绝策略。

上源码……

package java.util.concurrent;
    
    /**
     * A handler for tasks that cannot be executed by a {@link ThreadPoolExecutor}.
     *
     * @since 1.5
     * @author Doug Lea
     */
    public interface RejectedExecutionHandler {
    
        /**
         * Method that may be invoked by a {@link ThreadPoolExecutor} when
         * {@link ThreadPoolExecutor#execute execute} cannot accept a
         * task.  This may occur when no more threads or queue slots are
         * available because their bounds would be exceeded, or upon
         * shutdown of the Executor.
         *
         * <p>In the absence of other alternatives, the method may throw
         * an unchecked {@link RejectedExecutionException}, which will be
         * propagated to the caller of {@code execute}.
         *
         * @param r the runnable task requested to be executed
         * @param executor the executor attempting to execute this task
         * @throws RejectedExecutionException if there is no remedy
         */
        void rejectedExecution(Runnable r, ThreadPoolExecutor executor);
    }

上源码显示，JUC自带的拒绝策略有四个

#### AbortPolicy ####

抛出异常RejectedExecutionException

/**
         * A handler for rejected tasks that throws a
         * {@code RejectedExecutionException}.
         */
        public static class AbortPolicy implements RejectedExecutionHandler {
            /**
             * Creates an {@code AbortPolicy}.
             */
            public AbortPolicy() { }
    
            /**
             * Always throws RejectedExecutionException.
             *
             * @param r the runnable task requested to be executed
             * @param e the executor attempting to execute this task
             * @throws RejectedExecutionException always
             */
            public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
                throw new RejectedExecutionException("Task " + r.toString() +
                                                     " rejected from " +
                                                     e.toString());
            }
        }

AbortPolicy是JUC的默认拒绝策略

/**
         * The default rejected execution handler
         */
        private static final RejectedExecutionHandler defaultHandler =
            new AbortPolicy();

#### CallerRunsPolicy ####

如果线程池还没关闭，就在主线程中执行这个任务，否则任务将被丢弃

/**
         * A handler for rejected tasks that runs the rejected task
         * directly in the calling thread of the {@code execute} method,
         * unless the executor has been shut down, in which case the task
         * is discarded.
         */
        public static class CallerRunsPolicy implements RejectedExecutionHandler {
            /**
             * Creates a {@code CallerRunsPolicy}.
             */
            public CallerRunsPolicy() { }
    
            /**
             * Executes task r in the caller's thread, unless the executor
             * has been shut down, in which case the task is discarded.
             *
             * @param r the runnable task requested to be executed
             * @param e the executor attempting to execute this task
             */
            public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
                if (!e.isShutdown()) {
                    r.run();
                }
            }
        }

#### DiscardOldestPolicy ####

喜新厌旧，将任务队列中最久未处理(oldest unhandled)的任务丢弃，然后尝试执行当前任务，如果线程池关闭了，这个任务也将被丢弃

/**
         * A handler for rejected tasks that discards the oldest unhandled
         * request and then retries {@code execute}, unless the executor
         * is shut down, in which case the task is discarded.
         */
        public static class DiscardOldestPolicy implements RejectedExecutionHandler {
            /**
             * Creates a {@code DiscardOldestPolicy} for the given executor.
             */
            public DiscardOldestPolicy() { }
    
            /**
             * Obtains and ignores the next task that the executor
             * would otherwise execute, if one is immediately available,
             * and then retries execution of task r, unless the executor
             * is shut down, in which case task r is instead discarded.
             *
             * @param r the runnable task requested to be executed
             * @param e the executor attempting to execute this task
             */
            public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
                if (!e.isShutdown()) {
                    e.getQueue().poll();
                    e.execute(r);
                }
            }
        }

#### DiscardPilicy ####

佛系拒绝，什么也不做

/**
         * A handler for rejected tasks that silently discards the
         * rejected task.
         */
        public static class DiscardPolicy implements RejectedExecutionHandler {
            /**
             * Creates a {@code DiscardPolicy}.
             */
            public DiscardPolicy() { }
    
            /**
             * Does nothing, which has the effect of discarding task r.
             *
             * @param r the runnable task requested to be executed
             * @param e the executor attempting to execute this task
             */
            public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
            }
        }

### 揭晓答案 ###

#### 什么时候触发拒绝策略？ ####

一图胜千言

![cd7ccbd13ba84ae6a7f5756ce354528e.png][]

#### 拒绝策略有哪些？ ####

![f73e9d072efc46828d6fc7662b95f26c.png][]

#### 默认的拒绝策略是什么？ ####

![cbd7be22e2594222a19f53e3847ef819.png][]

#### 你实际开发经历中的拒绝策略是什么？ ####

分场景：看实际场景中任务重要性，如果是不重要的任务，默认的拒绝策略就好了，佛系，无为而治，什么也不做

如果任务很重要，不能丢，看了上面的四种拒绝策略，业务不允许丢弃任务的场景里，都不靠谱，就需要重写RejectedExecutionHandler接口自定义拒绝策略

**比如当发生拒绝时将任务转发到MQ中，稍后再处理**

**另外多提一句，这里就是23中设计模式中——策略模式，使用的是一个典型场景，面试八股文奇怪的知识又增加了！**

[cd7ccbd13ba84ae6a7f5756ce354528e.png]: https://img-blog.csdnimg.cn/cd7ccbd13ba84ae6a7f5756ce354528e.png
[f73e9d072efc46828d6fc7662b95f26c.png]: https://img-blog.csdnimg.cn/f73e9d072efc46828d6fc7662b95f26c.png
[cbd7be22e2594222a19f53e3847ef819.png]: https://img-blog.csdnimg.cn/cbd7be22e2594222a19f53e3847ef819.png