1 场景复现

服务调用时序如图1所示。中间件服务使用线程池ThreadPoolExecutor，配置丢弃策略为DiscardOldestPolicy（丢弃队列中等待最久的任务），队列容量为10。
在这里插入图片描述

图1 服务调用

public static ExecutorService threadPoolExecutorGenerate = new ThreadPoolExecutor(
            ThreadPoolConstant.CORE_THREAD_NUM,
            ThreadPoolConstant.MAX_THREAD_NUM,
            ThreadPoolConstant.KEEP_ALIVE_TIME_SECONDS,
            TimeUnit.SECONDS,
            new LinkedBlockingQueue<Runnable>(ThreadPoolConstant.QUEUE_LENGTH),
            Executors.defaultThreadFactory(),
            new ThreadPoolExecutor.AbortOldestPolicy());

2 原因

线程池配置丢弃策略为DiscardOldestPolicy（丢弃队列中等待最久的任务），队列容量为10，当队列达到上限时，丢弃任务，此时，后台服务无法获取到中间件丢弃任务的返回值，导致后台服务异常。

3 方案

中间件服务具有特殊性，必须要有返回值以确定当前任务是否正常，因此，使用线程池时，应该使用拒绝策略AbortPolicy，当拒绝任务后，抛出异常，在程序中捕获异常，并返回默认值或约定的值，保证中间件服务不会卡住，后台服务可以及时拿到数据，保证后台服务不会卡住。

3.1 AbortPolicy拒绝策略实现

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());
        }
    }

3.2 完整配置

 public static ExecutorService threadPoolExecutorGenerate = new ThreadPoolExecutor(
            ThreadPoolConstant.CORE_THREAD_NUM,
            ThreadPoolConstant.MAX_THREAD_NUM,
            ThreadPoolConstant.KEEP_ALIVE_TIME_SECONDS,
            TimeUnit.SECONDS,
            new LinkedBlockingQueue<Runnable>(ThreadPoolConstant.QUEUE_LENGTH),
            Executors.defaultThreadFactory(),
            new ThreadPoolExecutor.AbortPolicy());