ExecutorService问题

Question

我正在使用ThreadPoolExecutor服务来处理我代码中的项目列表。我会从DataBase上拿到这些商品清单。这里的问题是Tomcat线程nio-8080-exec-5连续查询DB，即使executor服务线程仍在处理先前获取的项。我只需要在Executor Service没有要处理的项时查询DB &当工作队列为空时。下面是我的示例代码：

private void start(){
            ThreadPoolExecutor executorPool = new ThreadPoolExecutor(2, 4, 240L, TimeUnit.SECONDS, new LinkedBlockingQueue<>());
            boolean itemsPresent =true;
            while(itemsPresent){
                log.info("Fetch Items from DB");
                List<Item> itemList = fetchItemsDB();
                log.info("Total Items fetched from DB: {}", itemList.size());
                    if(!itemList.isEmpty()){
                        itemList.forEach(itemInfo -> executorPool.execute(() -> processItems(itemInfo)));
                    }
                    else{
                    itemsPresent =false;
                    }
                }
            }

提前感谢您的帮助:)

请查看以下日志详细信息：

2018-12-25 19:03:54.189 INFO 4828 --- [     main] com.sample.MyApplication           : Running with Spring Boot v1.5.2.RELEASE, Spring v4.3.7.RELEASE
2018-12-25 19:03:54.189 INFO 4828 --- [     main] com.sample.MyApplication           : No active profile set, falling back to default profiles: default
2018-12-25 19:03:58.846 INFO 4828 --- [     main] com.sample.MyApplication           : Started MyApplication in 5.209 seconds (JVM running for 5.66)
2018-12-25 19:03:58.846 INFO 4828 --- [nio-8080-exec-5] o.a.c.c.C.[Tomcat].[localhost].[/]: Initializing Spring FrameworkServlet 'dispatcherServlet'
2018-12-25 19:03:58.846 INFO 4828 --- [nio-8080-exec-5] com.sample.MyApplication: Inside Start method !!!!
2018-12-25 19:03:58.846 INFO 4828 --- [nio-8080-exec-5] com.sample.MyApplication: Fetch Items from DB
2018-12-25 19:03:58.846 INFO 4828 --- [nio-8080-exec-5] com.sample.MyApplication: Total Items fetched from DB: 20
2018-12-25 19:03:58.846 INFO 4828 --- [pool-1-thread-5] com.sample.MyApplication: started to process: 1
2018-12-25 19:03:58.846 INFO 4828 --- [pool-1-thread-] com.sample.MyApplication: started to process: 2
2018-12-25 19:03:58.846 INFO 4828 --- [pool-1-thread-5] com.sample.MyApplication: started to process: 4
2018-12-25 19:03:58.846 INFO 4828 --- [pool-1-thread-1] com.sample.MyApplication: started to process: 6
2018-12-25 19:03:58.846 INFO 4828 --- [pool-1-thread-9] com.sample.MyApplication: started to process: 11
2018-12-25 19:03:58.846 INFO 4828 --- [pool-1-thread-7] com.sample.MyApplication: started to process: 12
2018-12-25 19:03:58.846 INFO 4828 --- [pool-1-thread-8] com.sample.MyApplication: started to process: 5
2018-12-25 19:03:58.846 INFO 4828 --- [pool-1-thread-3] com.sample.MyApplication: started to process: 9
2018-12-25 19:03:58.846 INFO 4828 --- [nio-8080-exec-5] com.sample.MyApplication: Fetch Items from DB
2018-12-25 19:03:58.846 INFO 4828 --- [nio-8080-exec-5] com.sample.MyApplication: Total Items fetched from DB: 20
2018-12-25 19:03:58.846 INFO 4828 --- [nio-8080-exec-5] com.sample.MyApplication: Fetch Items from DB
2018-12-25 19:03:58.846 INFO 4828 --- [nio-8080-exec-5] com.sample.MyApplication: Total Items fetched from DB: 20
2018-12-25 19:03:58.846 INFO 4828 --- [nio-8080-exec-5] com.sample.MyApplication: Fetch Items from DB
2018-12-25 19:03:58.846 INFO 4828 --- [nio-8080-exec-5] com.sample.MyApplication: Total Items fetched from DB: 20
2018-12-25 19:03:58.846 INFO 4828 --- [nio-8080-exec-5] com.sample.MyApplication: Fetch Items from DB
2018-12-25 19:03:58.846 INFO 4828 --- [nio-8080-exec-5] com.sample.MyApplication: Total Items fetched from DB: 15
2018-12-25 19:03:58.846 INFO 4828 --- [pool-1-thread-1] com.sample.MyApplication: started to process: 3
2018-12-25 19:03:58.846 INFO 4828 --- [pool-1-thread-1] com.sample.MyApplication: started to process: 7
2018-12-25 19:03:58.846 INFO 4828 --- [pool-1-thread-9] com.sample.MyApplication: started to process: 8
2018-12-25 19:03:58.846 INFO 4828 --- [pool-1-thread-7] com.sample.MyApplication: started to process: 10
2018-12-25 19:03:58.846 INFO 4828 --- [pool-1-thread-8] com.sample.MyApplication: started to process: 13
2018-12-25 19:03:58.846 INFO 4828 --- [pool-1-thread-3] com.sample.MyApplication: started to process: 14

Answer 1

如果项目处理任务和从数据库获取任务是互斥的，为什么不在单个线程中顺序编写它们呢？

Answer 2

据我所知，您需要在executor服务中完成所有可运行项目时触发一个事件(调用一个方法)。

没有一种干净的方法可以做到这一点。您可以使用ExecutorService.submit(Runnable)。此方法将返回一个Future<?>，它是runnable的结果的句柄。

也就是说，您可以为每个将来完成的操作增加一个AtomicInteger计数器。当此计数器值等于列表大小时，您可以轮询数据库以获取更多记录。