线程池使用不规范导致线程数大以及@Async的规范使用

文章详细内容来自：线程数突增！领导：谁再这么写就滚蛋！

下面是看完后文章的，一个总结

线程池的使用不规范，导致程序中线程数不下降，线程数量大。

临时变量的接口，通过下面简单的线程池执行，

private static void threadDontGcDemo(){ExecutorService executorService = Executors.newFixedThreadPool(10);executorService.submit(() -> {System.out.println("111");});}

线程不被GC回收，主要是线程池的gc root还是有可达路径的。这里讲个冷知识，这里的线程池的gc root是线程，具体的gc路径是thread->workers->线程池

如果临时采用线程池，需要手动设置线程池的shutdown，下面写法:

private static void threadDontGcDemo(){ExecutorService executorService = Executors.newFixedThreadPool(10);executorService.submit(() -> {System.out.println("111");});executorService.shutdown();}

原理是，shutdown方法是遍历所有线程，并且调用线程的interrupt()方法，通知线程中断，当worker进行getTask()时，让处于wait的线程打断，抛出异常，workers捕获该worker，workers.remove(w)，然后将worker移除workers，这样gc root就不可达了，便会被GC掉。

也就是说线程池的shutdownnow方法调用interruptIdleWorkers去对线程对象interrupt是为了让处于waiting或者是time_waiting的线程抛出异常。

总结为：

线程池调用shutdownnow方法是为了调用worker对象的interrupt方法，来打断那些沉睡中的线程（waiting或者time_waiting状态），使其抛出异常
线程池会把抛出异常的worker对象从workers集合中移除引用，此时被移除的worker对象因为没有到达gc root的路径已经可以被gc掉了
等到workers对象空了，并且当前tomcat线程也结束，此时线程池对象也可以被gc掉，整个线程池对象成功释放

@Async 线程控制

如果不进行线程控制，则异步执行程序越多，会导致线程用尽。

一种方式是全局配置，所有的异步线程共用线程池

@Configuration
public class AsyncConfig implements AsyncConfigurer {@Overridepublic Executor getAsyncExecutor() {ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();executor.setCorePoolSize(10);executor.setMaxPoolSize(20);executor.setQueueCapacity(100);return executor;}@Overridepublic AsyncUncaughtExceptionHandler getAsyncUncaughtExceptionHandler() {return new CustomAsyncExceptionHandler();}@Overridepublic Integer getAsyncExecutor() {return 30; // 单位为秒}}

另外一种是，@Async标签指定使用的线程池名称

@Configuration
@EnableAsync
public class TaskExcutorConfig {@Bean("taskExecutor")public Executor taskExecutro() {ThreadPoolTaskExecutor taskExecutor = new ThreadPoolTaskExecutor();taskExecutor.setCorePoolSize(30);taskExecutor.setMaxPoolSize(50);taskExecutor.setQueueCapacity(200);taskExecutor.setKeepAliveSeconds(60);taskExecutor.setThreadNamePrefix("taskExecutor--");taskExecutor.setWaitForTasksToCompleteOnShutdown(true);taskExecutor.setAwaitTerminationSeconds(60);return taskExecutor;}@Bean("commonExecutor")public Executor commonTaskExecutro() {ThreadPoolTaskExecutor taskExecutor = new ThreadPoolTaskExecutor();taskExecutor.setCorePoolSize(50);taskExecutor.setMaxPoolSize(100);taskExecutor.setQueueCapacity(2000);taskExecutor.setKeepAliveSeconds(60);taskExecutor.setThreadNamePrefix("commonExecutor--");taskExecutor.setWaitForTasksToCompleteOnShutdown(true);taskExecutor.setAwaitTerminationSeconds(60);return taskExecutor;}@Bean("notificationExecutor")public Executor notificationExecutor() {ThreadPoolTaskExecutor taskExecutor = new ThreadPoolTaskExecutor();taskExecutor.setCorePoolSize(30);taskExecutor.setMaxPoolSize(60);taskExecutor.setQueueCapacity(2000);taskExecutor.setKeepAliveSeconds(60);taskExecutor.setThreadNamePrefix("notificationExecutor--");taskExecutor.setWaitForTasksToCompleteOnShutdown(true);taskExecutor.setAwaitTerminationSeconds(60);return taskExecutor;}
}

使用时标明：