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 {

  @Override
  public Executor getAsyncExecutor() {
    ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
    executor.setCorePoolSize(10);
    executor.setMaxPoolSize(20);
    executor.setQueueCapacity(100);
    return executor;
  }

     @Override
  public AsyncUncaughtExceptionHandler getAsyncUncaughtExceptionHandler() {
    return new CustomAsyncExceptionHandler();
  }

    @Override
    public 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：

@Async("commonExecutor")