【SpringBoot】为什么我的定时任务不执行？

在 SpringBoot 中可以通过@Scheduled 注解来定义一个定时任务, 但是有时候你可能会发现有的定时任务到时间了却没有执行，但是又不是每次都不执行，这是怎么回事？



下面这段代码定义了一个每隔十秒钟执行一次的定时任务:



@Component
public class ScheduledTaskDemo {
    private static final Logger logger = LoggerFactory.getLogger(ScheduledTaskDemo.class);
    @Scheduled(cron = "0/10     *")
    public void execute() {
        logger.info("Scheduled task is running... ...");
    }
}



此时启动SpringBoot应用, 可以在控制台看到这个定时任务每隔10秒钟打印一条log



2017-12-21 22:20:20.832 INFO 7424 --- [ main] s.b.c.e.t.TomcatEmbeddedServletContainer : Tomcat started on port(s): 8000 (http)

2017-12-21 22:20:20.859 INFO 7424 --- [ main] com.example.demo.DemoApplication : Started DemoApplication in 12.134 seconds (JVM running for 14.156)

2017-12-21 22:20:30.002 INFO 7424 --- [pool-1-thread-1] c.e.demo.scheduled.ScheduledTaskDemo : Scheduled task is running... ...

2017-12-21 22:20:40.001 INFO 7424 --- [pool-1-thread-1] c.e.demo.scheduled.ScheduledTaskDemo : Scheduled task is running... ...

2017-12-21 22:20:50.002 INFO 7424 --- [pool-1-thread-1] c.e.demo.scheduled.ScheduledTaskDemo : Scheduled task is running... ...



但是, 一切还没结束.



如果没有相关log显示, 检查是否在入口类或者Configuration类上添加了@EnableScheduling注解



在上面的相关代码中, 我们使用cron表达式来指定定时任务的执行时间点, 即从0秒开始, 每隔10秒钟执行一次, 现在我们再加一个定时任务:



@Component
public class SecondScheduledTaskDemo {
    private static final Logger logger = LoggerFactory.getLogger(ScheduledTaskDemo.class);
    @Scheduled(cron = "0/10     *")
    public void second() {
        logger.info("Second scheduled task is starting... ...");
        logger.info("Second scheduled task is ending... ...");
    }
}



现在再启动SpringBoot应用, 再看log:



2017-12-21 23:12:20.007 INFO 13208 --- [pool-1-thread-1] c.e.demo.scheduled.ScheduledTaskDemo : First scheduled task is starting... ...

2017-12-21 23:12:35.013 INFO 13208 --- [pool-1-thread-1] c.e.demo.scheduled.ScheduledTaskDemo : First scheduled task is ending... ...

2017-12-21 23:12:35.016 INFO 13208 --- [pool-1-thread-1] c.e.demo.scheduled.SecondScheduledTaskDemo : Second scheduled task is starting... ...

2017-12-21 23:12:35.016 INFO 13208 --- [pool-1-thread-1] c.e.demo.scheduled.SecondScheduledTaskDemo : Second scheduled task is ending... ...

2017-12-21 23:12:40.000 INFO 13208 --- [pool-1-thread-1] c.e.demo.scheduled.ScheduledTaskDemo : First scheduled task is starting... ...

2017-12-21 23:12:55.001 INFO 13208 --- [pool-1-thread-1] c.e.demo.scheduled.ScheduledTaskDemo : First scheduled task is ending... ...

2017-12-21 23:12:55.002 INFO 13208 --- [pool-1-thread-1] c.e.demo.scheduled.SecondScheduledTaskDemo : Second scheduled task is starting... ...

2017-12-21 23:12:55.002 INFO 13208 --- [pool-1-thread-1] c.e.demo.scheduled.SecondScheduledTaskDemo : Second scheduled task is ending... ...



注意log中定时任务执行的时间点, 第二个定时任务原本应该每隔10秒钟执行一次, 但是从23:12:20到23:13:55, 本该执行4次, 确只执行了2次.



难道是cron表达式不对?



No.



为了找到原因, 我们从@Scheduled注解的源码开始找:



 
  <p>Processing of {@code @Scheduled} annotations is performed by
  registering a {@link ScheduledAnnotationBeanPostProcessor}. This can be
  done manually or, more conveniently, through the {@code <task:annotation-driven/>}
  element or @{@link EnableScheduling} annotation.
 



划重点, 每一个有@Scheduled注解的方法都会被注册为一个ScheduledAnnotationBeanPostProcessor, 再接着往下看ScheduledAnnotationBeanPostProcessor:



	/*
	  Set the {@link org.springframework.scheduling.TaskScheduler} that will invoke
	  the scheduled methods, or a {@link java.util.concurrent.ScheduledExecutorService}
	  to be wrapped as a TaskScheduler.
	  <p>If not specified, default scheduler resolution will apply: searching for a
	  unique {@link TaskScheduler} bean in the context, or for a {@link TaskScheduler}
	  bean named "taskScheduler" otherwise; the same lookup will also be performed for
	  a {@link ScheduledExecutorService} bean. If neither of the two is resolvable,
	  a local single-threaded default scheduler will be created within the registrar.
	  @see #DEFAULTTASKSCHEDULERBEANNAME
	 */
	public void setScheduler(Object scheduler) {
		this.scheduler = scheduler;
	}



重点来了, 注意这句话:



If neither of the two is resolvable, a local single-threaded default scheduler will be created within the registrar.



这句话意味着, 如果我们不主动配置我们需要的TaskScheduler, SpringBoot会默认使用一个单线程的scheduler来处理我们用@Scheduled注解实现的定时任务, 到此我们刚才的问题就可以理解了:



23:12:20, 第一个定时任务在线程pool-1-thread-1开始执行, 由于我们没有配置scheduler, 目前这个线程池pool-1里只有一个线程, 在打印了starting日志之后, 这个线程开始sleep;第二个定时任务也准备执行, 但是线程池已经没有多余线程了, 只能等待.



23:12:30, 第一个定时任务还在sleep, 第二个定时任务还在等待.



23:12:35, 第一个定时任务sleep结束, 打印ending日志并结束, 此时线程池空闲, 第二个定时任务从等待状态直接开始执行, 执行结束之后, 线程池空闲.



23:12:40, 线程池空闲, 第一个定时任务执行, 打印starting日志, 开始sleep.



注意, 这里也有可能执行第二个定时任务, 看CPU了.



... ...



搞清楚这个流程之后, 解决这个问题就很简单了.



根据刚才注释的描述, 我们只需要提供一个满足我们需要的TaskScheduler并注册到context中就可以了.



@Configuration
public class ScheduledTaskConfiguration implements SchedulingConfigurer {
    /*
      Callback allowing a {@link TaskScheduler
      TaskScheduler} and specific {@link Task Task}
      instances to be registered against the given the {@link ScheduledTaskRegistrar}
     
      @param taskRegistrar the registrar to be configured.
     */
    @Override
    public void configureTasks(ScheduledTaskRegistrar taskRegistrar) {
        final ThreadPoolTaskScheduler taskScheduler = new ThreadPoolTaskScheduler();
        taskScheduler.setPoolSize(2);
        taskScheduler.initialize();
        taskRegistrar.setTaskScheduler(taskScheduler);
    }
}



上面的代码提供了一个线程池大小为2的taskScheduler, 现在再启动下SpringBoot看看效果.



2017-12-21 23:28:30.001 INFO 7972 --- [TaskScheduler-1] c.e.demo.scheduled.ScheduledTaskDemo : Second scheduled task is starting... ...

2017-12-21 23:28:30.001 INFO 7972 --- [TaskScheduler-2] c.e.demo.scheduled.ScheduledTaskDemo : First scheduled task is starting... ...

2017-12-21 23:28:30.002 INFO 7972 --- [TaskScheduler-1] c.e.demo.scheduled.ScheduledTaskDemo : Second scheduled task is ending... ...

2017-12-21 23:28:40.001 INFO 7972 --- [TaskScheduler-1] c.e.demo.scheduled.ScheduledTaskDemo : Second scheduled task is starting... ...

2017-12-21 23:28:40.001 INFO 7972 --- [TaskScheduler-1] c.e.demo.scheduled.ScheduledTaskDemo : Second scheduled task is ending... ...

2017-12-21 23:28:45.002 INFO 7972 --- [TaskScheduler-2] c.e.demo.scheduled.ScheduledTaskDemo : First scheduled task is ending... ...



可以看到, 当线程池里有两个线程的时候, 这两个定时任务各自按照预定的时间进行触发, 互不影响了.