spring定时任务原理

参考文章： https://juejin.im/post/5b64448af265da0f7f44c201

https://juejin.im/post/5e338ebae51d4558864b1ca0

1、开发中使用时要注意的点

　　（0）spring定时任务执行原理实际使用的是 JDK 自带的 ScheduledExecutorService

　　（1）spring默认使用单线程的线程池去执行定时任务，所以如果某个任务执行时间过长，会导致其他定时任务阻塞无法执行。

　　（2）可以开启并行调度，springboot中的使用方式：这种模式每次任务执行都会创建一个线程去执行。

@EnableAsync
@EnableScheduling
@SpringBootApplication
public class QuickMediaApplication {

    public static void main(String[] args) {
        SpringApplication.run(QuickMediaApplication.class, args);
    }

    @Scheduled(cron = "0/1 * * * * ?")
    @Async
    public void sc1()  {
        System.out.println(Thread.currentThread().getName() + " | sc1 " + System.currentTimeMillis());
    }
}

　　风险：如果某个定时任务出现死循环或者执行时间过长而出发时间较短，会导致线程数量不可控。

　　（3）最稳妥的处理方式：自定义任务执行的线程池，如下：

　　普通spring配置：

@Bean
public AsyncTaskExecutor asyncTaskExecutor() {
    ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
    executor.setThreadNamePrefix("task-schedule-");
    executor.setMaxPoolSize(10);
    executor.setCorePoolSize(3);
    executor.setQueueCapacity(0);
    executor.setRejectedExecutionHandler(new ThreadPoolExecutor.AbortPolicy());
    return executor;
}

或

@Bean
public ScheduleExecutorService scheduleExecutorService{
    return Executors.new ScheduleThreadPool(10);    
}

　　springboot中配置：

spring.task.scheduling.pool.size=10
spring.task.scheduling.thread-name-prefix=task-schedule-

2、源码分析

　　通过监听IOC容器初始化事件，扫描所有 Bean 中带有 @Scheduled 注解的方法，然后封装成 Task 子类放置到 ScheduledTaskRegistrar中。如果自己定义了ScheduledExecutorService，会使用自己定义的线程池，否则ScheduledTaskRegistrar#afterPropertiesSet 创建一个单线程的定时任务执行器 ScheduledExecutorService，注入到 ConcurrentTaskScheduler中，然后通过 taskScheduler 执行定时任务。

public class ScheduledAnnotationBeanPostProcessor
        implements ScheduledTaskHolder, MergedBeanDefinitionPostProcessor, DestructionAwareBeanPostProcessor,
        Ordered, EmbeddedValueResolverAware, BeanNameAware, BeanFactoryAware, ApplicationContextAware,
        SmartInitializingSingleton, ApplicationListener<ContextRefreshedEvent>, DisposableBean {

　　................

    @Override
    public void onApplicationEvent(ContextRefreshedEvent event) {
        if (event.getApplicationContext() == this.applicationContext) {
            // Running in an ApplicationContext -> register tasks this late...
            // giving other ContextRefreshedEvent listeners a chance to perform
            // their work at the same time (e.g. Spring Batch's job registration).
            finishRegistration();
        }
    }

    private void finishRegistration() {
        if (this.scheduler != null) {
            this.registrar.setScheduler(this.scheduler);
        }

        .......................if (this.registrar.hasTasks() && this.registrar.getScheduler() == null) {
            Assert.state(this.beanFactory != null, "BeanFactory must be set to find scheduler by type");
            try {
                // Search for TaskScheduler bean...
                this.registrar.setTaskScheduler(resolveSchedulerBean(beanFactory, TaskScheduler.class, false));
            }
            catch (NoUniqueBeanDefinitionException ex) {
                logger.debug("Could not find unique TaskScheduler bean", ex);
                try {
                    this.registrar.setTaskScheduler(resolveSchedulerBean(beanFactory, TaskScheduler.class, true));
                }
                catch (NoSuchBeanDefinitionException ex2) {
                    if (logger.isInfoEnabled()) {
                        logger.info("More than one TaskScheduler bean exists within the context, and " +
                                "none is named 'taskScheduler'. Mark one of them as primary or name it 'taskScheduler' " +
                                "(possibly as an alias); or implement the SchedulingConfigurer interface and call " +
                                "ScheduledTaskRegistrar#setScheduler explicitly within the configureTasks() callback: " +
                                ex.getBeanNamesFound());
                    }
                }
            }
            catch (NoSuchBeanDefinitionException ex) {
                logger.debug("Could not find default TaskScheduler bean", ex);
                // Search for ScheduledExecutorService bean next...
                try {
                    this.registrar.setScheduler(resolveSchedulerBean(beanFactory, ScheduledExecutorService.class, false));
                }
                catch (NoUniqueBeanDefinitionException ex2) {
                    logger.debug("Could not find unique ScheduledExecutorService bean", ex2);
                    try {
                        this.registrar.setScheduler(resolveSchedulerBean(beanFactory, ScheduledExecutorService.class, true));
                    }
                    catch (NoSuchBeanDefinitionException ex3) {
                        if (logger.isInfoEnabled()) {
                            logger.info("More than one ScheduledExecutorService bean exists within the context, and " +
                                    "none is named 'taskScheduler'. Mark one of them as primary or name it 'taskScheduler' " +
                                    "(possibly as an alias); or implement the SchedulingConfigurer interface and call " +
                                    "ScheduledTaskRegistrar#setScheduler explicitly within the configureTasks() callback: " +
                                    ex2.getBeanNamesFound());
                        }
                    }
                }
                catch (NoSuchBeanDefinitionException ex2) {
                    logger.debug("Could not find default ScheduledExecutorService bean", ex2);
                    // Giving up -> falling back to default scheduler within the registrar...
                    logger.info("No TaskScheduler/ScheduledExecutorService bean found for scheduled processing");
                }
            }
        }

        this.registrar.afterPropertiesSet();
    }
}

if (this.taskScheduler == null) {
      this.localExecutor = Executors.newSingleThreadScheduledExecutor();
      this.taskScheduler = new ConcurrentTaskScheduler(this.localExecutor);
}

Spring 定时任务执行原理实际使用的是 JDK 自带的 ScheduledExecutorService