1. @Scheduled 可以将一个方法标识为可定时执行的。但必须指明cron()，fixedDelay(),或者fixedRate()属性。

注解的方法必须是无输入参数并返回空类型void的。

@Scheduled注解由注册的ScheduledAnnotationBeanPostProcessor来处理，该processor可以通过手动来注册，更方面的方式是通过<task:annotation-driven/>或者@EnableScheduling来注册。@EnableScheduling可以注册的原理是什么呢？先看定义：

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Import(SchedulingConfiguration.class)
@Documentedpublic @interfaceEnableScheduling {}

可以看到@EnableScheduling的实现由SchedulingConfiguration来完成。

@Configurationpublic classSchedulingConfiguration {@Bean(name=AnnotationConfigUtils.SCHEDULED_ANNOTATION_PROCESSOR_BEAN_NAME)@Role(BeanDefinition.ROLE_INFRASTRUCTURE)publicScheduledAnnotationBeanPostProcessor scheduledAnnotationProcessor() {return newScheduledAnnotationBeanPostProcessor();}}

从上述代码可以看出，SchedulingConfiguration注册了一个ScheduledAnnotationBeanPostProcessor。

来看一下ScheduledAnnotationBeanPostProcessor来如何处理定时任务的？

protected voidprocessScheduled(Scheduled scheduled, Method method, Object bean) {try{Assert.isTrue(void.class.equals(method.getReturnType()),"Only void-returning methods may be annotated with @Scheduled");Assert.isTrue(method.getParameterTypes().length== 0,"Only no-arg methods may be annotated with @Scheduled");if(AopUtils.isJdkDynamicProxy(bean)) {try{//Found a @Scheduled method on the target class for this JDK proxy ->//is it also present on the proxy itself?method =bean.getClass().getMethod(method.getName(), method.getParameterTypes());}catch(SecurityException ex) {ReflectionUtils.handleReflectionException(ex);}catch(NoSuchMethodException ex) {throw newIllegalStateException(String.format("@Scheduled method '%s' found on bean target class '%s' but not " +"found in any interface(s) for a dynamic proxy. Either pull the " +"method up to a declared interface or switch to subclass (CGLIB) " +"proxies by setting proxy-target-class/proxyTargetClass to 'true'",method.getName(), method.getDeclaringClass().getSimpleName()));}}else if(AopUtils.isCglibProxy(bean)) {//Common problem: private methods end up in the proxy instance, not getting delegated.if(Modifier.isPrivate(method.getModifiers())) {LogFactory.getLog(ScheduledAnnotationBeanPostProcessor.class).warn(String.format("@Scheduled method '%s' found on CGLIB proxy for target class '%s' but cannot " +"be delegated to target bean. Switch its visibility to package or protected.",method.getName(), method.getDeclaringClass().getSimpleName()));}}Runnable runnable= newScheduledMethodRunnable(bean, method);boolean processedSchedule = false;String errorMessage="Exactly one of the 'cron', 'fixedDelay(String)', or 'fixedRate(String)' attributes is required";//Determine initial delaylong initialDelay =scheduled.initialDelay();String initialDelayString=scheduled.initialDelayString();if(StringUtils.hasText(initialDelayString)) {Assert.isTrue(initialDelay< 0, "Specify 'initialDelay' or 'initialDelayString', not both");if (this.embeddedValueResolver != null) {initialDelayString= this.embeddedValueResolver.resolveStringValue(initialDelayString);}try{initialDelay=Integer.parseInt(initialDelayString);}catch(NumberFormatException ex) {throw newIllegalArgumentException("Invalid initialDelayString value \"" + initialDelayString + "\" - cannot parse into integer");}}//Check cron expressionString cron =scheduled.cron();if(StringUtils.hasText(cron)) {Assert.isTrue(initialDelay== -1, "'initialDelay' not supported for cron triggers");processedSchedule= true;String zone=scheduled.zone();if (this.embeddedValueResolver != null) {cron= this.embeddedValueResolver.resolveStringValue(cron);zone= this.embeddedValueResolver.resolveStringValue(zone);}TimeZone timeZone;if(StringUtils.hasText(zone)) {timeZone=StringUtils.parseTimeZoneString(zone);}else{timeZone=TimeZone.getDefault();}this.registrar.addCronTask(new CronTask(runnable, newCronTrigger(cron, timeZone)));}//At this point we don't need to differentiate between initial delay set or not anymoreif (initialDelay < 0) {initialDelay= 0;}//Check fixed delaylong fixedDelay =scheduled.fixedDelay();if (fixedDelay >= 0) {Assert.isTrue(!processedSchedule, errorMessage);processedSchedule= true;this.registrar.addFixedDelayTask(newIntervalTask(runnable, fixedDelay, initialDelay));}String fixedDelayString=scheduled.fixedDelayString();if(StringUtils.hasText(fixedDelayString)) {Assert.isTrue(!processedSchedule, errorMessage);processedSchedule= true;if (this.embeddedValueResolver != null) {fixedDelayString= this.embeddedValueResolver.resolveStringValue(fixedDelayString);}try{fixedDelay=Integer.parseInt(fixedDelayString);}catch(NumberFormatException ex) {throw newIllegalArgumentException("Invalid fixedDelayString value \"" + fixedDelayString + "\" - cannot parse into integer");}this.registrar.addFixedDelayTask(newIntervalTask(runnable, fixedDelay, initialDelay));}//Check fixed ratelong fixedRate =scheduled.fixedRate();if (fixedRate >= 0) {Assert.isTrue(!processedSchedule, errorMessage);processedSchedule= true;this.registrar.addFixedRateTask(newIntervalTask(runnable, fixedRate, initialDelay));}String fixedRateString=scheduled.fixedRateString();if(StringUtils.hasText(fixedRateString)) {Assert.isTrue(!processedSchedule, errorMessage);processedSchedule= true;if (this.embeddedValueResolver != null) {fixedRateString= this.embeddedValueResolver.resolveStringValue(fixedRateString);}try{fixedRate=Integer.parseInt(fixedRateString);}catch(NumberFormatException ex) {throw newIllegalArgumentException("Invalid fixedRateString value \"" + fixedRateString + "\" - cannot parse into integer");}this.registrar.addFixedRateTask(newIntervalTask(runnable, fixedRate, initialDelay));}//Check whether we had any attribute set
Assert.isTrue(processedSchedule, errorMessage);}catch(IllegalArgumentException ex) {throw newIllegalStateException("Encountered invalid @Scheduled method '" + method.getName() + "': " +ex.getMessage());}}

从上面的代码可以看出：@Scheduled有三个属性，分别是：

cron expression

fixedDelay

fixedRate 

根据这些属性的不同，都加入到ScheduledTaskRegistrar来管理定时任务：

/*** Schedule all registered tasks against the underlying {@linkplain* #setTaskScheduler(TaskScheduler) task scheduler}.*/protected voidscheduleTasks() {long now =System.currentTimeMillis();if (this.taskScheduler == null) {this.localExecutor =Executors.newSingleThreadScheduledExecutor();this.taskScheduler = new ConcurrentTaskScheduler(this.localExecutor);}if (this.triggerTasks != null) {for (TriggerTask task : this.triggerTasks) {this.scheduledFutures.add(this.taskScheduler.schedule(task.getRunnable(), task.getTrigger()));}}if (this.cronTasks != null) {for (CronTask task : this.cronTasks) {this.scheduledFutures.add(this.taskScheduler.schedule(task.getRunnable(), task.getTrigger()));}}if (this.fixedRateTasks != null) {for (IntervalTask task : this.fixedRateTasks) {if (task.getInitialDelay() > 0) {Date startTime= new Date(now +task.getInitialDelay());this.scheduledFutures.add(this.taskScheduler.scheduleAtFixedRate(task.getRunnable(), startTime, task.getInterval()));}else{this.scheduledFutures.add(this.taskScheduler.scheduleAtFixedRate(task.getRunnable(), task.getInterval()));}}}if (this.fixedDelayTasks != null) {for (IntervalTask task : this.fixedDelayTasks) {if (task.getInitialDelay() > 0) {Date startTime= new Date(now +task.getInitialDelay());this.scheduledFutures.add(this.taskScheduler.scheduleWithFixedDelay(task.getRunnable(), startTime, task.getInterval()));}else{this.scheduledFutures.add(this.taskScheduler.scheduleWithFixedDelay(task.getRunnable(), task.getInterval()));}}}}

从上面看出：

3种不同属性的task均由quartz的taskScheduler的不同方法来完成，

scheduleWithFixedDelay，

scheduleAtFixedRate，

schedule

即最终的实现由TaskScheduler来完成定时任务。