我们跑定时的时候，代码的结构大致是这个样子的。

@Scheduled(cron="0 0 4 * * ?")
public void taskCycle() {Calendar  cal  =  Calendar.getInstance();cal.add(Calendar.DATE, -1);yesterday = new SimpleDateFormat( "yyyyMMdd").format(cal.getTime());//默认5000int height = Integer.valueOf("5000");//主页heatMapDto = createZy("zhuye",height);createHotMap(heatMapDto);//我的账户heatMapDto = createZy("myacount",height);createHotMap(heatMapDto);LOGGER.info("今执行了热力图定时任务" + yesterday);
}

那么，这个Scheduled(cron="0 0 4 * * ?")是如何被系统识别并且去跑定时任务呢?@Scheduled的解析器是ScheduledAnnotationBeanPostProcessor 该类实现了类BeanPostProcessor，这样在容器启动的时候会在refresh()的时候调用到所有实现了BeanPostProcessor接口的类。如下：

AbstractBeanFactory (方法：createBean(beanName, mbd, args))--->AbstractAutowireCapableBeanFactory
(方法：createBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)) ---》AbstractAutowireCapableBeanFactory  （方法：protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd)）--》

protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {Object bean = null;if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {// Make sure bean class is actually resolved at this point.if (mbd.hasBeanClass() && !mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {//执行BeanPostProcessor的applyBeanPostProcessorsBeforeInstantiation接口bean = applyBeanPostProcessorsBeforeInstantiation(mbd.getBeanClass(), beanName);if (bean != null) {//执行BeanPostProcessor的applyBeanPostProcessorsBeforeInstantiation接口bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);}}mbd.beforeInstantiationResolved = (bean != null);}return bean;
}

---》AbstractAutowireCapableBeanFactory
public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)throws BeansException {Object result = existingBean;for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {//bean会走下每一个配置到系统中实现了接口BeanPostProcessor的类的postProcessAfterIn//itialization方法result = beanProcessor.postProcessAfterInitialization(result, beanName);if (result == null) {return result;}}return result;
}

到此我们知道了接口实现类的实现时机。那么@Scheduled的解析器是ScheduledAnnotationBeanPostProcessor在执行的时候又做了什么呢？让我们来看看ScheduledAnnotationBeanPostProcessor的postProcessAfterInitialization方法是实现了什么。

public Object postProcessAfterInitialization(final Object bean, String beanName) {
//获取目标类final Class<?> targetClass = AopUtils.getTargetClass(bean);//利用类反射为目标类方法做回调函数ReflectionUtils.doWithMethods(targetClass, new MethodCallback() {public void doWith(Method method) throws IllegalArgumentException, IllegalAccessException {//首先判断是否有@Scheduled标签Scheduled annotation = AnnotationUtils.getAnnotation(method, Scheduled.class);//如果有标签if (annotation != null) {try {//判断返回类型是否为空  （Scheduled表示不允许有返回值）Assert.isTrue(void.class.equals(method.getReturnType()),"Only void-returning methods may be annotated with @Scheduled");//Scheduled方法不允许有参数Assert.isTrue(method.getParameterTypes().length == 0,"Only no-arg methods may be annotated with @Scheduled");//判断目标类是否是jdk代理类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 new IllegalStateException(String.format("@Scheduled method '%s' found on bean target class '%s', " +"but not found in any interface(s) for bean JDK proxy. Either " +"pull the method up to an interface or switch to subclass (CGLIB) " +"proxies by setting proxy-target-class/proxyTargetClass " +"attribute to 'true'", method.getName(), targetClass.getSimpleName()));}}//生成一个定时器任务Runnable runnable = new ScheduledMethodRunnable(bean, method);boolean processedSchedule = false;String errorMessage = "Exactly one of the 'cron', 'fixedDelay(String)', or 'fixedRate(String)' attributes is required";// Determine initial delay//首先实例化一个定时器延时时间 initialDelay =-1long initialDelay = annotation.initialDelay();//initialDelayString =""String initialDelayString = annotation.initialDelayString();if (!"".equals(initialDelayString)) {Assert.isTrue(initialDelay < 0, "Specify 'initialDelay' or 'initialDelayString', not both");if (embeddedValueResolver != null) {initialDelayString = embeddedValueResolver.resolveStringValue(initialDelayString);}try {initialDelay = Integer.parseInt(initialDelayString);}catch (NumberFormatException ex) {throw new IllegalArgumentException("Invalid initialDelayString value \"" + initialDelayString + "\" - cannot parse into integer");}}//获取cron表达式0 * * * * *String cron = annotation.cron();//检查cron表达式非空if (!"".equals(cron)) {Assert.isTrue(initialDelay == -1, "'initialDelay' not supported for cron triggers");processedSchedule = true;if (embeddedValueResolver != null) {//解析cron表达式cron = embeddedValueResolver.resolveStringValue(cron);}//注册定时任务registrar.addCronTask(new CronTask(runnable, cron));}// At this point we don't need to differentiate between initial delay set or not anymoreif (initialDelay < 0) {initialDelay = 0;}// Check fixed delay fixedDelay =-1long fixedDelay = annotation.fixedDelay();if (fixedDelay >= 0) {Assert.isTrue(!processedSchedule, errorMessage);processedSchedule = true;registrar.addFixedDelayTask(new IntervalTask(runnable, fixedDelay, initialDelay));}String fixedDelayString = annotation.fixedDelayString();if (!"".equals(fixedDelayString)) {Assert.isTrue(!processedSchedule, errorMessage);processedSchedule = true;if (embeddedValueResolver != null) {fixedDelayString = embeddedValueResolver.resolveStringValue(fixedDelayString);}try {fixedDelay = Integer.parseInt(fixedDelayString);}catch (NumberFormatException ex) {throw new IllegalArgumentException("Invalid fixedDelayString value \"" + fixedDelayString + "\" - cannot parse into integer");}registrar.addFixedDelayTask(new IntervalTask(runnable, fixedDelay, initialDelay));}// Check fixed ratelong fixedRate = annotation.fixedRate();if (fixedRate >= 0) {Assert.isTrue(!processedSchedule, errorMessage);processedSchedule = true;registrar.addFixedRateTask(new IntervalTask(runnable, fixedRate, initialDelay));}String fixedRateString = annotation.fixedRateString();if (!"".equals(fixedRateString)) {Assert.isTrue(!processedSchedule, errorMessage);processedSchedule = true;if (embeddedValueResolver != null) {fixedRateString = embeddedValueResolver.resolveStringValue(fixedRateString);}try {fixedRate = Integer.parseInt(fixedRateString);}catch (NumberFormatException ex) {throw new IllegalArgumentException("Invalid fixedRateString value \"" + fixedRateString + "\" - cannot parse into integer");}registrar.addFixedRateTask(new IntervalTask(runnable, fixedRate, initialDelay));}// Check whether we had any attribute setAssert.isTrue(processedSchedule, errorMessage);}catch (IllegalArgumentException ex) {throw new IllegalStateException("Encountered invalid @Scheduled method '" + method.getName() + "': " + ex.getMessage());}}}});return bean;
}

到此我们把定时任务注册到了定时列表中了。那么定时列表是如何启动的呢？

ScheduledTaskRegistrar类实现了InitializingBean接口，在容器启动的时候实现了该方法

public void afterPropertiesSet() {
//定时任务启动   scheduleTasks();
}

定时任务都放到了List<ScheduledFuture>.该接口扩展了延时队列接口Delayed和异步接口Future。我们看一个ScheduledFuture的实现类。周期性的队列其实就是在再延时队列基础上的。我们知道延时队列能够实现在任务执行时间到了的时候去执行一次任务。而周期性队列的任务就是将延时队列的务执行之后把它按照周期时间继续放入到延时队列中，等待下一个执行时间到了继续该循环操作。