@Target({ElementType.METHOD, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Repeatable(Schedules.class)
public @interface Scheduled {
 /**
  * A cron-like expression, extending the usual UN*X definition to include
  * triggers on the second as well as minute, hour, day of month, month
  * and day of week.  e.g. {@code "0 * * * * MON-FRI"} means once per minute on
  * weekdays (at the top of the minute - the 0th second).
  * @return an expression that can be parsed to a cron schedule
  * @see org.springframework.scheduling.support.CronSequenceGenerator
  */
 String cron() default "";
 /**
  * A time zone for which the cron expression will be resolved. By default, this
  * attribute is the empty String (i.e. the server's local time zone will be used).
  * @return a zone id accepted by {@link java.util.TimeZone#getTimeZone(String)},
  * or an empty String to indicate the server's default time zone
  * @since 4.0
  * @see org.springframework.scheduling.support.CronTrigger#CronTrigger(String, java.util.TimeZone)
  * @see java.util.TimeZone
  */
 String zone() default "";
 /**
  * Execute the annotated method with a fixed period in milliseconds between the
  * end of the last invocation and the start of the next.
  * @return the delay in milliseconds
  */
 long fixedDelay() default -1;
 /**
  * Execute the annotated method with a fixed period in milliseconds between the
  * end of the last invocation and the start of the next.
  * @return the delay in milliseconds as a String value, e.g. a placeholder
  * @since 3.2.2
  */
 String fixedDelayString() default "";
 /**
  * Execute the annotated method with a fixed period in milliseconds between
  * invocations.
  * @return the period in milliseconds
  */
 long fixedRate() default -1;
 /**
  * Execute the annotated method with a fixed period in milliseconds between
  * invocations.
  * @return the period in milliseconds as a String value, e.g. a placeholder
  * @since 3.2.2
  */
 String fixedRateString() default "";
 /**
  * Number of milliseconds to delay before the first execution of a
  * {@link #fixedRate()} or {@link #fixedDelay()} task.
  * @return the initial delay in milliseconds
  * @since 3.2
  */
 long initialDelay() default -1;
 /**
  * Number of milliseconds to delay before the first execution of a
  * {@link #fixedRate()} or {@link #fixedDelay()} task.
  * @return the initial delay in milliseconds as a String value, e.g. a placeholder
  * @since 3.2.2
  */
 String initialDelayString() default "";
}

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

 @Override
 public Object postProcessAfterInitialization(final Object bean, String beanName) {
  Class<?> targetClass = AopProxyUtils.ultimateTargetClass(bean);
  if (!this.nonAnnotatedClasses.contains(targetClass)) {
   //獲取含有@Scheduled注解的方法
   Map<Method, Set<Scheduled>> annotatedMethods = MethodIntrospector.selectMethods(targetClass,
     (MethodIntrospector.MetadataLookup<Set<Scheduled>>) method -> {
      Set<Scheduled> scheduledMethods = AnnotatedElementUtils.getMergedRepeatableAnnotations(
        method, Scheduled.class, Schedules.class);
      return (!scheduledMethods.isEmpty() ? scheduledMethods : null);
     });
   if (annotatedMethods.isEmpty()) {
    this.nonAnnotatedClasses.add(targetClass);
    if (logger.isTraceEnabled()) {
     logger.trace("No @Scheduled annotations found on bean class: " + bean.getClass());
    }
   }
   else {
   
    // 循環(huán)處理包含@Scheduled注解的方法
    annotatedMethods.forEach((method, scheduledMethods) ->
      scheduledMethods.forEach(scheduled -> processScheduled(scheduled, method, bean)));
    if (logger.isDebugEnabled()) {
     logger.debug(annotatedMethods.size() + " @Scheduled methods processed on bean '" + beanName +
       "': " + annotatedMethods);
    }
   }
  }
  return bean;
 }

protected void processScheduled(Scheduled scheduled, Method method, Object bean) {
  try {
   Assert.isTrue(method.getParameterCount() == 0,
     "Only no-arg methods may be annotated with @Scheduled");
   //獲取調(diào)用的方法
   Method invocableMethod = AopUtils.selectInvocableMethod(method, bean.getClass());
   //處理線程
   Runnable runnable = new ScheduledMethodRunnable(bean, invocableMethod);
   boolean processedSchedule = false;
   String errorMessage =
     "Exactly one of the 'cron', 'fixedDelay(String)', or 'fixedRate(String)' attributes is required";
   Set<ScheduledTask> tasks = new LinkedHashSet<>(4);
   // Determine initial delay
   long 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);
    }
    if (StringUtils.hasLength(initialDelayString)) {
     try {
      initialDelay = parseDelayAsLong(initialDelayString);
     }
     catch (RuntimeException ex) {
      throw new IllegalArgumentException(
        "Invalid initialDelayString value \"" + initialDelayString + "\" - cannot parse into long");
     }
    }
   }
   // 獲取cron參數(shù)
   String cron = scheduled.cron();
   if (StringUtils.hasText(cron)) {
    String zone = scheduled.zone();
    if (this.embeddedValueResolver != null) {
     cron = this.embeddedValueResolver.resolveStringValue(cron);
     zone = this.embeddedValueResolver.resolveStringValue(zone);
    }
    if (StringUtils.hasLength(cron)) {
     Assert.isTrue(initialDelay == -1, "'initialDelay' not supported for cron triggers");
     processedSchedule = true;
     TimeZone timeZone;
     if (StringUtils.hasText(zone)) {
      timeZone = StringUtils.parseTimeZoneString(zone);
     }
     else {
      timeZone = TimeZone.getDefault();
     }
     //加入到定時(shí)任務(wù)列表中
     tasks.add(this.registrar.scheduleCronTask(new CronTask(runnable, new CronTrigger(cron, timeZone))));
    }
   }
   // At this point we don't need to differentiate between initial delay set or not anymore
   if (initialDelay < 0) {
    initialDelay = 0;
   }
   // Check fixed delay
   long fixedDelay = scheduled.fixedDelay();
   if (fixedDelay >= 0) {
    Assert.isTrue(!processedSchedule, errorMessage);
    processedSchedule = true;
    tasks.add(this.registrar.scheduleFixedDelayTask(new FixedDelayTask(runnable, fixedDelay, initialDelay)));
   }
   String fixedDelayString = scheduled.fixedDelayString();
   if (StringUtils.hasText(fixedDelayString)) {
    if (this.embeddedValueResolver != null) {
     fixedDelayString = this.embeddedValueResolver.resolveStringValue(fixedDelayString);
    }
    if (StringUtils.hasLength(fixedDelayString)) {
     Assert.isTrue(!processedSchedule, errorMessage);
     processedSchedule = true;
     try {
      fixedDelay = parseDelayAsLong(fixedDelayString);
     }
     catch (RuntimeException ex) {
      throw new IllegalArgumentException(
        "Invalid fixedDelayString value \"" + fixedDelayString + "\" - cannot parse into long");
     }
     tasks.add(this.registrar.scheduleFixedDelayTask(new FixedDelayTask(runnable, fixedDelay, initialDelay)));
    }
   }
   // 執(zhí)行頻率的類型為long
   long fixedRate = scheduled.fixedRate();
   if (fixedRate >= 0) {
    Assert.isTrue(!processedSchedule, errorMessage);
    processedSchedule = true;
    tasks.add(this.registrar.scheduleFixedRateTask(new FixedRateTask(runnable, fixedRate, initialDelay)));
   }
   String fixedRateString = scheduled.fixedRateString();
   if (StringUtils.hasText(fixedRateString)) {
    if (this.embeddedValueResolver != null) {
     fixedRateString = this.embeddedValueResolver.resolveStringValue(fixedRateString);
    }
    if (StringUtils.hasLength(fixedRateString)) {
     Assert.isTrue(!processedSchedule, errorMessage);
     processedSchedule = true;
     try {
      fixedRate = parseDelayAsLong(fixedRateString);
     }
     catch (RuntimeException ex) {
      throw new IllegalArgumentException(
        "Invalid fixedRateString value \"" + fixedRateString + "\" - cannot parse into long");
     }
     tasks.add(this.registrar.scheduleFixedRateTask(new FixedRateTask(runnable, fixedRate, initialDelay)));
    }
   }
   // Check whether we had any attribute set
   Assert.isTrue(processedSchedule, errorMessage);
   // Finally register the scheduled tasks
   synchronized (this.scheduledTasks) {
    Set<ScheduledTask> registeredTasks = this.scheduledTasks.get(bean);
    if (registeredTasks == null) {
     registeredTasks = new LinkedHashSet<>(4);
     this.scheduledTasks.put(bean, registeredTasks);
    }
    registeredTasks.addAll(tasks);
   }
  }
  catch (IllegalArgumentException ex) {
   throw new IllegalStateException(
     "Encountered invalid @Scheduled method '" + method.getName() + "': " + ex.getMessage());
  }
 }

public class ScheduledTaskRegistrar implements InitializingBean, DisposableBean {
 private TaskScheduler taskScheduler;
 private ScheduledExecutorService localExecutor;
 private List<TriggerTask> triggerTasks;
 private List<CronTask> cronTasks;
 private List<IntervalTask> fixedRateTasks;
 private List<IntervalTask> fixedDelayTasks;
 private final Map<Task, ScheduledTask> unresolvedTasks = new HashMap<Task, ScheduledTask>(16);
 private final Set<ScheduledTask> scheduledTasks = new LinkedHashSet<ScheduledTask>(16);
 
 ......
}

public ScheduledTask scheduleCronTask(CronTask task) {
  ScheduledTask scheduledTask = this.unresolvedTasks.remove(task);
  boolean newTask = false;
  if (scheduledTask == null) {
   scheduledTask = new ScheduledTask();
   newTask = true;
  }
  if (this.taskScheduler != null) {
   scheduledTask.future = this.taskScheduler.schedule(task.getRunnable(), task.getTrigger());
  }
  else {
   addCronTask(task);
   this.unresolvedTasks.put(task, scheduledTask);
  }
  return (newTask ? scheduledTask : null);
 }

@Override
 protected ExecutorService initializeExecutor(
   ThreadFactory threadFactory, RejectedExecutionHandler rejectedExecutionHandler) {
  this.scheduledExecutor = createExecutor(this.poolSize, threadFactory, rejectedExecutionHandler);
  if (this.removeOnCancelPolicy) {
   if (this.scheduledExecutor instanceof ScheduledThreadPoolExecutor) {
    ((ScheduledThreadPoolExecutor) this.scheduledExecutor).setRemoveOnCancelPolicy(true);
   }
   else {
    logger.info("Could not apply remove-on-cancel policy - not a Java 7+ ScheduledThreadPoolExecutor");
   }
  }
  return this.scheduledExecutor;
 }

public ScheduledFuture<?> schedule() {
  synchronized (this.triggerContextMonitor) {
   this.scheduledExecutionTime = this.trigger.nextExecutionTime(this.triggerContext);
   if (this.scheduledExecutionTime == null) {
    return null;
   }
   //獲取時(shí)間差
   long initialDelay = this.scheduledExecutionTime.getTime() - System.currentTimeMillis();
   this.currentFuture = this.executor.schedule(this, initialDelay, TimeUnit.MILLISECONDS);
   return this;
  }
 }