@EnableScheduling // <==
@SpringBootApplication
public class Application {
    public static void main(String[] args)  {
        SpringApplication.run(Application.class, args);
    }
}

@Import(SchedulingConfiguration.class)  // <==
public @interface EnableScheduling {
}

@Configuration
@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
public class SchedulingConfiguration {

    // Bean 名稱使用 : 
    // org.springframework.context.annotation.internalScheduledAnnotationProcessor
	@Bean(name = TaskManagementConfigUtils.SCHEDULED_ANNOTATION_PROCESSOR_BEAN_NAME)
	@Role(BeanDefinition.ROLE_INFRASTRUCTURE) // 定義為基礎設施bean
	public ScheduledAnnotationBeanPostProcessor scheduledAnnotationProcessor() {
		return new ScheduledAnnotationBeanPostProcessor();
	}

}

@Override
	public Object postProcessAfterInitialization(Object bean, String beanName) {
		if (bean instanceof AopInfrastructureBean || bean instanceof TaskScheduler ||
				bean instanceof ScheduledExecutorService) {
			// Ignore AOP infrastructure such as scoped proxies.
			return bean;
		}
		Class<?> targetClass = AopProxyUtils.ultimateTargetClass(bean);
       // this.nonAnnotatedClasses 是一個緩存，用于記錄處理過程中所發(fā)現的不包含任何被@Scheduled注解的方法的類
		if (!this.nonAnnotatedClasses.contains(targetClass)) {
          // 獲取類  targetClass 上所有使用注解  @Scheduled 的方法
          // 注意 : 某個方法上可能同時使用多個注解  @Scheduled ，所以以下 annotatedMethods 的每個 Entry 是
          // 一個方法對應一個 @cheduled 集合
			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()) {
              // 如果當前類 targetClass 不包含任何使用注解  @Scheduled 的方法，將其添加到 this.nonAnnotatedClasses
				this.nonAnnotatedClasses.add(targetClass);
				if (logger.isTraceEnabled()) {
					logger.trace("No @Scheduled annotations found on bean class: " + targetClass);
				}
			}
			else {
				// Non-empty set of methods
              // 當前類 targetClass 上找到了使用注解 @Scheduled 的方法，記錄在  annotatedMethods 中，
              // 現在將它們逐個處理，使用的處理為方法 processScheduled             
				annotatedMethods.forEach((method, scheduledMethods) ->
						scheduledMethods.forEach(scheduled -> processScheduled(scheduled, method, bean)));
				if (logger.isTraceEnabled()) {
					logger.trace(annotatedMethods.size() + " @Scheduled methods processed on bean '" + beanName +
							"': " + annotatedMethods);
				}
			}
		}
		return bean;
	}

/**
	 * Process the given {@code @Scheduled} method declaration on the given bean.
	 * @param scheduled the @Scheduled annotation
	 * @param method the method that the annotation has been declared on
	 * @param bean the target bean instance
	 * @see #createRunnable(Object, Method)
	 */
	protected void processScheduled(Scheduled scheduled, Method method, Object bean) {
		try {
           // 將使用了 @Scheduled 注解的方法包裝成一個 Runnable 對象  ， 隨后構建 ScheduledTask 對象時
           // 會用得到
			Runnable runnable = createRunnable(bean, method);
          // 用于記錄當前 @Scheduled 注解是否已經被處理，初始化為 false  
			boolean processedSchedule = false;
			String errorMessage =
			"Exactly one of the 'cron', 'fixedDelay(String)', or 'fixedRate(String)' attributes is required";
          // 用于保存針對當前 @Scheduled  注解生成的 ScheduledTask，
          // 該方法完成時，該集合內元素數量通常為 1
			Set<ScheduledTask> tasks = new LinkedHashSet<>(4);
			// Determine initial delay
          // 確定 initial delay 屬性 ： 基于注解屬性 initialDelay 或者  initialDelayString 分析得到,
          // 二者只能使用其中之一
			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");
					}
				}
			}
			// Check cron expression
          // 檢查這是否是一個 cron 表達式類型的注解  
			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;
					if (!Scheduled.CRON_DISABLED.equals(cron)) {
						TimeZone timeZone;
						if (StringUtils.hasText(zone)) {
							timeZone = StringUtils.parseTimeZoneString(zone);
						}
						else {
							timeZone = TimeZone.getDefault();
						}
	                    // 包裝成為一個 CronTask 
						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
           // 檢查這是否是一個固定延遲類型的注解    
			long fixedDelay = scheduled.fixedDelay();
			if (fixedDelay >= 0) {
				Assert.isTrue(!processedSchedule, errorMessage);
				processedSchedule = true;
				// 包裝成為一個 FixedDelayTask 
				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");
					}
	                 // 包裝成為一個 FixedDelayTask    
					tasks.add(this.registrar.scheduleFixedDelayTask(
						new FixedDelayTask(runnable, fixedDelay, initialDelay)));
				}
			}
			// Check fixed rate
           // 檢查這是否是一個固定周期執(zhí)行類型的注解    
			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");
					}
					// 包裝成為一個 FixedRateTask       
					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> regTasks = 
					this.scheduledTasks.computeIfAbsent(bean, key -> new LinkedHashSet<>(4));
				regTasks.addAll(tasks);
			}
		}
		catch (IllegalArgumentException ex) {
			throw new IllegalStateException(
					"Encountered invalid @Scheduled method '" + method.getName() + "': " + ex.getMessage());
		}
	}