public class TimerTest {
    public static void main(String[] args) {
        Timer timer = new Timer("aa");
        Task task = new Task();
        timer.schedule(task,new Date(),1000);
    }
}
class Task extends TimerTask{
    @Override
    public void run() {
        System.out.println(new Date());
    }
}

public class TimerTest {
    public static void main(String[] args) throws Exception{
        ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(10);
        scheduledExecutorService.scheduleAtFixedRate(
                () -> System.out.println(new Date()),
                1,3, TimeUnit.SECONDS);
    }
}

@Target({ElementType.METHOD, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Repeatable(Schedules.class)
public @interface Scheduled {
	String CRON_DISABLED = ScheduledTaskRegistrar.CRON_DISABLED;
    // cron表達(dá)式
	String cron() default "";
    // 時(shí)區(qū)
	String zone() default "";
    // 從上一次調(diào)用結(jié)束到下一次調(diào)用之間的固定時(shí)間
	long fixedDelay() default -1;
    // 和fixedDelay意思相同，只是使用字符傳格式，支持占位符。例如：fixedDelayString = "${time.fixedDelay}"
	String fixedDelayString() default "";
    // 兩次調(diào)用之間固定的毫秒數(shù)（不需要等待上次任務(wù)完成）
	long fixedRate() default -1;
    // 同上，支持占位符
	String fixedRateString() default "";
    // 第一次執(zhí)行任務(wù)前延遲的毫秒數(shù)
	long initialDelay() default -1;
    // 同上，支持占位符
	String initialDelayString() default "";
}

@Component
@EnableScheduling
public class ScheduledTask {
    @Scheduled(fixedDelay = 1000)
    public void task(){
        System.out.println("aaa");
    }
}

	@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);
		if (!this.nonAnnotatedClasses.contains(targetClass) &&
				AnnotationUtils.isCandidateClass(targetClass, Arrays.asList(Scheduled.class, Schedules.class))) {
			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: " + targetClass);
				}
			}
			else {
				// Non-empty set of methods
				annotatedMethods.forEach((method, scheduledMethods) ->
                        // 調(diào)用processScheduled方法將定時(shí)任務(wù)的方法存放到任務(wù)隊(duì)列中
						scheduledMethods.forEach(scheduled -> processScheduled(scheduled, method, bean)));
				if (logger.isTraceEnabled()) {
					logger.trace(annotatedMethods.size() + " @Scheduled methods processed on bean '" + beanName +
							"': " + annotatedMethods);
				}
			}
		}
		return bean;
	}

	protected void processScheduled(Scheduled scheduled, Method method, Object bean) {
		try {
            // 創(chuàng)建任務(wù)線程
			Runnable runnable = createRunnable(bean, method);
            // 解析到定時(shí)任務(wù)方式的標(biāo)記，解析到正確的參數(shù)后會(huì)設(shè)置為TRUE，如果在解析到了其他的參數(shù)就會(huì)拋出異常
			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 解析第一次的延遲（解析initialDelay參數(shù)）
			long initialDelay = scheduled.initialDelay();
			String initialDelayString = scheduled.initialDelayString();
			if (StringUtils.hasText(initialDelayString)) {
                // initialDelay不能小于0
				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 解析cron表達(dá)式
			String cron = scheduled.cron();
			if (StringUtils.hasText(cron)) {
                // 解析時(shí)區(qū)
				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();
						}
						tasks.add(this.registrar.scheduleCronTask(new CronTask(runnable, new CronTrigger(cron, timeZone))));
					}
				}
			}
            // 第一次延遲參數(shù)小于0，默認(rèn)為0
			// At this point we don't need to differentiate between initial delay set or not anymore
			if (initialDelay < 0) {
				initialDelay = 0;
			}
			// Check fixed delay 解析fixedDelay參數(shù)
			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)));
				}
			}
			// Check fixed rate 解析fixedRate參數(shù)
			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
            // 如果五個(gè)參數(shù)一個(gè)也沒解析到，拋出異常
			Assert.isTrue(processedSchedule, errorMessage);
			// Finally register the scheduled tasks
            // 并發(fā)控制將任務(wù)隊(duì)列存入注冊(cè)任務(wù)列表
			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());
		}
	}

protected void scheduleTasks() {
   if (this.taskScheduler == null) {
       //獲取ScheduledExecutorService對(duì)象，實(shí)際上都是使用ScheduledThreadPoolExecutor執(zhí)行定時(shí)任務(wù)調(diào)度
      this.localExecutor = Executors.newSingleThreadScheduledExecutor();
      this.taskScheduler = new ConcurrentTaskScheduler(this.localExecutor);
   }
   if (this.triggerTasks != null) {
      for (TriggerTask task : this.triggerTasks) {
         addScheduledTask(scheduleTriggerTask(task));
      }
   }
   if (this.cronTasks != null) {
      for (CronTask task : this.cronTasks) {
         addScheduledTask(scheduleCronTask(task));
      }
   }
   if (this.fixedRateTasks != null) {
      for (IntervalTask task : this.fixedRateTasks) {
         addScheduledTask(scheduleFixedRateTask(task));
      }
   }
   if (this.fixedDelayTasks != null) {
      for (IntervalTask task : this.fixedDelayTasks) {
         addScheduledTask(scheduleFixedDelayTask(task));
      }
   }
}
private void addScheduledTask(@Nullable ScheduledTask task) {
   if (task != null) {
      this.scheduledTasks.add(task);
   }
}