public class Timer {
    /**
     * The timer task queue.  This data structure is shared with the timer
     * thread.  The timer produces tasks, via its various schedule calls,
     * and the timer thread consumes, executing timer tasks as appropriate,
     * and removing them from the queue when they're obsolete.
     */
    private final TaskQueue queue = new TaskQueue();
    /**
     * The timer thread.
     */
    private final TimerThread thread = new TimerThread(queue);
    。。。。。。
}

//其中的delay是延時(shí)時(shí)間，表示多少毫秒后執(zhí)行一次task
public void schedule(TimerTask task, long delay) {
    if (delay < 0)
        throw new IllegalArgumentException("Negative delay.");
    sched(task, System.currentTimeMillis()+delay, 0);
}

//到達(dá)指定時(shí)間time的時(shí)候執(zhí)行一次task
public void schedule(TimerTask task, Date time) {
    sched(task, time.getTime(), 0);
}

//經(jīng)過(guò)delay毫秒后按每period毫秒執(zhí)行一次的周期執(zhí)行task
public void schedule(TimerTask task, long delay, long period) {
    if (delay < 0)
        throw new IllegalArgumentException("Negative delay.");
    if (period <= 0)
        throw new IllegalArgumentException("Non-positive period.");
    sched(task, System.currentTimeMillis()+delay, -period);
}

//到達(dá)指定時(shí)間firstTime之后按照每period毫秒執(zhí)行一次的周期執(zhí)行task
public void schedule(TimerTask task, Date firstTime, long period) {
    if (period <= 0)
        throw new IllegalArgumentException("Non-positive period.");
    sched(task, firstTime.getTime(), -period);
}

public abstract class TimerTask implements Runnable {
    /**
     * This object is used to control access to the TimerTask internals.
     */
    final Object lock = new Object();
    
    。。。。。。
}

public class JavaTimerJob {
    public static void main(String[] args) {
        Timer timer = new Timer();
        Task task = new Task();
        //當(dāng)前時(shí)間開(kāi)始，每1秒執(zhí)行一次
        timer.schedule(task, new Date(),1000);
    }
 }
class Task extends TimerTask {
    @Override
    public void run() {
        System.out.println(new Date()+":  This is my job...");
    }
}

//command是待執(zhí)行的線程，delay表示延時(shí)時(shí)長(zhǎng)，unit代表時(shí)間單位
public ScheduledFuture<?> schedule(Runnable command,
                                   long delay,
                                   TimeUnit unit) {
    if (command == null || unit == null)
        throw new NullPointerException();
    RunnableScheduledFuture<?> t = decorateTask(command,
        new ScheduledFutureTask<Void>(command, null,
                                      triggerTime(delay, unit)));
    delayedExecute(t);
    return t;
}

//command是待執(zhí)行的線程，initialDelay表示延時(shí)時(shí)長(zhǎng)，period代表執(zhí)行間隔時(shí)長(zhǎng)，unit代表時(shí)間單位
public ScheduledFuture<?> scheduleAtFixedRate(Runnable command,
                                              long initialDelay,
                                              long period,
                                              TimeUnit unit) {
    if (command == null || unit == null)
        throw new NullPointerException();
    if (period <= 0)
        throw new IllegalArgumentException();
    ScheduledFutureTask<Void> sft =
        new ScheduledFutureTask<Void>(command,
                                      null,
                                      triggerTime(initialDelay, unit),
                                      unit.toNanos(period));
    RunnableScheduledFuture<Void> t = decorateTask(command, sft);
    sft.outerTask = t;
    delayedExecute(t);
    return t;
}

//command是待執(zhí)行的線程，initialDelay表示延時(shí)時(shí)長(zhǎng)，delay代表每次執(zhí)行線程前的延時(shí)時(shí)長(zhǎng)，unit代表時(shí)間單位
public ScheduledFuture<?> scheduleWithFixedDelay(Runnable command,
                                                 long initialDelay,
                                                 long delay,
                                                 TimeUnit unit) {
    if (command == null || unit == null)
        throw new NullPointerException();
    if (delay <= 0)
        throw new IllegalArgumentException();
    ScheduledFutureTask<Void> sft =
        new ScheduledFutureTask<Void>(command,
                                      null,
                                      triggerTime(initialDelay, unit),
                                      unit.toNanos(-delay));
    RunnableScheduledFuture<Void> t = decorateTask(command, sft);
    sft.outerTask = t;
    delayedExecute(t);
    return t;
}

public class JavaScheduledThreadPoolExecutor {
    public static void main(String[] args) {
        ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(8);
        //延時(shí)1秒后開(kāi)始執(zhí)行，每3秒執(zhí)行一次
        scheduledExecutorService.scheduleAtFixedRate(new Runnable() {
            @Override
            public void run() {
                System.out.println(new Date()+": This is my job...");
            }
        }, 1, 3, TimeUnit.SECONDS);
    }
}

@Target({ElementType.METHOD, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Repeatable(Schedules.class)
public @interface Scheduled {
   String CRON_DISABLED = "-";
   String cron() default "";  //類似于corn表達(dá)式，可以指定定時(shí)任務(wù)執(zhí)行的延遲及周期規(guī)則
   
   String zone() default "";  //指明解析cron表達(dá)式的時(shí)區(qū)。
   long fixedDelay() default -1;  //在最后一次調(diào)用結(jié)束和下一次調(diào)用開(kāi)始之間以固定周期（以毫秒為單位）執(zhí)行帶注解的方法。（要等待上次任務(wù)完成后）
   String fixedDelayString() default "";  //同上面作用一樣，只是String類型
   long fixedRate() default -1;  //在調(diào)用之間以固定的周期（以毫秒為單位）執(zhí)行帶注解的方法。（不需要等待上次任務(wù)完成）
   
   String fixedRateString() default "";  //同上面作用一樣，只是String類型
   long initialDelay() default -1;  //第一次執(zhí)行fixedRate()或fixedDelay()任務(wù)之前延遲的毫秒數(shù) 。
   String initialDelayString() default "";  //同上面作用一樣，只是String類型
}

@Slf4j
@Component
public class TestJob {
   //每40秒執(zhí)行一次
    @Scheduled(cron = "0/40 * * * * ?")
    public void logJob(){
        if(log.isDebugEnabled()){
            log.debug("現(xiàn)在是：{}",LocalDateTime.now());
        }
    }
}

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)) {
       //獲取定時(shí)任務(wù)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: " + targetClass);
         }
      }
      else {
         // Non-empty set of methods
         annotatedMethods.forEach((method, scheduledMethods) ->
               //調(diào)用processScheduled方法將定時(shí)任務(wù)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);
      boolean processedSchedule = false;
      String errorMessage =
            "Exactly one of the 'cron', 'fixedDelay(String)', or 'fixedRate(String)' attributes is required";
      Set<ScheduledTask> tasks = new LinkedHashSet<>(4);
      //解析任務(wù)執(zhí)行初始延遲
      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");
            }
         }
      }
      //解析cron表達(dá)式
      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();
               }
               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;
      }
      //解析fixedDelay參數(shù)
      long fixedDelay = scheduled.fixedDelay();
      if (fixedDelay >= 0) {
         Assert.isTrue(!processedSchedule, errorMessage);
         processedSchedule = true;
         
         //存放任務(wù)到任務(wù)隊(duì)列中
         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)));
         }
      }
      //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)));
         }
      }
      // 斷言檢查
      Assert.isTrue(processedSchedule, errorMessage);
      //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);
   }
}