為什么需要定時任務

定時任務的應用場景十分廣泛，如定時清理文件、定時生成報表、定時數據同步備份等。

JAVA定時任務的原理

jdk自帶的庫中，有兩種技術可以實現定時任務，一種是Timer,另一種是
ScheduledThreadPoolExecutor

Timer+TimerTask

Timer是一個線程，控制執行TimerTask所需要執行的內容

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，需要注意，Timer類有幾個方法創建不同的線程執行:

延時執行

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

指定時間點執行

//到達指定時間time的時候執行一次task
public void schedule(TimerTask task, Date time) {
    sched(task, time.getTime(), 0);
}

延時周期執行

//經過delay毫秒后按每period毫秒執行一次的周期執行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);
}

指定時間點后周期執行

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

TimerTask是一個實現了Runable接口的類，所以能夠放到線程去執行:

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

示例：

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

執行結果：

Tue May 30 13:45:47 CST 2022:  This is my job...
Tue May 30 13:45:48 CST 2022:  This is my job...
Tue May 30 13:45:49 CST 2022:  This is my job...
Tue May 30 13:45:50 CST 2022:  This is my job...
。。。。

弊端：Timer是單線程的，一旦定時任務中某一過程時刻拋出異常，將會導致整體線程停止，定時任務停止。

ScheduledThreadPoolExecutor

繼承了ThreadPoolExecutor,,是一個基于線程池的調度器通過實現ScheduledExecutorService接口方法去實現任務調度，主要方法如下:

延時執行

//command是待執行的線程，delay表示延時時長，unit代表時間單位
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;
}

延時周期執行

//command是待執行的線程，initialDelay表示延時時長，period代表執行間隔時長，unit代表時間單位
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;
}

每段延時間隔執行

//command是待執行的線程，initialDelay表示延時時長，delay代表每次執行線程前的延時時長，unit代表時間單位
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;
}

示例：

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

執行結果：

Tue May 30 15:05:16 CST 2022: This is my job...
Tue May 30 15:05:19 CST 2022: This is my job...
Tue May 30 15:05:22 CST 2022: This is my job...
Tue May 30 15:05:25 CST 2022: This is my job...
。。。。。

Timer VS ScheduledThreadPoolExecutor

Timer

是單線程，如果開啟多個線程服務，將會出現競爭，一旦出現異常，線程停止，定時任務停止；
兼容性更高，jdk1.3后使用

ScheduledThreadPoolExecutor

基于線程池實現多線程，且自動調整線程數，線程出錯并不會影響整體定時任務執行。
在jdk1.5后可使用

Spring定時任務

Spring原生定時任務主要依靠@Scheduled注解實現：

@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表達式，可以指定定時任務執行的延遲及周期規則
   
   String zone() default "";  //指明解析cron表達式的時區。
   long fixedDelay() default -1;  //在最后一次調用結束和下一次調用開始之間以固定周期（以毫秒為單位）執行帶注解的方法。（要等待上次任務完成后）
   String fixedDelayString() default "";  //同上面作用一樣，只是String類型
   long fixedRate() default -1;  //在調用之間以固定的周期（以毫秒為單位）執行帶注解的方法。（不需要等待上次任務完成）
   
   String fixedRateString() default "";  //同上面作用一樣，只是String類型
   long initialDelay() default -1;  //第一次執行fixedRate()或fixedDelay()任務之前延遲的毫秒數 。
   String initialDelayString() default "";  //同上面作用一樣，只是String類型
}

Spring靜態定時任務示例:

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

執行結果：

現在是：2022-05-30T16:03:40.006
現在是：2022-05-30T16:04
現在是：2022-05-30T16:04:40.003

@Scheduled定時任務原理（源碼）

①項目啟動掃描帶有注解@Scheduled的所有方法信息由
ScheduledAnnotationBeanPostProcessor的postProcessAfterInitialization方法實現功能：

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)) {
       //獲取定時任務的方法
      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) ->
               //調用processScheduled方法將定時任務方法存放到任務隊列中
               scheduledMethods.forEach(scheduled -> processScheduled(scheduled, method, bean)));
         if (logger.isTraceEnabled()) {
            logger.trace(annotatedMethods.size() + " @Scheduled methods processed on bean '" + beanName +
                  "': " + annotatedMethods);
         }
      }
   }
   return bean;
}

②調用processScheduled方法將定時任務方法存放到任務隊列中

protected void processScheduled(Scheduled scheduled, Method method, Object bean) {
   try {
       //創建任務線程
      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);
      //解析任務執行初始延遲
      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
      long fixedDelay = scheduled.fixedDelay();
      if (fixedDelay >= 0) {
         Assert.isTrue(!processedSchedule, errorMessage);
         processedSchedule = true;
         
         //存放任務到任務隊列中
         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
      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);
      //并發控制將任務隊列存入注冊任務列表
      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());
   }
}

③將任務解析并添加到任務隊列后，交由ScheduledTaskRegistrar類的scheduleTasks方法添加（注冊）定時任務到環境中:

protected void scheduleTasks() {
   if (this.taskScheduler == null) {
       //獲取ScheduledExecutorService對象，實際上都是使用ScheduledThreadPoolExecutor執行定時任務調度
      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);
   }
}

由上述源碼可以看出，Spring原生定時任務的大概步驟如下： 1.掃描帶@Scheduled注解的類和方法（
ScheduledAnnotationBeanPostProcessor.postProcessAfterInitialization(........)) 2.將定時任務解析完成后加入任務隊列（ScheduledAnnotationBeanPostProcessor.processScheduled(........)) 3.將定時任務注冊到當前運行環境，等待執行（ScheduledTaskRegistrar.scheduleTasks(.......)) 且@Scheduled的底層調度實現是ScheduledThreadPoolExecutor