java implements scheduled tasks

In the library that comes with Jdk, there are two ways to implement scheduled tasks, one is Timer, the other is ScheduledThreadPoolExecutor.

Timer TimerTask

Creating a Timer creates a thread, which can be used to schedule TimerTask tasks

Timer has four construction methods, which can specify the name of the Timer thread and whether to set it as a daemon thread. The default name is Timer-number , which is not a daemon thread by default.

There are three main methods:

cancel(): Terminate task scheduling, cancel all currently scheduled tasks, and running tasks will not be affected

purge(): Remove all canceled tasks from the task queue

schedule: Start scheduling tasks, providing several overloaded methods ：

schedule(TimerTask task, long delay) Delayed execution, which means delay will be executed once after tasktask

schedule(TimerTask task, Date time)`Execute at the specified time, and execute `task once at `time` time
#schedule(TimerTask task, long delay, long period)`Delay cycle execution, after ` `task
schedule(TimerTask task, Date firstTime, long period)` is executed periodically after the specified time `firstTime` and is executed once every `period` milliseconds after the specified time `firstTime` is reached.

Example

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());
    }
}

Output:
Thu Jul 07 14:50:02 CST 2022
Thu Jul 07 14:50:03 CST 2022
Thu Jul 07 14:50:04 CST 2022
Thu Jul 07 14:50:05 CST 2022
…………

Disadvantages

Timer is single-threaded and does not throw exceptions. If an exception occurs in a scheduled task, the entire thread will stop, causing the scheduled task to terminate.

ScheduledThreadPoolExecutor

Due to the defects of Timer, it is not recommended to use Timer. It is recommended to use ScheduledThreadPoolExecutor.

ScheduledThreadPoolExecutor is a replacement for Timer. It was introduced in JDK1.5 and inherits ThreadPoolExecutor. It is a scheduled task class based on thread pool design. .

Main scheduling method:

scheduleExecute scheduling only once, (task, delay time, delay time unit)

scheduleAtFixedRateScheduling at a fixed frequency. If the execution time is too long, the next scheduling will be delayed, (task, delay time of the first execution, period, time unit)

scheduleWithFixedDelay Delay scheduling, after a task is executed, a delay time is added to execute the next task, (task, delay time of the first execution, interval time, time unit)

Example

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);
    }
}

Spring scheduled tasks

Spring scheduled tasks are mainly implemented by @Scheduled annotations, corn, fixedDelay, fixedDelayString, fixedRate, fixedRateString The five parameters are required Specifying one of them, passing two or three will throw an exception

@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 "";
}

Example

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

Principle

Project startupScheduledAnnotationBeanPostProcessorpostProcessAfterInitialization () Method scans methods with @Scheduled annotations:

	@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方法將定時任務(wù)的方法存放到任務(wù)隊列中
						scheduledMethods.forEach(scheduled -> processScheduled(scheduled, method, bean)));
				if (logger.isTraceEnabled()) {
					logger.trace(annotatedMethods.size() + " @Scheduled methods processed on bean &#39;" + beanName +
							"&#39;: " + 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);
            // 解析到定時任務(wù)方式的標(biāo)記，解析到正確的參數(shù)后會設(shè)置為TRUE，如果在解析到了其他的參數(shù)就會拋出異常
			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 &#39;initialDelay&#39; or &#39;initialDelayString&#39;, 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, "&#39;initialDelay&#39; 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&#39;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
            // 如果五個參數(shù)一個也沒解析到，拋出異常
			Assert.isTrue(processedSchedule, errorMessage);
			// Finally register the scheduled tasks
            // 并發(fā)控制將任務(wù)隊列存入注冊任務(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 &#39;" + method.getName() + "&#39;: " + ex.getMessage());
		}
	}

After the task is parsed and added to the task queue, it is handed over to the ScheduledTaskRegistrar class ##scheduleTasksMethod to add (register) scheduled tasks to the environment

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

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