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Spring自带的定时任务是如何实现的?有何注意点?

Spring自带的定时任务是如何实现的?有何注意点?

作者: 技术栈 | 来源:发表于2022-08-05 09:41 被阅读0次

    实现案例

    Spring Task封装的比较好,使用非常简单。

    @EnableScheduling+@Scheduled

    • 通过@EnableScheduling启用定时任务,@Scheduled定义任务
    @EnableScheduling
    @Configuration
    public class ScheduleDemo {
    
        /**
         * 每隔1分钟执行一次。
         */
        @Scheduled(fixedRate = 1000 * 60 * 1)
        public void runScheduleFixedRate() {
            log.info("runScheduleFixedRate: current DateTime, {}", LocalDateTime.now());
        }
    
        /**
         * 每个整点小时执行一次。
         */
        @Scheduled(cron = "0 0 */1 * * ?")
        public void runScheduleCron() {
            log.info("runScheduleCron: current DateTime, {}", LocalDateTime.now());
        }
    
    }
    
    • @Scheduled所支持的参数
    1. cron:cron表达式,指定任务在特定时间执行;
    2. fixedDelay:表示上一次任务执行完成后多久再次执行,参数类型为long,单位ms;
    3. fixedDelayString:与fixedDelay含义一样,只是参数类型变为String;
    4. fixedRate:表示按一定的频率执行任务,参数类型为long,单位ms;
    5. fixedRateString: 与fixedRate的含义一样,只是将参数类型变为String;
    6. initialDelay:表示延迟多久再第一次执行任务,参数类型为long,单位ms;
    7. initialDelayString:与initialDelay的含义一样,只是将参数类型变为String;
    8. zone:时区,默认为当前时区,一般没有用到。

    进一步理解

    我们再通过一些问题来帮助你更深入理解Spring Task实现方式。@pdai

    使用Spring Task要注意什么?

    • 关于异常处理

    建议自行处理异常

    • 关于超时处理

    在实际的开发中,这个问题经常会出现,比如执行一段时间后定时任务不再执行了; 这种情况会发生在,比如你调用一个第三方接口,没有设置调用超时,继而引发异常,这时候当前任务便阻塞了。

    Spring Task的原理?

    Spring Task的源码在这里:

    @EnableScheduling注解

    添加@EnableScheduling注解会自动注入SchedulingConfiguration

     * @author Chris Beams
     * @author Juergen Hoeller
     * @since 3.1
     * @see Scheduled
     * @see SchedulingConfiguration
     * @see SchedulingConfigurer
     * @see ScheduledTaskRegistrar
     * @see Trigger
     * @see ScheduledAnnotationBeanPostProcessor
     */
    @Target(ElementType.TYPE)
    @Retention(RetentionPolicy.RUNTIME)
    @Import(SchedulingConfiguration.class)
    @Documented
    public @interface EnableScheduling {
    
    }
    

    SchedulingConfiguration中初始化ScheduledAnnotationBeanPostProcessor

    SchedulingConfiguration配置中自动初始化ScheduledAnnotationBeanPostProcessor

    @Configuration(proxyBeanMethods = false)
    @Role(BeanDefinition.ROLE_INFRASTRUCTURE)
    public class SchedulingConfiguration {
    
        @Bean(name = TaskManagementConfigUtils.SCHEDULED_ANNOTATION_PROCESSOR_BEAN_NAME)
        @Role(BeanDefinition.ROLE_INFRASTRUCTURE)
        public ScheduledAnnotationBeanPostProcessor scheduledAnnotationProcessor() {
            return new ScheduledAnnotationBeanPostProcessor();
        }
    
    }
    

    什么是BeanPostProcessor? 我们在前文中有详解的讲解,具体看Spring核心之控制反转(IOC)源码解析

    Spring 容器中 Bean 的生命周期流程

    ScheduledTaskRegistrar注册task

    在ScheduledAnnotationBeanPostProcessor构造函数中初始化了ScheduledTaskRegistrar

    /**
        * Create a default {@code ScheduledAnnotationBeanPostProcessor}.
        */
    public ScheduledAnnotationBeanPostProcessor() {
        this.registrar = new ScheduledTaskRegistrar();
    }
    
    

    ScheduledTaskRegistrar最主要的是注册各种类型的task (这种方式在新的版本中已经废弃了)

    protected void scheduleTasks() {
        if (this.taskScheduler == null) {
            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));
            }
        }
    }
    
    

    注册哪些Task,怎么设计类的呢?

    ScheduledAnnotationBeanPostProcessor加载Scheduled注解

    在BeanPostProcessor的postProcessAfterInitialization阶段加载Scheduled注解

    @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> scheduledAnnotations = AnnotatedElementUtils.getMergedRepeatableAnnotations(
                                method, Scheduled.class, Schedules.class);
                        return (!scheduledAnnotations.isEmpty() ? scheduledAnnotations : 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, scheduledAnnotations) ->
                        scheduledAnnotations.forEach(scheduled -> processScheduled(scheduled, method, bean)));
                if (logger.isTraceEnabled()) {
                    logger.trace(annotatedMethods.size() + " @Scheduled methods processed on bean '" + beanName +
                            "': " + annotatedMethods);
                }
            }
        }
        return bean;
    }
    

    Scheduled注解是添加到方法级别,具体如下

    @Target({ElementType.METHOD, ElementType.ANNOTATION_TYPE})
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Repeatable(Schedules.class)
    public @interface Scheduled {
    
        /**
         * A special cron expression value that indicates a disabled trigger: {@value}.
         * <p>This is primarily meant for use with <code>${...}</code> placeholders,
         * allowing for external disabling of corresponding scheduled methods.
         * @since 5.1
         * @see ScheduledTaskRegistrar#CRON_DISABLED
         */
        String CRON_DISABLED = ScheduledTaskRegistrar.CRON_DISABLED;
    
        /**
         * A cron-like expression, extending the usual UN*X definition to include triggers
         * on the second, minute, hour, day of month, month, and day of week.
         * <p>For example, {@code "0 * * * * MON-FRI"} means once per minute on weekdays
         * (at the top of the minute - the 0th second).
         * <p>The fields read from left to right are interpreted as follows.
         * <ul>
         * <li>second</li>
         * <li>minute</li>
         * <li>hour</li>
         * <li>day of month</li>
         * <li>month</li>
         * <li>day of week</li>
         * </ul>
         * <p>The special value {@link #CRON_DISABLED "-"} indicates a disabled cron
         * trigger, primarily meant for externally specified values resolved by a
         * <code>${...}</code> placeholder.
         * @return an expression that can be parsed to a cron schedule
         * @see org.springframework.scheduling.support.CronExpression#parse(String)
         */
        String cron() default "";
    
        /**
         * A time zone for which the cron expression will be resolved. By default, this
         * attribute is the empty String (i.e. the server's local time zone will be used).
         * @return a zone id accepted by {@link java.util.TimeZone#getTimeZone(String)},
         * or an empty String to indicate the server's default time zone
         * @since 4.0
         * @see org.springframework.scheduling.support.CronTrigger#CronTrigger(String, java.util.TimeZone)
         * @see java.util.TimeZone
         */
        String zone() default "";
    
        /**
         * Execute the annotated method with a fixed period between the end of the
         * last invocation and the start of the next.
         * <p>The time unit is milliseconds by default but can be overridden via
         * {@link #timeUnit}.
         * @return the delay
         */
        long fixedDelay() default -1;
    
        /**
         * Execute the annotated method with a fixed period between the end of the
         * last invocation and the start of the next.
         * <p>The time unit is milliseconds by default but can be overridden via
         * {@link #timeUnit}.
         * @return the delay as a String value &mdash; for example, a placeholder
         * or a {@link java.time.Duration#parse java.time.Duration} compliant value
         * @since 3.2.2
         */
        String fixedDelayString() default "";
    
        /**
         * Execute the annotated method with a fixed period between invocations.
         * <p>The time unit is milliseconds by default but can be overridden via
         * {@link #timeUnit}.
         * @return the period
         */
        long fixedRate() default -1;
    
        /**
         * Execute the annotated method with a fixed period between invocations.
         * <p>The time unit is milliseconds by default but can be overridden via
         * {@link #timeUnit}.
         * @return the period as a String value &mdash; for example, a placeholder
         * or a {@link java.time.Duration#parse java.time.Duration} compliant value
         * @since 3.2.2
         */
        String fixedRateString() default "";
    
        /**
         * Number of units of time to delay before the first execution of a
         * {@link #fixedRate} or {@link #fixedDelay} task.
         * <p>The time unit is milliseconds by default but can be overridden via
         * {@link #timeUnit}.
         * @return the initial
         * @since 3.2
         */
        long initialDelay() default -1;
    
        /**
         * Number of units of time to delay before the first execution of a
         * {@link #fixedRate} or {@link #fixedDelay} task.
         * <p>The time unit is milliseconds by default but can be overridden via
         * {@link #timeUnit}.
         * @return the initial delay as a String value &mdash; for example, a placeholder
         * or a {@link java.time.Duration#parse java.time.Duration} compliant value
         * @since 3.2.2
         */
        String initialDelayString() default "";
    
        /**
         * The {@link TimeUnit} to use for {@link #fixedDelay}, {@link #fixedDelayString},
         * {@link #fixedRate}, {@link #fixedRateString}, {@link #initialDelay}, and
         * {@link #initialDelayString}.
         * <p>Defaults to {@link TimeUnit#MICROSECONDS}.
         * <p>This attribute is ignored for {@linkplain #cron() cron expressions}
         * and for {@link java.time.Duration} values supplied via {@link #fixedDelayString},
         * {@link #fixedRateString}, or {@link #initialDelayString}.
         * @return the {@code TimeUnit} to use
         * @since 5.3.10
         */
        TimeUnit timeUnit() default TimeUnit.MILLISECONDS;
    
    }
    

    @Scheduled所支持的参数:

    1. cron:cron表达式,指定任务在特定时间执行;
    2. fixedDelay:表示上一次任务执行完成后多久再次执行,参数类型为long,单位ms;
    3. fixedDelayString:与fixedDelay含义一样,只是参数类型变为String;
    4. fixedRate:表示按一定的频率执行任务,参数类型为long,单位ms;
    5. fixedRateString: 与fixedRate的含义一样,只是将参数类型变为String;
    6. initialDelay:表示延迟多久再第一次执行任务,参数类型为long,单位ms;
    7. initialDelayString:与initialDelay的含义一样,只是将参数类型变为String;
    8. zone:时区,默认为当前时区,一般没有用到。

    获取到方法上Scheduled注解(对任务的定义),通过processScheduled处理具体类型的task

    /**
        * Process the given {@code @Scheduled} method declaration on the given bean.
        * @param scheduled the {@code @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 {
            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);
    
            // Determine initial delay
            long initialDelay = convertToMillis(scheduled.initialDelay(), scheduled.timeUnit());
            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 = convertToMillis(initialDelayString, scheduled.timeUnit());
                    }
                    catch (RuntimeException ex) {
                        throw new IllegalArgumentException(
                                "Invalid initialDelayString value \"" + initialDelayString + "\" - cannot parse into long");
                    }
                }
            }
    
            // Check cron expression
            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;
            }
    
            // Check fixed delay
            long fixedDelay = convertToMillis(scheduled.fixedDelay(), scheduled.timeUnit());
            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 = convertToMillis(fixedDelayString, scheduled.timeUnit());
                    }
                    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
            long fixedRate = convertToMillis(scheduled.fixedRate(), scheduled.timeUnit());
            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 = convertToMillis(fixedRateString, scheduled.timeUnit());
                    }
                    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
            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());
        }
    }
    

    ScheduledTaskRegistrar 中解析task

    以CronTask为例,如果定义了taskScheduler则由taskScheduler执行,如果没有放到unresolvedTasks中。

    /**
        * Schedule the specified cron task, either right away if possible
        * or on initialization of the scheduler.
        * @return a handle to the scheduled task, allowing to cancel it
        * (or {@code null} if processing a previously registered task)
        * @since 4.3
        */
    @Nullable
    public ScheduledTask scheduleCronTask(CronTask task) {
        ScheduledTask scheduledTask = this.unresolvedTasks.remove(task);
        boolean newTask = false;
        if (scheduledTask == null) {
            scheduledTask = new ScheduledTask(task);
            newTask = true;
        }
        if (this.taskScheduler != null) {
            scheduledTask.future = this.taskScheduler.schedule(task.getRunnable(), task.getTrigger());
        }
        else {
            addCronTask(task);
            this.unresolvedTasks.put(task, scheduledTask);
        }
        return (newTask ? scheduledTask : null);
    }
    

    TaskScheduler对Task处理

    默认是ConcurrentTaskScheduler, 处理方法如下

    @Override
    @Nullable
    public ScheduledFuture<?> schedule(Runnable task, Trigger trigger) {
        try {
            if (this.enterpriseConcurrentScheduler) {
                return new EnterpriseConcurrentTriggerScheduler().schedule(decorateTask(task, true), trigger);
            }
            else {
                ErrorHandler errorHandler =
                        (this.errorHandler != null ? this.errorHandler : TaskUtils.getDefaultErrorHandler(true));
                return new ReschedulingRunnable(task, trigger, this.clock, this.scheduledExecutor, errorHandler).schedule();
            }
        }
        catch (RejectedExecutionException ex) {
            throw new TaskRejectedException("Executor [" + this.scheduledExecutor + "] did not accept task: " + task, ex);
        }
    }
    

    EnterpriseConcurrentTriggerScheduler 是 JSR-236 Trigger标准,它的处理方法如下

    /**
        * Delegate that adapts a Spring Trigger to a JSR-236 Trigger.
        * Separated into an inner class in order to avoid a hard dependency on the JSR-236 API.
        */
    private class EnterpriseConcurrentTriggerScheduler {
    
        public ScheduledFuture<?> schedule(Runnable task, final Trigger trigger) {
            ManagedScheduledExecutorService executor = (ManagedScheduledExecutorService) scheduledExecutor;
            return executor.schedule(task, new javax.enterprise.concurrent.Trigger() {
                @Override
                @Nullable
                public Date getNextRunTime(@Nullable LastExecution le, Date taskScheduledTime) {
                    return (trigger.nextExecutionTime(le != null ?
                            new SimpleTriggerContext(le.getScheduledStart(), le.getRunStart(), le.getRunEnd()) :
                            new SimpleTriggerContext()));
                }
                @Override
                public boolean skipRun(LastExecution lastExecution, Date scheduledRunTime) {
                    return false;
                }
            });
        }
    }
    

    如果没有使用EnterpriseConcurrentTriggerScheduler, 则使用ReschedulingRunnable,本质上由ScheduledExecutorService处理

    public ReschedulingRunnable(Runnable delegate, Trigger trigger, Clock clock,
                ScheduledExecutorService executor, ErrorHandler errorHandler) {
    
        super(delegate, errorHandler);
        this.trigger = trigger;
        this.triggerContext = new SimpleTriggerContext(clock);
        this.executor = executor;
    }
    
    @Nullable
    public ScheduledFuture<?> schedule() {
        synchronized (this.triggerContextMonitor) {
            this.scheduledExecutionTime = this.trigger.nextExecutionTime(this.triggerContext);
            if (this.scheduledExecutionTime == null) {
                return null;
            }
            long initialDelay = this.scheduledExecutionTime.getTime() - this.triggerContext.getClock().millis();
            this.currentFuture = this.executor.schedule(this, initialDelay, TimeUnit.MILLISECONDS);
            return this;
        }
    }
    

    示例源码

    https://github.com/realpdai/tech-pdai-spring-demos

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