概述

在SpringBoot中可以通過@Scheduled來注解定義一個定時任務，但是有時候你可能發(fā)現(xiàn)有的定時任務道理時間卻沒有執(zhí)行，但是又不是每次都不執(zhí)行，為什么呢？？？

舉例說明

下面這段diam定義了一個沒隔10s執(zhí)行一次的定時任務：

package com.study.practice.schedule;

import lombok.extern.slf4j.Slf4j;
import org.springframework.scheduling.annotation.Scheduled;
import org.springframework.stereotype.Component;

/**
?* @Description : schedule測試
?* @Version : V1.0.0
?* @Date : 2021/12/1 11:22
?*/
@Component
@Slf4j
public class ScheduleTest {
? ? @Scheduled(cron = "0/10 * * * * ?")
? ? public void execute() {
? ? ? ? log.info("Scheduled task is running ... ...");
? ? }
}

記得在啟動類或者Configuration類上添加了@EnableScheduling注解。

啟動應用，控制臺每隔10秒打印一條日志：

2021-12-01 11:27:00.002  INFO 97876 --- [   scheduling-1] c.study.practice.schedule.ScheduleTest   : Scheduled task is running ... ...
2021-12-01 11:27:10.001  INFO 97876 --- [   scheduling-1] c.study.practice.schedule.ScheduleTest   : Scheduled task is running ... ...
2021-12-01 11:27:20.002  INFO 97876 --- [   scheduling-1] c.study.practice.schedule.ScheduleTest   : Scheduled task is running ... ...
2021-12-01 11:27:30.001  INFO 97876 --- [   scheduling-1] c.study.practice.schedule.ScheduleTest   : Scheduled task is running ... ...
2021-12-01 11:27:40.002  INFO 97876 --- [   scheduling-1] c.study.practice.schedule.ScheduleTest   : Scheduled task is running ... ...
2021-12-01 11:27:50.001  INFO 97876 --- [   scheduling-1] c.study.practice.schedule.ScheduleTest   : Scheduled task is running ... ...
2021-12-01 11:28:00.002  INFO 97876 --- [   scheduling-1] c.study.practice.schedule.ScheduleTest   : Scheduled task is running ... ...
2021-12-01 11:28:10.002  INFO 97876 --- [   scheduling-1] c.study.practice.schedule.ScheduleTest   : Scheduled task is running ... ...

但是，問題才剛剛開始。

在上面的相關(guān)代碼中，我們使用cron表達式指定的定時任務執(zhí)行時間點從0秒開始，每隔10s執(zhí)行一次，現(xiàn)在我們再加一個定時任務：

package com.study.practice.schedule;

import lombok.extern.slf4j.Slf4j;
import org.springframework.scheduling.annotation.Scheduled;
import org.springframework.stereotype.Component;

import java.util.concurrent.TimeUnit;

/**
?* @Description : schedule測試
?* @Version : V1.0.0
?* @Date : 2021/12/1 11:22
?*/
@Component
@Slf4j
public class ScheduleTestTwo {
? ? @Scheduled(cron = "0/10 * * * * *")
? ? public void second() throws InterruptedException {
? ? ? ? log.info("Second scheduled task is running... ...");
? ? ? ? TimeUnit.SECONDS.sleep(5);
? ? }
}

啟動項目，執(zhí)行結(jié)果如下：

2021-12-01 11:36:30.001  INFO 134576 --- [   scheduling-1] c.study.practice.schedule.ScheduleTest   : Scheduled task is running ... ...
2021-12-01 11:36:30.002  INFO 134576 --- [   scheduling-1] c.s.practice.schedule.ScheduleTestTwo    : Second scheduled task is running... ...
2021-12-01 11:36:40.002  INFO 134576 --- [   scheduling-1] c.s.practice.schedule.ScheduleTestTwo    : Second scheduled task is running... ...
2021-12-01 11:36:45.002  INFO 134576 --- [   scheduling-1] c.study.practice.schedule.ScheduleTest   : Scheduled task is running ... ...
2021-12-01 11:36:50.001  INFO 134576 --- [   scheduling-1] c.s.practice.schedule.ScheduleTestTwo    : Second scheduled task is running... ...
2021-12-01 11:36:55.002  INFO 134576 --- [   scheduling-1] c.study.practice.schedule.ScheduleTest   : Scheduled task is running ... ...
2021-12-01 11:37:00.001  INFO 134576 --- [   scheduling-1] c.s.practice.schedule.ScheduleTestTwo    : Second scheduled task is running... ...
2021-12-01 11:37:05.001  INFO 134576 --- [   scheduling-1] c.study.practice.schedule.ScheduleTest   : Scheduled task is running ... ...

可以看到任務1和任務2本該都是每個10s執(zhí)行，但是卻發(fā)現(xiàn)只有任務2執(zhí)行了，任務1卻等待了5s之后才執(zhí)行。

原因分析

為了找到原因，我們從@Scheduled注解的源碼開始分析：

下面是@Scheduled注解的注釋：

?*
?* <p>Processing of {@code @Scheduled} annotations is performed by
?* registering a {@link ScheduledAnnotationBeanPostProcessor}. This can be
?* done manually or, more conveniently, through the {@code <task:annotation-driven/>}
?* element or @{@link EnableScheduling} annotation.
?*

劃重點, 每一個有@Scheduled注解的方法都會被注冊為一個ScheduledAnnotationBeanPostProcessor, 再接著往下看ScheduledAnnotationBeanPostProcessor:

?? ?/**
?? ? * Set the {@link org.springframework.scheduling.TaskScheduler} that will invoke
?? ? * the scheduled methods, or a {@link java.util.concurrent.ScheduledExecutorService}
?? ? * to be wrapped as a TaskScheduler.
?? ? * <p>If not specified, default scheduler resolution will apply: searching for a
?? ? * unique {@link TaskScheduler} bean in the context, or for a {@link TaskScheduler}
?? ? * bean named "taskScheduler" otherwise; the same lookup will also be performed for
?? ? * a {@link ScheduledExecutorService} bean. If neither of the two is resolvable,
?? ? * a local single-threaded default scheduler will be created within the registrar.
?? ? * @see #DEFAULT_TASK_SCHEDULER_BEAN_NAME
?? ? */
?? ?public void setScheduler(Object scheduler) {
?? ??? ?this.scheduler = scheduler;
?? ?}

重點來了，注意這句話：

if neither of two is resolvable, a local single-thread default scheduler will be created within in the registrar.

這句話意味著，如果我們不主動配置我們需要的TaskScheduler，SpringBoot會默認使用一個單線程的scheduler來處理我們用@Scheduled注解實現(xiàn)的定時任務，到此我們剛才的問題就可以理解了。

因為是單個線程執(zhí)行所有的定時任務，所有task2如果先執(zhí)行，因為執(zhí)行中等待了5s，所以task2執(zhí)行完后，task1接著繼續(xù)執(zhí)行。

解決方案

搞清楚這個流程后，解決這個問題就很簡單了。

根據(jù)剛才注釋的描述，我們只需要提供一個滿足自己需要的TaskScheduler并注冊到context容器中就可以了。

package com.study.practice.config;

import org.springframework.context.annotation.Configuration;
import org.springframework.scheduling.annotation.SchedulingConfigurer;
import org.springframework.scheduling.concurrent.ThreadPoolTaskScheduler;
import org.springframework.scheduling.config.ScheduledTaskRegistrar;

/**
?* @Description : scheduler配置類
?* @Version : V1.0.0
?* @Date : 2021/12/1 14:00
?*/
@Configuration
public class ScheduledTaskConfig implements SchedulingConfigurer {
? ? @Override
? ? public void configureTasks(ScheduledTaskRegistrar taskRegistrar) {
? ? ? ? ThreadPoolTaskScheduler taskScheduler = new ThreadPoolTaskScheduler();
? ? ? ? taskScheduler.setPoolSize(2);
? ? ? ? taskScheduler.initialize();
? ? ? ? taskRegistrar.setTaskScheduler(taskScheduler);
? ? }
}

上面的代碼提供了一個線程池大小為2的taskScheduler，再次啟動SpringBoot查看效果。

2021-12-01 14:05:20.001  INFO 39768 --- [TaskScheduler-2] c.s.practice.schedule.ScheduleTestTwo    : Second scheduled task is running... ...
2021-12-01 14:05:20.001  INFO 39768 --- [TaskScheduler-1] c.study.practice.schedule.ScheduleTest   : Scheduled task is running ... ...
2021-12-01 14:05:30.002  INFO 39768 --- [TaskScheduler-1] c.study.practice.schedule.ScheduleTest   : Scheduled task is running ... ...
2021-12-01 14:05:30.002  INFO 39768 --- [TaskScheduler-2] c.s.practice.schedule.ScheduleTestTwo    : Second scheduled task is running... ...
2021-12-01 14:05:40.001  INFO 39768 --- [TaskScheduler-1] c.study.practice.schedule.ScheduleTest   : Scheduled task is running ... ...
2021-12-01 14:05:40.001  INFO 39768 --- [TaskScheduler-2] c.s.practice.schedule.ScheduleTestTwo    : Second scheduled task is running... ...

可以看到，當線程池里有兩個線程時，這兩個任務各種按照預定的時間進行觸發(fā)，互不影響了。

以上為個人經(jīng)驗，希望能給大家一個參考，也希望大家多多支持腳本之家。

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