在工程中时常会遇到一些需求，例如定时刷新一下配置、隔一段时间检查下网络状态并发送邮件等诸如此类的定时任务。

定时任务本质就是一个异步的线程，线程可以查询或修改并执行一系列的操作。由于本质是线程，在 Java 中可以自行编写一个线程池对定时任务进行控制，但这样效率太低了，且功能有限，属于重复造轮子。

事实上，当前实现定时任务已经有了比较好的解决方案，大致有以下几种：

1. Spring Scheduler 框架
2. Quartz 框架，功能强大，配置灵活（自然更繁琐 =。=）

本文将总结 Spring 定时任务。Let's Begin

Spring Scheduler

Spring 于 3.0 版本引入了 TaskScheduler，相比较于 Spring 2.0 时的 TaskExecutor，简化了对线程的管理，线程均由框架管理，不需要指定调度器 scheduler 实现，可以自定义调度的间隔和时间，相对的对线程池等的功能较简单。

<u>TaskExecutor 与 TaskScheduler 的区别</u>

<u>Scheduler 只是任务的简单调度，可以指定任务的执行时间，但对任务队列和线程池的管控较弱，定时调度的主要目的还是控制执行时间。</u>

<u> Executor 则提供了更细化线程池配置，如等待队列容量、存活时间控制，也支持异步执行任务。TaskExecutor 可以理解为是 Spring 框架下的线程池管理，是从 JDK 5 中对 Executor 接口的抽离，主要职责并不在于几点几分几秒去执行什么任务。</u>

定时调度实现依赖于 spring-context 包。因此 Maven 中需加入以下依赖配置。

<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-context</artifactId>
    <version>${spring.version}</version>
</dependency>

与定时任务相关的代码，位于包 org.springframework.scheduling 下。源码并不复杂，可以花点时间读一读。

<br />

Spring TaskScheduler实践

作为 Spring 下的框架，一些 Spring 的基本配置就此略过。

XML 配置

1. 定义 Task 类

@Lazy
@Service
public class DemoTask {
    public void job1() {
        System.out.println("Task job1: " + System.currentTimeMillis());
    }

    public void job2() {
        System.out.println("Task job2: " + System.currentTimeMillis());
    }
}

其中 @Lazy 注解表明不执行 Bean 的初始化。

2. XML 配置

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:task="http://www.springframework.org/schema/task"
       xsi:schemaLocation="http://www.springframework.org/schema/task http://www.springframework.org/schema/task/spring-task.xsd
        http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">

    <task:scheduler id="demoTaskBean" pool-size="2"/>
    <task:scheduled-tasks scheduler="demoTaskBean">
        <task:scheduled ref="demoTask" method="job1" fixed-rate="100" initial-delay="200" cron="" fixed-delay="300"/>
        <task:scheduled ref="demoTask" method="job2" cron="*/5 * * * * ?"/>
    </task:scheduled-tasks>
</beans>

XML 的配置，定义一个 scheduler，然后定义具体任务 scheduled-task。

1. task:scheduler 定义一个 ThreadPoolTaskScheduler, 提供唯一参数指定了池大小。
   • pool-size: 任务池大小。池的大小控制着 task:scheduled 的执行，例如以上例子，若 pool-size = 1，则两个 task 只能依次运行，无法并行执行。

2. task:scheduled-tasks ：指定所采用的任务池

   • scheduler: 定义的任务池 Bean 的 ID，若不指定，则会被包装为一个单线程 Executor
   • task:scheduled : 指定具体任务，至少有一个，其参数如下表

cron 表达式请参考附录

注解配置

1. 定义 Task 类

@Service
public class DemoTask {
    @Scheduled(cron = "5 * * * * ?")
    public void job1() {
        System.out.println("Task job1: " + System.currentTimeMillis());
    }

    @Scheduled(initialDelay = 100, fixedDelay = 1000)
    public void job2() {
        System.out.println("Task job2: " + System.currentTimeMillis());
    }
}

2. 添加注解驱动配置

<task:annotation-driven />

更多细节，建议查看 xsd 文件和 Spring 源码

Scheduling 框架源码分析

Scheduleing 部分的源码是比较清晰和简单的。

Scheduleing 中最重要的类是 ThreadPoolTaskScheduler ，这是使用 schedule 时的默认的线程池，其类图如下：

image

在 ThreadPoolTaskScheduler 中定义了一些内部变量

// ScheduledThreadPoolExecutor.setRemoveOnCancelPolicy(boolean) only available on JDK 7+
private static final boolean setRemoveOnCancelPolicyAvailable =
    ClassUtils.hasMethod(ScheduledThreadPoolExecutor.class, "setRemoveOnCancelPolicy", boolean.class);

private volatile int poolSize = 1;
private volatile boolean removeOnCancelPolicy = false;
private volatile ScheduledExecutorService scheduledExecutor;
private volatile ErrorHandler errorHandler;

需要关注的是 ScheduledExecutorService scheduledExecutor，这是一个继承了 ExecutorService 接口的一个接口，是在 ExecutorService 基础上新增了定时调度的几个方法，scheduledExecutor 的具体实现是 java.util.concurrent.ScheduledThreadPoolExecutor，具体可以查看 JDK 源码。

除了定义 Executor 类，还实现了AsyncListenableTaskExecutor,SchedulingTaskExecutor, TaskScheduler 三个接口的相关方法，此处不再赘述。

存在的问题

1. 注意线程池和任务数的关系

   在实践中，若出现线程池数小于任务数时，任务将进入队列，按照配置中的顺序执行。此时，若对任务的执行时间有严格要求的话，将无法被满足。例如定义了两个任务均需要在整分钟时执行，任务执行分别需要花费 20s 和 30s。那么在一个线程池下，将先执行第一个 20s 的任务，完成后再执行第二个 30s 的任务。

2. 注意 fixedDelay 和 fixedRate 之间的区别。前者是任务完成后计时，后者是从任务开始时计时。如果每分钟执行一次任务，但任务执行时间大于一分钟的话，fixedRate 将会出现任务在队列中的堆积的问题

3. 多实例执行的问题。由于 Spring Task 是集成与 Spring 框架之内的，若部署采取了多实例部署方案，并设计了定时调度单一资源的任务，如定时刷新 Redis 数据，定时更新 MySQL 数据等等，将会出现问题。解决此问题，一是放弃实例中使用 Spring Task，将定时调度单独部署与一台机器，但这种方案的缺点是增加了维护的成本而且造成了单点故障的问题；二是采用锁机制，一般而言可以将锁至于缓存数据库中，每次定时任务执行时，先请求锁资源，若失败，则不执行，只有请求到锁资源的定时任务才执行。

进阶学学 Quartz

// TBD

总结

Scheduling 是专注于定时任务调度的框架，但 Spring Task 不仅仅是这一块。事实上，Task 部分更多阐述了 TaskExecutor 的内容，这也是 Scheduling 所依赖的。

并发、线程池、异步等等，均在 TaskExecutor 中做了详细的说明，这一块需要在之后的学习中做进一步的总结。

附录

Cron 表达式 (摘自 Quartz)

因此 cron 表达式可以是 6-7 位。

特殊字符

• * (“all values”) - used to select all values within a field. For example, * in the minute field means “every minute”.
• ? (“no specific value”) - useful when you need to specify something in one of the two fields in which the character is allowed, but not the other. For example, if I want my trigger to fire on a particular day of the month (say, the 10th), but don’t care what day of the week that happens to be, I would put 10 in the day-of-month field, and ? in the day-of-week field. See the examples below for clarification.
• - - used to specify ranges. For example, 10-12 in the hour field means “the hours 10, 11 and 12”.
• , - used to specify additional values. For example, MON,WED,FRI in the day-of-week field means “the days Monday, Wednesday, and Friday”.
• / - used to specify increments. For example, 0/15 in the seconds field means “the seconds 0, 15, 30, and 45”. And 5/15 in the seconds field means “the seconds 5, 20, 35, and 50”. You can also specify / after the * character - in this case * is equivalent to having 0 before the /. 1/3 in the day-of-month field means “fire every 3 days starting on the first day of the month”.
• L (“last”) - has different meaning in each of the two fields in which it is allowed. For example, the value L in the day-of-month field means “the last day of the month” - day 31 for January, day 28 for February on non-leap years. If used in the day-of-week field by itself, it simply means 7 or SAT. But if used in the day-of-week field after another value, it means “the last xxx day of the month” - for example 6L means “the last friday of the month”. You can also specify an offset from the last day of the month, such as L-3 which would mean the third-to-last day of the calendar month. When using the L option, it is important not to specify lists, or ranges of values, as you’ll get confusing/unexpected results.
• W (“weekday”) - used to specify the weekday (Monday-Friday) nearest the given day. As an example, if you were to specify 15W as the value for the day-of-month field, the meaning is: “the nearest weekday to the 15th of the month”. So if the 15th is a Saturday, the trigger will fire on Friday the 14th. If the 15th is a Sunday, the trigger will fire on Monday the 16th. If the 15th is a Tuesday, then it will fire on Tuesday the 15th. However if you specify 1W as the value for day-of-month, and the 1st is a Saturday, the trigger will fire on Monday the 3rd, as it will not ‘jump’ over the boundary of a month’s days. The W character can only be specified when the day-of-month is a single day, not a range or list of days.

The L and W characters can also be combined in the day-of-month field to yield LW, which translates to "last weekday of the month".

• # - used to specify “the nth” XXX day of the month. For example, the value of 6#3 in the day-of-week field means “the third Friday of the month” (day 6 = Friday and #3 = the 3rd one in the month). Other examples: 2#1 = the first Monday of the month and 4#5 = the fifth Wednesday of the month. Note that if you specify #5 and there is not 5 of the given day-of-week in the month, then no firing will occur that month.

The legal characters and the names of months and days of the week are not case sensitive. MON is the same as mon.

举例

以下是摘自 Quartz 的完整例子

Pay attention to the effects of '?' and '*' in the day-of-week and day-of-month fields!

参考资料

[1] Spring docs, Task Execution and Scheduling. https://docs.spring.io/autorepo/docs/spring-framework/4.2.x/spring-framework-reference/html/scheduling.html

[2] Spring ThreadPoolTaskScheduler vs ThreadPoolTaskExecutor
https://stackoverflow.com/questions/33453722/spring-threadpooltaskscheduler-vs-threadpooltaskexecutor
[3] Cron Trigger Tutorial， http://www.quartz-scheduler.org/documentation/quartz-2.2.x/tutorials/crontrigger