背景
最近开始做个小项目，用到了Spring Boot，在看前辈书写的代码时，发现其中有许多注解是我之前从来没接触过的。于是决定每天抽点时间，学习总结一下这些陌生的注解，让以后的工作更加有效率。
在Spring Boot 的入口类 XXXApplication 中,必然会有@SpringBootApplication，用来标注项目入口，以及完成一些基本的自动自动配置。但是，在这次项目中，XXXApplication 类中除了@SpringBootApplication注解，还用到了另一个注解，也就是下面要学习的@EnableScheduling注解。
分析
在没有开始学习该注解相关的源码之前，可以从其名称先分析一下作用。
Enable，能够、使...可以。
Schedule, 任务计划、日程。ing形式也就是取其动词形式的语义。
那么也就是说这个注解是用来使计划任务功能可以使用的注解。
那么，新的问题就来了，对于没有接触过 Spring 中“计划任务”的我而言，计划任务又是什么呢？
下来，就可以带着问题去看看这个注解的定义类了。
问题：
- 这个注解的功能是不是如分析的一样，是启动某种功能的注解？
- 关于 “计划任务” 的定义是什么？
话不多说，直接上定义类源码：
/*
* Copyright 2002-2016 the original author or authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.springframework.scheduling.annotation;
import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import java.util.concurrent.Executor;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Import;
import org.springframework.scheduling.Trigger;
import org.springframework.scheduling.config.ScheduledTaskRegistrar;
/**
* Enables Spring's scheduled task execution capability, similar to
* functionality found in Spring's {@code <task:*>} XML namespace. To be used
* on @{@link Configuration} classes as follows:
*
* <pre class="code">
* @Configuration
* @EnableScheduling
* public class AppConfig {
*
* // various @Bean definitions
* }</pre>
*
* This enables detection of @{@link Scheduled} annotations on any Spring-managed
* bean in the container. For example, given a class {@code MyTask}
*
* <pre class="code">
* package com.myco.tasks;
*
* public class MyTask {
*
* @Scheduled(fixedRate=1000)
* public void work() {
* // task execution logic
* }
* }</pre>
*
* the following configuration would ensure that {@code MyTask.work()} is called
* once every 1000 ms:
*
* <pre class="code">
* @Configuration
* @EnableScheduling
* public class AppConfig {
*
* @Bean
* public MyTask task() {
* return new MyTask();
* }
* }</pre>
*
* Alternatively, if {@code MyTask} were annotated with {@code @Component}, the
* following configuration would ensure that its {@code @Scheduled} method is
* invoked at the desired interval:
*
* <pre class="code">
* @Configuration
* @EnableScheduling
* @ComponentScan(basePackages="com.myco.tasks")
* public class AppConfig {
* }</pre>
*
* Methods annotated with {@code @Scheduled} may even be declared directly within
* {@code @Configuration} classes:
*
* <pre class="code">
* @Configuration
* @EnableScheduling
* public class AppConfig {
*
* @Scheduled(fixedRate=1000)
* public void work() {
* // task execution logic
* }
* }</pre>
*
* <p>By default, will be searching for an associated scheduler definition: either
* a unique {@link org.springframework.scheduling.TaskScheduler} bean in the context,
* or a {@code TaskScheduler} bean named "taskScheduler" otherwise; the same lookup
* will also be performed for a {@link java.util.concurrent.ScheduledExecutorService}
* bean. If neither of the two is resolvable, a local single-threaded default
* scheduler will be created and used within the registrar.
*
* <p>When more control is desired, a {@code @Configuration} class may implement
* {@link SchedulingConfigurer}. This allows access to the underlying
* {@link ScheduledTaskRegistrar} instance. For example, the following example
* demonstrates how to customize the {@link Executor} used to execute scheduled
* tasks:
*
* <pre class="code">
* @Configuration
* @EnableScheduling
* public class AppConfig implements SchedulingConfigurer {
*
* @Override
* public void configureTasks(ScheduledTaskRegistrar taskRegistrar) {
* taskRegistrar.setScheduler(taskExecutor());
* }
*
* @Bean(destroyMethod="shutdown")
* public Executor taskExecutor() {
* return Executors.newScheduledThreadPool(100);
* }
* }</pre>
*
* <p>Note in the example above the use of {@code @Bean(destroyMethod="shutdown")}.
* This ensures that the task executor is properly shut down when the Spring
* application context itself is closed.
*
* <p>Implementing {@code SchedulingConfigurer} also allows for fine-grained
* control over task registration via the {@code ScheduledTaskRegistrar}.
* For example, the following configures the execution of a particular bean
* method per a custom {@code Trigger} implementation:
*
* <pre class="code">
* @Configuration
* @EnableScheduling
* public class AppConfig implements SchedulingConfigurer {
*
* @Override
* public void configureTasks(ScheduledTaskRegistrar taskRegistrar) {
* taskRegistrar.setScheduler(taskScheduler());
* taskRegistrar.addTriggerTask(
* new Runnable() {
* public void run() {
* myTask().work();
* }
* },
* new CustomTrigger()
* );
* }
*
* @Bean(destroyMethod="shutdown")
* public Executor taskScheduler() {
* return Executors.newScheduledThreadPool(42);
* }
*
* @Bean
* public MyTask myTask() {
* return new MyTask();
* }
* }</pre>
*
* <p>For reference, the example above can be compared to the following Spring XML
* configuration:
*
* <pre class="code">
* {@code
* <beans>
*
* <task:annotation-driven scheduler="taskScheduler"/>
*
* <task:scheduler id="taskScheduler" pool-size="42"/>
*
* <task:scheduled-tasks scheduler="taskScheduler">
* <task:scheduled ref="myTask" method="work" fixed-rate="1000"/>
* </task:scheduled-tasks>
*
* <bean id="myTask" class="com.foo.MyTask"/>
*
* </beans>
* }</pre>
*
* The examples are equivalent save that in XML a <em>fixed-rate</em> period is used
* instead of a custom <em>{@code Trigger}</em> implementation; this is because the
* {@code task:} namespace {@code scheduled} cannot easily expose such support. This is
* but one demonstration how the code-based approach allows for maximum configurability
* through direct access to actual componentry.<p>
*
* @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 {
}
从注释的第一句话就可以解答第一个问题，这个注解确实是启动前面所说的“计划任务”功能的，类似于用XML配置 Spring 时的 task 标签的作用。
可是这个执行“计划任务”的功能又是什么呢？
范例下面有解释：
当开发者在AppConfig类中使用了本注解，并在某个Task类中使用了@Schedule注解，那么被@Schedule注解的标注的方法就可以在指定时间自动执行。
那么这个“计划任务”其实被称为“定时任务”更为合适。
总结
关于@EnableScheduling，其实就是用来使@Schedule注解功能可用的注解。在 Spring Boot 的配置类中，标注上这个注解，就可以对项目中的方法某些方法使用@Schedule注解，将其变为定时自动执行。
但只有两种注解共同使用时，才能达到本注解应有的作用。
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