Spring中@DependsOn注解的作用及实现原理解析

系统管理员 2023-01-16 09:57 86阅读 0赞

**官方文档解释**

> Beans on which the current bean depends. Any beans specified are guaranteed to be created by the container before this bean. Used infrequently in cases where a bean does not explicitly depend on another through properties or constructor arguments, but rather depends on the side effects of another bean's initialization. 
> A depends-on declaration can specify both an initialization-time dependency and, in the case of singleton beans only, a corresponding destruction-time dependency. Dependent beans that define a depends-on relationship with a given bean are destroyed first, prior to the given bean itself being destroyed. Thus, a depends-on declaration can also control shutdown order. 
> May be used on any class directly or indirectly annotated with org.springframework.stereotype.Component or on methods annotated with Bean. 
> Using DependsOn at the class level has no effect unless component-scanning is being used. If a DependsOn-annotated class is declared via XML, DependsOn annotation metadata is ignored, and <bean depends-on="..."/> is respected instead.

## @DependsOn注解的作用 ##

`org.springframework.context.annotation.DependsOn`  
***该注解的属性是一个字符串数组，数组的元素是每个依赖的bean的名称。***

<table> 
 <tbody> 
 <tr> 
 <td> 1 2 3 4 5 6 7 8 </td> 
 <td> <code>@Target</code><code>({ElementType.TYPE, ElementType.METHOD})</code> <code>@Retention</code><code>(RetentionPolicy.RUNTIME)</code> <code>@Documented</code> <code>public</code> <code>@interface</code> <code>DependsOn { </code> &nbsp; <code>&nbsp;&nbsp;&nbsp;&nbsp;</code><code>String[] value() </code><code>default</code> <code>{};</code> &nbsp; <code>}</code> </td> 
 </tr> 
 </tbody> 
</table>

`@DependsOn`注解主要用于指定当前bean所依赖的beans。任何被指定依赖的bean都由Spring保证在当前bean之前创建。在少数情况下，bean不是通过**属性**或**构造函数参数**显式依赖于另一个bean，但却需要要求另一个bean优先完成初始化，则可以使用`@DependsOn`这个注解。

`@DependsOn`既可以指定**初始化依赖顺序**，也可以指定bean相应的**销毁执行顺序**（仅在单例bean的情况下）。

可用于任何直接或间接带`@Component`注解的bean或在用`@Bean`注释的方法上。 
如果使用的是xml配置，则需要使用`<bean dependens on=“…”/>`标签.

**简单描述就是`@DependsOn`可以控制bean的创建、初始化（InitializingBean）、销毁方法执行顺序**。

示例：假如有三个Bean类叫Aaa、Bbb、Ccc分别实现了如下两个接口。  
`org.springframework.beans.factory.InitializingBean`  
`org.springframework.beans.factory.DisposableBean`

Ccc通过@DependsOn指定依赖bean创建的顺序为Bbb > Aaa

<table> 
 <tbody> 
 <tr> 
 <td> 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 </td> 
 <td> <code>@DependsOn</code><code>({ </code><code>"bbb"</code><code>,</code><code>"ccc"</code><code>})</code> <code>@Service</code> <code>public</code> <code>class</code> <code>Aaa </code><code>implements</code> <code>InitializingBean, DisposableBean { </code> <code>&nbsp;</code><code>private</code> <code>static</code> <code>final</code> <code>Logger logger = LoggerFactory.getLogger(Aaa.</code><code>class</code><code>);</code> &nbsp; <code>&nbsp;</code><code>public</code> <code>Aaa() { </code> <code>&nbsp;&nbsp;</code><code>logger.info(</code><code>this</code><code>.getClass().getName() + </code><code>" Construction"</code><code>);</code> <code>&nbsp;</code><code>}</code> &nbsp; <code>&nbsp;</code><code>@Override</code> <code>&nbsp;</code><code>public</code> <code>void</code> <code>afterPropertiesSet() </code><code>throws</code> <code>Exception { </code> <code>&nbsp;&nbsp;</code><code>logger.info(</code><code>this</code><code>.getClass().getName() + </code><code>" afterPropertiesSet"</code><code>);</code> <code>&nbsp;</code><code>}</code> &nbsp; <code>&nbsp;</code><code>@Override</code> <code>&nbsp;</code><code>public</code> <code>void</code> <code>destroy() </code><code>throws</code> <code>Exception { </code> <code>&nbsp;&nbsp;</code><code>logger.info(</code><code>this</code><code>.getClass().getName() + </code><code>" destroy"</code><code>);</code> <code>&nbsp;</code><code>}</code> <code>}</code> </td> 
 </tr> 
 </tbody> 
</table>

Bbb Ccc类实现如下

<table> 
 <tbody> 
 <tr> 
 <td> 1 2 3 4 5 6 7 8 </td> 
 <td> <code>@Service</code> <code>public</code> <code>class</code> <code>Bbb </code><code>implements</code> <code>InitializingBean, DisposableBean { </code> <code>&nbsp;</code><code>//实现和Aaa相同</code> <code>}</code> <code>@Service</code> <code>public</code> <code>class</code> <code>Ccc </code><code>implements</code> <code>InitializingBean, DisposableBean { </code> <code>&nbsp;</code><code>//实现和Aaa相同</code> <code>}</code> </td> 
 </tr> 
 </tbody> 
</table>

那么初始顺序如下: `bbb --> ccc --> aaa`

![在这里插入图片描述][5ab2534ddbe7f0f6dcd4e5513080cfbe.png]

而销毁方法执行顺序正好相反如下: `aaa --> ccc --> bbb`

![在这里插入图片描述][57fd7d93527695f53c30b510836d5632.png]

## @DependsOn注解的实现原理 ##

Spring在启动时扫描到一个bean，会封装成一个BeanDefinition，如果是`AnnotatedBeanDefinition`则解析类上的注解信息，发现`@DependsOn`注解，则读取value值，调用`BeanDefinition#setDependsOn`保存。  
源码见`ClassPathBeanDefinitionScanner#doScan`，`AnnotationConfigUtils#processCommonDefinitionAnnotations(AnnotatedBeanDefinition, AnnotatedTypeMetadata)`

创建bean时，也就是调用`AbstractBeanFactory#doGetBean`时，会获取这些被依赖的**beanName**，按照数组顺序，再调用`AbstractBeanFactory#getBean(beanName)`来优先创建被依赖的bean，从而达到控制依赖顺序。

除此之外，在创建bean时，还会调用`AbstractBeanFactory#registerDisposableBeanIfNecessary`来向Spring中注册带有销毁方法的bean，源码见`DefaultSingletonBeanRegistry#registerDisposableBean`，内部通过LinkedHashMap保存。key为bean名称。进程退出时，会逆序调用销毁方法。  
源码见`DefaultSingletonBeanRegistry#destroySingletons`

<table> 
 <tbody> 
 <tr> 
 <td> 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 </td> 
 <td> <code>public</code> <code>void</code> <code>destroySingletons() { </code> <code>&nbsp;&nbsp;&nbsp;&nbsp;</code><code>if</code> <code>(logger.isTraceEnabled()) { </code> <code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code><code>logger.trace(</code><code>"Destroying singletons in "</code> <code>+ </code><code>this</code><code>);</code> <code>&nbsp;&nbsp;&nbsp;&nbsp;</code><code>}</code> <code>&nbsp;&nbsp;&nbsp;&nbsp;</code><code>synchronized</code> <code>(</code><code>this</code><code>.singletonObjects) { </code> <code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code><code>this</code><code>.singletonsCurrentlyInDestruction = </code><code>true</code><code>;</code> <code>&nbsp;&nbsp;&nbsp;&nbsp;</code><code>}</code> &nbsp; <code>&nbsp;&nbsp;&nbsp;&nbsp;</code><code>String[] disposableBeanNames;</code> <code>&nbsp;&nbsp;&nbsp;&nbsp;</code><code>synchronized</code> <code>(</code><code>this</code><code>.disposableBeans) { </code> <code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code><code>disposableBeanNames = StringUtils.toStringArray(</code><code>this</code><code>.disposableBeans.keySet());</code> <code>&nbsp;&nbsp;&nbsp;&nbsp;</code><code>}</code> <code>&nbsp;&nbsp;&nbsp;&nbsp;</code><code>for</code> <code>(</code><code>int</code> <code>i = disposableBeanNames.length - </code><code>1</code><code>; i &gt;= </code><code>0</code><code>; i--) { </code> <code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code><code>destroySingleton(disposableBeanNames[i]);</code> <code>&nbsp;&nbsp;&nbsp;&nbsp;</code><code>}</code> &nbsp; <code>&nbsp;&nbsp;&nbsp;&nbsp;</code><code>this</code><code>.containedBeanMap.clear();</code> <code>&nbsp;&nbsp;&nbsp;&nbsp;</code><code>this</code><code>.dependentBeanMap.clear();</code> <code>&nbsp;&nbsp;&nbsp;&nbsp;</code><code>this</code><code>.dependenciesForBeanMap.clear();</code> &nbsp; <code>&nbsp;&nbsp;&nbsp;&nbsp;</code><code>clearSingletonCache();</code> <code>}</code> </td> 
 </tr> 
 </tbody> 
</table>

[5ab2534ddbe7f0f6dcd4e5513080cfbe.png]: /images/20221022/f93c323c4c52426b882f228b8e9decba.png
[57fd7d93527695f53c30b510836d5632.png]: /images/20221022/8ed1d8da9e6049e8ada46929a01032df.png