@SpringBootApplication解析

迷南。 2022-04-14 04:19 365阅读 0赞

我们经常写的@SpringBootApplication 是什么意思呢？\*  
@SpringBootApplication 源码点开瞟一眼：  
![fgf][]

》我们发现@SpringBootApplication 中包含  
@SpringBootConfiguration、@ EnableAutoConfiguration、@ ComponentScan ，这一个注解代替了这三个注解； \*

>  *  @Configuration 和 @bean 注入定义了一个实体类；
>  *  @EnableAutoConfiguration 启用了上下文自动配置；
>  *  @ComponentScan 扫描指定的包，若未指定值，默认扫包范围：被注释的 class所在的包；\*

》1、继续查看@SpringBootConfiguration  
![sdfd][]

> @Configuration注解
> 
>  *  声明了这是一个配置类，而springboot 会通过@ComponentScan 扫描添加  
>     带有@Configuration注解的配置类，并读取配置类的信息；
> 
> @SpringBootConfiguration注解
> 
>  *  是作用于 springboot应用的，@SpringBootConfiguration中可以添加N个@Configuration注解的配置类，但是对于一个springboot项目，只能存在一个@SpringBootConfiguration注解；

》2、查看@EnableAutoConfiguration  
![注解][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MjMyMzgwMg_size_16_color_FFFFFF_t_70]

> 2.1、能完成自动配置，主要是  
> @Inherited注解、@ConditionalOnMissingBean注解等
> 
>  *  @ConditionalOnMissingBean注解生效，其上有@Conditional注解；
>  *  @EnableAutoConfiguration注解上存在@Import(AutoConfigurationImportSelector.class)自动导配置类的包，可以把所有带有@Configuration的类的包导入；

附上一段注释，说只要依靠@ConditionalOnMissingBean注解生效自动配置；  
当missing 某一个bean时候，满足自动配置的条件，@EnableAutoConfiguration 会生效；

Auto-configuration classes are regular Spring { @link Configuration} beans. They are
     * located using the { @link SpringFactoriesLoader} mechanism (keyed against this class).
     * Generally auto-configuration beans are { @link Conditional @Conditional} beans (most
     * often using { @link ConditionalOnClass @ConditionalOnClass} and
     * { @link ConditionalOnMissingBean @ConditionalOnMissingBean} annotations).

> 2.2、跟进@ConditionalOnMissingBean  
> @ConditionalOnMissingBean中有@Conditional注解,该注解意思是条件匹配；  
> 若满足@Conditional要求的条件，才会被注册到组件中；当我们自定义一些配置时候，就会默认配置就会失效；因为这个bean 已经存在了；

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MjMyMzgwMg_size_16_color_FFFFFF_t_70 1]

> 2.3、  
> @Inherited注解

![sdgdgh][]

》会通过注解超类来找依赖中的类，完成自动初始化类加载

* Indicates that an annotation type is automatically inherited.  If
     * an Inherited meta-annotation is present on an annotation type
     * declaration, and the user queries the annotation type on a class
     * declaration, and the class declaration has no annotation for this type,
     * then the class's superclass will automatically be queried for the
     * annotation type.  This process will be repeated until an annotation for this
     * type is found, or the top of the class hierarchy (Object)
     * is reached.  If no superclass has an annotation for this type, then
     * the query will indicate that the class in question has no such annotation.
     *
     * <p>Note that this meta-annotation type has no effect if the annotated
     * type is used to annotate anything other than a class.  Note also
     * that this meta-annotation only causes annotations to be inherited
     * from superclasses; annotations on implemented interfaces have no
     * effect.

》3、@ComponentScan如何扫包的  
![56][]

》贴上一段注释,第二段意思是，如没有指明要扫的包，会从当前包路径向下扫包；  
所以，main函数放在顶层；

* Configures component scanning directives for use with @{ @link Configuration} classes.
     * Provides support parallel with Spring XML's { @code <context:component-scan>} element.
     *
     * <p>Either { @link #basePackageClasses} or {@link #basePackages} (or its alias
     * { @link #value}) may be specified to define specific packages to scan. If specific
     * packages are not defined, scanning will occur from the package of the
     * class that declares this annotation.

》小结：  
@SpringBootApplication 中包含  
@SpringBootConfiguration、@ EnableAutoConfiguration、@ ComponentScan

@ EnableAutoConfiguration

[fgf]: /images/20220414/fd21ad66fd6541f7942ddbf5b8e8d149.png
[sdfd]: /images/20220414/edc39fdc0f434cebbad398599e4457af.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MjMyMzgwMg_size_16_color_FFFFFF_t_70]: /images/20220414/e6fbdc19cc954bd0a4be5662bc393f2f.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MjMyMzgwMg_size_16_color_FFFFFF_t_70 1]: /images/20220414/648e4696eb7240899e5c28e6da9069c5.png
[sdgdgh]: /images/20220414/9593599d9a3f4f10b9c5aee8c55a500b.png
[56]: /images/20220414/a852d5beb0f44ee4a3f9a5e08d509233.png