我們經常寫的@SpringBootApplication 是什麼意思呢？*
@SpringBootApplication 源碼點開瞟一眼：
》我們發現@SpringBootApplication 中包含
@SpringBootConfiguration、@ EnableAutoConfiguration、@ ComponentScan ，這一個注解代替了這三個注解； *
- @Configuration 和 @bean 注入定義了一個實體類；
- @EnableAutoConfiguration 啟用了上下文自動配置；
- @ComponentScan 掃描指定的包，若未指定值，預設掃包範圍：被注釋的 class所在的包；*
》1、繼續檢視@SpringBootConfiguration
@Configuration注解@SpringBootConfiguration注解
聲明了這是一個配置類，而springboot 會通過@ComponentScan 掃描添加
帶有@Configuration注解的配置類，并讀取配置類的資訊；
- 是作用于 springboot應用的，@SpringBootConfiguration中可以添加N個@Configuration注解的配置類，但是對于一個springboot項目，隻能存在一個@SpringBootConfiguration注解；
》2、檢視@EnableAutoConfiguration
2.1、能完成自動配置，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)自動導配置類的包，可以把所有帶有@Configuration的類的包導入；
附上一段注釋，說隻要依靠@ConditionalOnMissingBean注解生效自動配置；
當missing 某一個bean時候，滿足自動配置的條件，@EnableAutoConfiguration 會生效；
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、跟進@ConditionalOnMissingBean
@ConditionalOnMissingBean中有@Conditional注解,該注解意思是條件比對；
若滿足@Conditional要求的條件，才會被注冊到元件中；當我們自定義一些配置時候，就會預設配置就會失效；因為這個bean 已經存在了；
2.3、
@Inherited注解
》會通過注解超類來找依賴中的類，完成自動初始化類加載
* 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如何掃包的
》貼上一段注釋,第二段意思是，如沒有指明要掃的包，會從目前包路徑向下掃包；
是以，main函數放在頂層；
* 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.
》小結：
@SpringBootApplication 中包含
@SpringBootConfiguration、@ EnableAutoConfiguration、@ ComponentScan
@ EnableAutoConfiguration