@SpringBootApplication解析

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

》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

@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

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；

2.3、
@Inherited注解

》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

》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