一、简单构建一个springboot项目

@SpringBootApplication
public class SpringbootApplication {
    public static void main(String[] args) {
        SpringApplication.run(SpringbootApplication.class, args);
    }
}

  只要启动上述代码，就能启动一个基本的springboot项目，springboot启动的核心就在于  @SpringBootApplication 注解以及 SpringApplication.run(SpringbootApplication.class, args); 

下面也将围绕这两个方法进行讲解。

 

二、@SpringBootApplication 原理

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
        @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {

  源码如上，可以看到@SpringBootApplication 是个符合注解，其中重点是

• @ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class), @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })

• @EnableAutoConfiguration

 

　　2.1　　第一个注解@ComponentScan

@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class), @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })  

  扫码根路径下所有@Controller @Service @Respository @componet 并注入到spring容器中，然后排除TypeExcludeFilter和AutoConfigurationExcludeFilter中定义的bean。

 

　　2.2　　第二个注解：@EnableAutoConfiguration （核心）

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {

　　查看源码可知，@EnableAutoConfiguration也是一个符合注解，其中

• @Import(AutoConfigurationImportSelector.class)
• @AutoConfigurationPackage

　　　　2.2.1　　@Import(AutoConfigurationImportSelector.class)

 　　@import注解用于将bean导入到spring容器中，具体哪些bean需要导入容器则查看AutoConfigurationImportSelector 源码，查看AutoConfigurationImportSelector 的

 selectImports 方法，该方法加载META-INF/spring.factories下EnableAutoConfiguration的类(常用的JdbcTemplateAutoConfiguration等自动注解类)加载到容器中，如下图spring.factories

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

• @EnableAutoConfiguration