Interview talk: understand the principles of Springboot start

SpringBoot: Operating principle Inquiry

Speaking from pom.xml springboot we did not like javaweb, ssm as a tomcat to perform, because it embeds itself on a tomcat
and then we only need to configure the dependent pom.xml inside
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into the parent project, here is the real application management SpringBoot All versions of which depend on the place, SpringBoot version control center;
later we import dependence is no need to write the default version; however, if the imported package is not managed in dependence on the need to manually configure version;

Launcher

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>

pringboot-boot-starter: the scene is springboot starter
springboot start essential, here we need web functionality to import starter-web

Startup class

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

/@SpringBootApplication 来标注一个主程序类 ， 说明这是一个Spring Boot应用
@SpringBootApplication
public class SpringbootDemo02Application {
     public static void main(String[] args) {
        //将Spring应用启动起来
        SpringApplication.run(SpringbootDemo02Application.class, args);
    }
}

First, we learned people know, do not know it does not matter, why start class called startup class, and we ourselves can also write start classes, but if you create a Springboot project then will be built one and here the main difference is that * * @ SpringBootApplication **

@SpringBootApplication : SpringBoot application marked on a class == Description This class is SpringBoot main configuration class ==, SpringBoot you should run the main method of this class to start SpringBoot application; enter this comment:
Entered SpringBootApplication I found three more important notes other notes if you do not know if I have another offer to go to a foundation to explain: Notes and reflection

@ComponentScan

The elements corresponding to annotation xml configuration function is to automatically scan and load match type of bean, bean definition is loaded to the IOC container

@SpringBootConfiguration

SpringBoot的配置类；标注在某个类上，表示这是一个SpringBoot的配置类；我们继续进去这个注解查看
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@Configuration：配置类上来标注这个注解，配置类-----配置文件；我们继续点进去，发现配置类也是容器中的一个组件。@Component

这就说明了SpringbootConfiguration其实就是为了标注这个类是个组件，那我们接下来看第三个注解

@EnableAutoConfiguration

以前我们需要配置的东西，SpringBoot可以自动帮我们配置； @EnableAutoConfiguration告诉SpringBoot开启自动配置功能，这样自动配置才能生效；
Here Insert Picture Description 但是到底自动配置啥，我们任然不知道所以继续点进去看

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@AutoConfigurationPackage ：自动配置包，点进去看到一个 @Import({Registrar.class})

@import ：Spring底层注解@import ，给容器中导入一个组件，导入的组件由 {Registrar.class} 将主配置类【即**@SpringBootApplication**标注的类】的所在包及包下面所有子包里面的所有组件扫描到Spring容器= ；

然后回来继续看：@Import({AutoConfigurationImportSelector.class}) ：给容器导入组件；

AutoConfigurationImportSelector类

导入哪些组件的选择器 ;
它将所有需要导入的组件以全类名的方式返回，这些组件就会被添加到容器中
它会给容器中导入非常多的自动配置类（xxxAutoConfiguration）, 就是给容器中导入这个场景需要的所有组件，并配置好这些组件；

/AutoConfigurationImportSelector中获取配置文件信息；
protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
   List<String> configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(),
         getBeanClassLoader());
   Assert.notEmpty(configurations, "No auto configuration classes found in META-INF/spring.factories. If you "
         + "are using a custom packaging, make sure that file is correct.");
   return configurations;
}

SpringFactoriesLoader.loadFactoryNames（EnableAutoConfiguation.class，classLoader）;
这里的getSpringFactoriesLoaderFactoryClass（）方法返回的就是我们最开始看的启动自动导入配置文件的注解类；
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进入 SpringFactoriesLoader 查看