1、自动配置原理

配置的是一些基本属性

一、代码路径

1、SpringBootApplication注解

public @interface SpringBootApplication {
    @AliasFor(
        annotation = EnableAutoConfiguration.class
    )
}

2、EnableAutoConfiguration注解

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import({AutoConfigurationImportSelector.class})
public @interface EnableAutoConfiguration {
    String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";
    Class<?>[] exclude() default {};
    String[] excludeName() default {};
}

3、AutoConfigurationImportSelector类

getCandidateConfigurations调用 SpringFactoriesLoader类的loadFactoryNames方法。

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
        List<String> configurations = SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.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;
    }

4、SpringFactoriesLoader类

getResources(“META-INF/spring.factories”)获取配置信息。

private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
        MultiValueMap<String, String> result = (MultiValueMap)cache.get(classLoader);
        if (result != null) {
            return result;
        } else {
            try {
                Enumeration<URL> urls = classLoader != null ? classLoader.getResources("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories");
                LinkedMultiValueMap result = new LinkedMultiValueMap();

   ![image.png](https://cdn.nlark.com/yuque/0/2021/png/5365888/1627027494375-2db09b85-0980-48ae-b921-1f3356ed315f.png#clientId=ud79fe82f-80d8-4&from=paste&height=232&id=uc95ee75c&margin=%5Bobject%20Object%5D&name=image.png&originHeight=464&originWidth=1162&originalType=binary&ratio=1&size=75467&status=done&style=none&taskId=ufa129cf9-e3d9-49ad-8735-1f4a6750564&width=581)

二、Spring.factories的几个常用自动配置

org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration

三、总结

spring-boot-autoconfigure包中：
xxxAutoConfiguration类用来自动加载配置、xxxProperties类是定义具体配置属性

在application.properties中设置debug=true，可以打印配置信息。

debug=true   
server.port=8000
## sqlite 数据库
spring.datasource.driver-class-name=org.sqlite.JDBC
#数据库地址
spring.datasource.url=jdbc:sqlite:app.db
#每次启动更改数据表结构
spring.jpa.hibernate.ddl-auto=update
#显示数据库操作记录
spring.jpa.show-sql=true

四、扩展配置

五、参考

1、SpringBoot自动配置源码分析
2、狂神-MVC自动配置原理
3、官方：https://docs.spring.io/spring-boot/docs/2.2.5.RELEASE/reference/htmlsingle/#boot-features-spring-mvc-auto-configuration