springboot 与 spring 相比，有一个劣势就是大部分配置都有一套默认设定，不用再写大量的配置文件，即“约定优于配置 ”。
而这是靠主动拆卸实现的。

一、主动加载入口

springboot 的启动形式如下：

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

察看 @SpringBootApplication 注解

// ## 此注解非常重要
@EnableAutoConfiguration
@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
        @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {...}


// ## 此注解同样重要
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {...}

从下面的代码看出，最终会通过 @Import 注解进入 AutoConfigurationImportSelector 类（调用链：@SpringBootApplication->@EnableAutoConfiguration->@Import(AutoConfigurationImportSelector.class)）

持续察看：

// == 1.AutoConfigurationImportSelector 实现 DeferredImportSelector 接
//      调用 getImportGroup()办法，返回 AutoConfigurationGroup 类
public class AutoConfigurationImportSelector implements DeferredImportSelector{
    @Override
    public Class<? extends Group> getImportGroup() {return AutoConfigurationGroup.class;}
}

// == 2. 实现 DeferredImportSelector.Group 接口    
private static class AutoConfigurationGroup
            implements DeferredImportSelector.Group{..}
}

Group 接口有两个办法：

// 加载配置信息
void process(AnnotationMetadata metadata, DeferredImportSelector selector);

// 包装成 Entry 对象并返回
Iterable<Entry> selectImports();

接下来别离看一下这两个办法的实现。

1、process()办法——配置文件加载

@Override
public void process(AnnotationMetadata annotationMetadata, DeferredImportSelector deferredImportSelector) {AutoConfigurationEntry autoConfigurationEntry = ((AutoConfigurationImportSelector) deferredImportSelector)
      // == 配置加载
      .getAutoConfigurationEntry(annotationMetadata);

  this.autoConfigurationEntries.add(autoConfigurationEntry);
  for (String importClassName : autoConfigurationEntry.getConfigurations()) {this.entries.putIfAbsent(importClassName, annotationMetadata);
  }
}

// org.springframework.boot.autoconfigure.AutoConfigurationImportSelector#getAutoConfigurationEntry
protected AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {AnnotationAttributes attributes = getAttributes(annotationMetadata);
    // 获取配置信息
    List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
    
    //...
}

// org.springframework.boot.autoconfigure.AutoConfigurationImportSelector#getCandidateConfigurations
protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
    // ## spi 形式加载配置
    List<String> configurations = SpringFactoriesLoader.loadFactoryNames(EnableAutoConfiguration.class,
                getBeanClassLoader());
    return configurations;
}

之前介绍了 spi 配置加载，spi 形式会查找 META-INF 下的 spring.factories 文件
文件内容：

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\
org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,
org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.jdbc.JdbcRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration,\
org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration,\
org.springframework.boot.autoconfigure.gson.GsonAutoConfiguration,\
// ... 省略...

依据 org.springframework.boot.autoconfigure.EnableAutoConfiguration 主动加载 Redis、Mongo 等 156 个默认配置

2、selectImports()办法——包装成 Entry

@Override
public Iterable<Entry> selectImports() {
    // 配置排序，并包装成 Entry 返回
    return sortAutoConfigurations(processedConfigurations, getAutoConfigurationMetadata()).stream()
            .map((importClassName) -> new Entry(this.entries.get(importClassName), importClassName))
            .collect(Collectors.toList());
}

二、spring 初始化时加载 Entry 对象

Group 接口 selectImports()办法返回的 Entry 对象去了哪里？
其实 spring 初始化加载时会用到它们。

从 spring 的 refresh 办法（初始化外围办法）一路追踪，整个调用链如下：

AbstractApplicationContext # refresh
AbstractApplicationContext # invokeBeanFactoryPostProcessors
PostProcessorRegistrationDelegate # invokeBeanFactoryPostProcessors
PostProcessorRegistrationDelegate # invokeBeanDefinitionRegistryPostProcessors
ConfigurationClassPostProcessor # postProcessBeanDefinitionRegistry
ConfigurationClassPostProcessor # processConfigBeanDefinitions
ConfigurationClassParser # parse
ConfigurationClassParser$DeferredImportSelectorHandler # process
ConfigurationClassParser$DeferredImportSelectorGroupingHandler # processGroupImports      
ConfigurationClassParser$DeferredImportSelectorGrouping # getImports

这些 Entry 最终会以 BeanDefinition 对象的模式，放入到 BeanDefinitionMap 中。
后续进入 spring bean 对象初始化、属性赋值流程了，不再细说。