面试官：谈谈你对 Spring Boot 自动装配机制的理解

@SpringBootApplication
public class SpringBootDemoApplication {
}

@SpringBootApplication(exclude = DataSourceAutoConfiguration.class)
public class SpringBootDemoApplication {
}

@SpringBootApplication(excludeName = "org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration")
public class SpringBootDemoApplication {
}

spring.autoconfigure.exclude=org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration

@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
  String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";
  Class<?>[] exclude() default {};
  String[] excludeName() default {};
}

public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware,
    ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {
}

public interface DeferredImportSelector extends ImportSelector {
  default Class<? extends Group> getImportGroup() {
    return null;
  }
  interface Group {
    // 使用指定的 DeferredImportSelector 处理配置类的注解元数据
    void process(AnnotationMetadata metadata, DeferredImportSelector selector);
    // 返回应该为当前分组导入哪些类的 Entry
    Iterable<Entry> selectImports();
    class Entry {
      // 正在导入的配置类的注解元数据
      private final AnnotationMetadata metadata;
      // 要导入的类的完全限定符
      private final String importClassName;
    }
  }
}

public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware,
    ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {
  @Override
  public Class<? extends Group> getImportGroup() {
    return AutoConfigurationGroup.class;
  }
  private static class AutoConfigurationGroup
      implements DeferredImportSelector.Group, BeanClassLoaderAware, BeanFactoryAware, ResourceLoaderAware {
    // @Import 导入的类 -> @Import 注解所在配置类注解元数据
    private final Map<String, AnnotationMetadata> entries = new LinkedHashMap<>();
    // @Import 导入配置类列表
    private final List<AutoConfigurationEntry> autoConfigurationEntries = new ArrayList<>();
    @Override
    public void process(AnnotationMetadata annotationMetadata, DeferredImportSelector deferredImportSelector) {
      // 收集配置类
      AutoConfigurationEntry autoConfigurationEntry = ((AutoConfigurationImportSelector) deferredImportSelector)
          .getAutoConfigurationEntry(getAutoConfigurationMetadata(), annotationMetadata);
      this.autoConfigurationEntries.add(autoConfigurationEntry);
      for (String importClassName : autoConfigurationEntry.getConfigurations()) {
        this.entries.putIfAbsent(importClassName, annotationMetadata);
      }
    }
  }
}

  protected AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata,
                                 AnnotationMetadata annotationMetadata) {
    if (!isEnabled(annotationMetadata)) {
      return EMPTY_ENTRY;
    }
    // 获取配置类上的注解属性
    AnnotationAttributes attributes = getAttributes(annotationMetadata);
    // 获取候配置类
    List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
    // 移除重复的配置类
    configurations = removeDuplicates(configurations);
    // 获取排除的配置类
    Set<String> exclusions = getExclusions(annotationMetadata, attributes);
    // 检查排除类
    checkExcludedClasses(configurations, exclusions);
    // 移除排除类
    configurations.removeAll(exclusions);
    // 过滤排除类
    configurations = filter(configurations, autoConfigurationMetadata);
    // 发布事件
    fireAutoConfigurationImportEvents(configurations, exclusions);
    return new AutoConfigurationEntry(configurations, exclusions);
  }

  protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
    List<String> configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(),
        getBeanClassLoader());
    return configurations;
  }
  protected Class<?> getSpringFactoriesLoaderFactoryClass() {
    return EnableAutoConfiguration.class;
  }

  private static class AutoConfigurationGroup
      implements DeferredImportSelector.Group, BeanClassLoaderAware, BeanFactoryAware, ResourceLoaderAware {
    @Override
    public Iterable<Entry> selectImports() {
      if (this.autoConfigurationEntries.isEmpty()) {
        return Collections.emptyList();
      }
      Set<String> allExclusions = this.autoConfigurationEntries.stream()
          .map(AutoConfigurationEntry::getExclusions).flatMap(Collection::stream).collect(Collectors.toSet());
      Set<String> processedConfigurations = this.autoConfigurationEntries.stream()
          .map(AutoConfigurationEntry::getConfigurations).flatMap(Collection::stream)
          .collect(Collectors.toCollection(LinkedHashSet::new));
      processedConfigurations.removeAll(allExclusions);
      // 排序
      return sortAutoConfigurations(processedConfigurations, getAutoConfigurationMetadata()).stream()
          .map((importClassName) -> new Entry(this.entries.get(importClassName), importClassName))
          .collect(Collectors.toList());
    }
  }