解析SpringBoot @EnableAutoConfiguration的使用

package com.wangzhi;

import org.springframework.stereotype.Service;

@Service
public class Dog {
}

org.springframework.boot.autoconfigure.EnableAutoConfiguration=com.wangzhi.Dog

package com.wangzhi.springbootdemo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.context.annotation.ComponentScan;

@EnableAutoConfiguration
public class SpringBootDemoApplication {

  public static void main(String[] args) {
    ConfigurableApplicationContext context = SpringApplication.run(SpringBootDemoApplication.class, args);
    System.out.println(context.getBean(com.wangzhi.Dog.class));
  }

}

protected AutoConfigurationEntry getAutoConfigurationEntry(
    AutoConfigurationMetadata autoConfigurationMetadata,
    AnnotationMetadata annotationMetadata) {
  if (!isEnabled(annotationMetadata)) {
    return EMPTY_ENTRY;
  }
  // EnableAutoConfiguration注解的属性：exclude，excludeName等
  AnnotationAttributes attributes = getAttributes(annotationMetadata);
  // 得到所有的Configurations
  List<String> configurations = getCandidateConfigurations(annotationMetadata,
      attributes);
  // 去重
  configurations = removeDuplicates(configurations);
  // 删除掉exclude中指定的类
  Set<String> exclusions = getExclusions(annotationMetadata, attributes);
  checkExcludedClasses(configurations, exclusions);
  configurations.removeAll(exclusions);
  configurations = filter(configurations, autoConfigurationMetadata);
  fireAutoConfigurationImportEvents(configurations, exclusions);
  return new AutoConfigurationEntry(configurations, exclusions);
}

public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {
    // factoryClassName为org.springframework.boot.autoconfigure.EnableAutoConfiguration
 String factoryClassName = factoryClass.getName();
    // 该方法返回的是所有spring.factories文件中key为org.springframework.boot.autoconfigure.EnableAutoConfiguration的类路径
 return loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());
 }


public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";

private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
 MultiValueMap<String, String> result = cache.get(classLoader);
 if (result != null) {
  return result;
 }

 try {
      // 找到所有的"META-INF/spring.factories"
  Enumeration<URL> urls = (classLoader != null ?
   classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
   ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
  result = new LinkedMultiValueMap<>();
  while (urls.hasMoreElements()) {
  URL url = urls.nextElement();
  UrlResource resource = new UrlResource(url);
        // 读取文件内容，properties类似于HashMap，包含了属性的key和value
  Properties properties = PropertiesLoaderUtils.loadProperties(resource);
  for (Map.Entry<?, ?> entry : properties.entrySet()) {
   String factoryClassName = ((String) entry.getKey()).trim();
          // 属性文件中可以用','分割多个value
   for (String factoryName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {
   result.add(factoryClassName, factoryName.trim());
   }
  }
  }
  cache.put(classLoader, result);
  return result;
 }
 catch (IOException ex) {
  throw new IllegalArgumentException("Unable to load factories from location [" +
   FACTORIES_RESOURCE_LOCATION + "]", ex);
 }
 }

public void processGroupImports() {
  for (DeferredImportSelectorGrouping grouping : this.groupings.values()) {
    // getImports即上面得到的所有类路径的封装
    grouping.getImports().forEach(entry -> {
      ConfigurationClass configurationClass = this.configurationClasses.get(
          entry.getMetadata());
      try {
        // 和处理@Import注解一样
        processImports(configurationClass, asSourceClass(configurationClass),
            asSourceClasses(entry.getImportClassName()), false);
      }
      catch (BeanDefinitionStoreException ex) {
        throw ex;
      }
      catch (Throwable ex) {
        throw new BeanDefinitionStoreException(
            "Failed to process import candidates for configuration class [" +
                configurationClass.getMetadata().getClassName() + "]", ex);
      }
    });
  }
}

private void processImports(ConfigurationClass configClass, SourceClass currentSourceClass,
  Collection<SourceClass> importCandidates, boolean checkForCircularImports) {
 ...
  // 遍历收集到的类路径
  for (SourceClass candidate : importCandidates) {
    ...
    //如果candidate是ImportSelector或ImportBeanDefinitionRegistrar类型其处理逻辑会不一样，这里不关注
   // Candidate class not an ImportSelector or ImportBeanDefinitionRegistrar ->
   // process it as an @Configuration class
   this.importStack.registerImport(
    currentSourceClass.getMetadata(), candidate.getMetadata().getClassName());
 // 当作 @Configuration 处理  
    processConfigurationClass(candidate.asConfigClass(configClass));
  ...
}
      
  ...
}