Spring Boot @EnableAutoConfiguration和 @Configuration的区别

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程序那些事 2022-09-19 16:44:17 博主文章分类：Spring Boot ©著作权

文章标签 Spring Spring Boot spring 自动加载实例化 文章分类 spring boot 后端开发

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Spring Boot @EnableAutoConfiguration和
@Configuration的区别

在Spring Boot中，我们会使用@SpringBootApplication来开启Spring Boot程序。在之前的文章中我们讲到了@SpringBootApplication相当于@EnableAutoConfiguration，@ComponentScan，@Configuration三者的集合。

其中@Configuration用在类上面，表明这个是个配置类，如下所示：

@Configuration
public class MySQLAutoconfiguration {
    ...
}

而@EnableAutoConfiguration则是开启Spring Boot的自动配置功能。什么是自动配置功能呢？简单点说就是Spring Boot根据依赖中的jar包，自动选择实例化某些配置。

接下来我们看一下@EnableAutoConfiguration是怎么工作的。

先看一下@EnableAutoConfiguration的定义：

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {

  String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

  /**
   * Exclude specific auto-configuration classes such that they will never be applied.
   * @return the classes to exclude
   */
  Class<?>[] exclude() default {};

  /**
   * Exclude specific auto-configuration class names such that they will never be
   * applied.
   * @return the class names to exclude
   * @since 1.3.0
   */
  String[] excludeName() default {};

}

注意这一行： @Import(AutoConfigurationImportSelector.class)

AutoConfigurationImportSelector实现了ImportSelector接口，并会在实例化时调用selectImports。下面是其方法：

public String[] selectImports(AnnotationMetadata annotationMetadata) {
    if (!isEnabled(annotationMetadata)) {
      return NO_IMPORTS;
    }
    AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
        .loadMetadata(this.beanClassLoader);
    AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(autoConfigurationMetadata,
        annotationMetadata);
    return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
  }

这个方法中的getCandidateConfigurations会从类加载器中查找所有的META-INF/spring.factories，并加载其中实现了@EnableAutoConfiguration的类。有兴趣的朋友可以具体研究一下这个方法的实现。

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

    try {
      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);
        Properties properties = PropertiesLoaderUtils.loadProperties(resource);
        for (Map.Entry<?, ?> entry : properties.entrySet()) {
          String factoryTypeName = ((String) entry.getKey()).trim();
          for (String factoryImplementationName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {
            result.add(factoryTypeName, factoryImplementationName.trim());
          }
        }
      }
      cache.put(classLoader, result);
      return result;
    }
    catch (IOException ex) {
      throw new IllegalArgumentException("Unable to load factories from location [" +
          FACTORIES_RESOURCE_LOCATION + "]", ex);
    }
  }

我们再看一下spring-boot-autoconfigure-2.2.2.RELEASE.jar中的META-INF/spring.factories。

spring.factories里面的内容是key=value形式的，我们重点关注一下EnableAutoConfiguration：

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\
org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\
org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\
org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\
org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,\
org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,\
org.springframework.boot.autoconfigure.cloud.CloudServiceConnectorsAutoConfiguration,\
org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\
...

这里只列出了一部分内容，根据上面的代码，所有的EnableAutoConfiguration的实现都会被自动加载。这就是自动加载的原理了。

如果我们仔细去看具体的实现：

@Configuration(proxyBeanMethods = false)
@AutoConfigureAfter(JmxAutoConfiguration.class)
@ConditionalOnProperty(prefix = "spring.application.admin", value = "enabled", havingValue = "true",
    matchIfMissing = false)
public class SpringApplicationAdminJmxAutoConfiguration {