Spring Boot 自动配置的实现

Spring Boot 自动配置

来看下 spring boot中自动配置的注解

@SuppressWarnings("deprecation")
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(EnableAutoConfigurationImportSelector.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 {};

}

1. exclude() 可以排除一些自动配置的内容
2. excludeName 通过名称排除自动配置内容

再来看下， @EnableAutoConfiguration 是怎么处理自动配置的呢？

注意到@Import(EnableAutoConfigurationImportSelector.class)

public class EnableAutoConfigurationImportSelector
    extends AutoConfigurationImportSelector {

  @Override
  protected boolean isEnabled(AnnotationMetadata metadata) {
    if (getClass().equals(EnableAutoConfigurationImportSelector.class)) {
      return getEnvironment().getProperty(
          EnableAutoConfiguration.ENABLED_OVERRIDE_PROPERTY, Boolean.class,
          true);
    }
    return true;
  }
}

}

再来看下 AutoConfigurationImportSelector ，主要是 接口的 ImportSelector 的实现

@Override
  public String[] selectImports(AnnotationMetadata annotationMetadata) {
    if (!isEnabled(annotationMetadata)) {
      return NO_IMPORTS;
    }
    try {
      //1、 自动配置的元数据 spring-autocomfigure-metadata.properties
      // 自动配置的开启条件
      AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
          .loadMetadata(this.beanClassLoader);
      AnnotationAttributes attributes = getAttributes(annotationMetadata);
      // 获取设置的自动配置列表 spring.factories
      List<String> configurations = getCandidateConfigurations(annotationMetadata,
          attributes);
      configurations = removeDuplicates(configurations);
      configurations = sort(configurations, autoConfigurationMetadata);
      // 获取要排除的自动配置列表，可以通过 注解@EnableAutoConfiguration 的exclude和
       // 配置文件设置 spring.autoconfigure.exclude key的值
      Set<String> exclusions = getExclusions(annotationMetadata, attributes);
      checkExcludedClasses(configurations, exclusions);
      configurations.removeAll(exclusions);
       // 通过 spring-autocomfigure-metadata.properties ConditionOnClass 条件进行过滤
      configurations = filter(configurations, autoConfigurationMetadata);
      fireAutoConfigurationImportEvents(configurations, exclusions);
      return configurations.toArray(new String[configurations.size()]);
    }
    catch (IOException ex) {
      throw new IllegalStateException(ex);
    }
  }

看下 spring.factories 文件

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.CloudAutoConfiguration,\
org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.context.MessageSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration,\
org.springframework.boot.autoconfigure.couchbase.CouchbaseAutoConfiguration,\
org.springframework.boot.autoconfigure.dao.PersistenceExceptionTranslationAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.ldap.LdapDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.ldap.LdapRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.neo4j.Neo4jDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.neo4j.Neo4jRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.solr.SolrRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.rest.RepositoryRestMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.data.web.SpringDataWebAutoConfiguration,\
org.springframework.boot.autoconfigure.elasticsearch.jest.JestAutoConfiguration,\
org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration,\
org.springframework.boot.autoconfigure.gson.GsonAutoConfiguration,\
org.springframework.boot.autoconfigure.h2.H2ConsoleAutoConfiguration,\
org.springframework.boot.autoconfigure.hateoas.HypermediaAutoConfiguration,\

再看下 spring framework中 ConfigurationClassParser 的处理方式，会解析 @Import 里的接口 ImportSelector 返回的所有配置类,那是怎么配置的呢，如 JpaRepositoriesAutoConfiguration

@Configuration
@ConditionalOnBean(DataSource.class)
@ConditionalOnClass(JpaRepository.class)
@ConditionalOnMissingBean({ JpaRepositoryFactoryBean.class,
    JpaRepositoryConfigExtension.class })
@ConditionalOnProperty(prefix = "spring.data.jpa.repositories", name = "enabled", havingValue = "true", matchIfMissing = true)
@Import(JpaRepositoriesAutoConfigureRegistrar.class)
@AutoConfigureAfter(HibernateJpaAutoConfiguration.class)
public class JpaRepositoriesAutoConfiguration {

}

从上面可以看到，有很多的@ConditionalOn**的注解，我们来看下 ConditionEvaluator这个 条件计算器，会去计算出当前这个配置类 是否要开启，而这些 @ConditionalOn** 是依赖于 @Conditional 这个注解，如  @ConditionalOnBean 最终是通过 Condition 接口来作条件选择

@Target({ ElementType.TYPE, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional(OnBeanCondition.class)
public @interface ConditionalOnBean {

  /**
   * The class type of bean that should be checked. The condition matches when any of
   * the classes specified is contained in the {@link ApplicationContext}.
   * @return the class types of beans to check
   */
  Class<?>[] value() default {};

  /**
   * The class type names of bean that should be checked. The condition matches when any
   * of the classes specified is contained in the {@link ApplicationContext}.
   * @return the class type names of beans to check
   */
  String[] type() default {};

  /**
   * The annotation type decorating a bean that should be checked. The condition matches
   * when any of the annotations specified is defined on a bean in the
   * {@link ApplicationContext}.
   * @return the class-level annotation types to check
   */
  Class<? extends Annotation>[] annotation() default {};

  /**
   * The names of beans to check. The condition matches when any of the bean names
   * specified is contained in the {@link ApplicationContext}.
   * @return the name of beans to check
   */
  String[] name() default {};

  /**
   * Strategy to decide if the application context hierarchy (parent contexts) should be
   * considered.
   * @return the search strategy
   */
  SearchStrategy search() default SearchStrategy.ALL;

}

Spring boot 的autoconfigure 是囊括了所有可以和spring 整合的项目，但大部分情况下，并不是所以的项目都会启用，通过 Condition和@Conditional 来判断条件

1. 判断classPath 是否存在指定的类  @ConditionalOnClass
2. 判断 ApplicationContext 中是否存在指定的 Bean  @ConditionalOnBean
3. 配置环境中是否存在特定的配置项  @ConditionalOnProperty
4. 配置环境中指定的配置项是否存在指定的值

禁用配置

当前 也是可以禁用某些我们不想要的默认配置，如上面加载时说到，会排除一些配置(exclude)

1. 设置 @EnableAutoConfiguration 的exclude 配置
2. 在配置文件增加 spring.autoconfigure.exclude 配置

以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持我们。