SpringBoot自动装配原理

SpringBoot自动装配原理是怎么样的呢，不同的人可能有不同的理解，但作为程序员，最权威的就是看源码了，下面看下springboot相关的源码，理解一下SpringBoot自动装配原理

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = {
      @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
      @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {
}

进入SpringBootApplication注解可以看到EnableAutoConfiguration注解

@Enable注解的主要作用把相关组件的Bean装配到IOC容器中

EnableAutoConfiguration注解

进入EnableAutoConfiguration注解 :

@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 {};

}

@AutoConfigurationPackage的作用是把使用了该注解的类所在的包和子包下所有的组件扫描到SpringIOC容器中。@Import注解导入了一个AutoConfigurationImportSelector类，AutoConfigurationImportSelector实现配置类的导入

AutoConfigurationImportSelector

AutoConfigurationImportSelector实现了ImportSelector接口 selectImports方法返回一个String数组，数组中指定需要装配到IOC容器中

@Override
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());
}

1. AutoConfigurationMetadataLoader.loadMetadata从META-INF/spring-autoconfigure-metadata.properties中加载自动装配的条件元数据，就是只有满足条件的Bean才能够进行装配。
2. 收集符合条件的配置类autoConfigurationEntry.getConfigurations()，完成自动装配

AutoConfigurationImportSelector中不执行selectImports方法，而是通过ConfigurationClassPostProcessor的processConfigBeanDefinitions方法进行扫描和注册所有配置类的Bean。

protected AutoConfigurationEntry getAutoConfigurationEntry(
      AutoConfigurationMetadata autoConfigurationMetadata,
      AnnotationMetadata annotationMetadata) {
   if (!isEnabled(annotationMetadata)) {
      return EMPTY_ENTRY;
   }
   AnnotationAttributes attributes = getAttributes(annotationMetadata);
   List<String> configurations = getCandidateConfigurations(annotationMetadata,
         attributes);
   configurations = removeDuplicates(configurations);
   Set<String> exclusions = getExclusions(annotationMetadata, attributes);
   checkExcludedClasses(configurations, exclusions);
   configurations.removeAll(exclusions);
   configurations = filter(configurations, autoConfigurationMetadata);
   fireAutoConfigurationImportEvents(configurations, exclusions);
   return new AutoConfigurationEntry(configurations, exclusions);
}

1. getAttributes获得@EnableAutoConfiguration注解中的属性exclude excludeName等
2. getCandidateConfigurations()获得所有自动装配的配置类
3. removeDuplicates()方法去除重复的配置项
4. getExclusions方法根据@EnableAutoConfiguration注解中配置的exclude属性，把不需要自动装配的配置类移除。
5. fireAutoConfigurationImportEvents方法广播事件
6. 最后返回经过多层判断和过滤之后的配置类集合。

getCandidateConfigurations()方法

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata,
      AnnotationAttributes attributes) {
   List<String> configurations = SpringFactoriesLoader.loadFactoryNames(
         getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader());
   Assert.notEmpty(configurations,
         "No auto configuration classes found in META-INF/spring.factories. If you "
               + "are using a custom packaging, make sure that file is correct.");
   return configurations;
}

SpringFactoriesLoader扫描classpath下的META-INF/spring.factories文件，根据Key得到对应的value值。

总结

1. 通过@Import实现配置类的导入
2. AutoConfigurationImportSelector类实现ImportSelector接口，重写selectImports方法，用于实现选择性批量配置类的装配
3. 通过Spring提供的SpringFactoriesLoader机制，扫描classpath路径下的META-INF/spring.factories文件，读取需要实现自动装配的配置类
4. 通过条件筛选，把不符合条件的配置类移除，最终完成自动装配。

好了 这就是SpringBoot自动装配原理，给大家简简单单的分析了一下，如果你觉得还不错的话，欢迎给大家留言和点赞，大家的赞是对我最大的鼓励，我将继续更文，提升自己的同时分享更多的好文章。