Springboot @Import 详解

SpringBoot 的 @Import 用于将指定的类实例注入之Spring IOC Container中。

今天抽空在仔细看了下Springboot 关于 @Import 的处理过程， 记下来以后看。

1. @Import

先看Spring对它的注释 (文档贴过来的）， 总结下来作用就是和xml配置的 <import />标签作用一样，允许通过它引入 @Configuration 注解的类 (java config)， 引入ImportSelector接口(这个比较重要， 因为要通过它去判定要引入哪些@Configuration) 和 ImportBeanDefinitionRegistrar 接口的实现， 也包括 @Component注解的普通类。

但是如果要引入另一个xml 文件形式配置的 bean, 则需要通过 @ImportResource 注解。

/**
 * Indicates one or more {@link Configuration @Configuration} classes to import.
 *
 * <p>Provides functionality equivalent to the {@code <import/>} element in Spring XML.
 * Allows for importing {@code @Configuration} classes, {@link ImportSelector} and
 * {@link ImportBeanDefinitionRegistrar} implementations, as well as regular component
 * classes (as of 4.2; analogous to {@link AnnotationConfigApplicationContext#register}).
 *
 * <p>{@code @Bean} definitions declared in imported {@code @Configuration} classes should be
 * accessed by using {@link org.springframework.beans.factory.annotation.Autowired @Autowired}
 * injection. Either the bean itself can be autowired, or the configuration class instance
 * declaring the bean can be autowired. The latter approach allows for explicit, IDE-friendly
 * navigation between {@code @Configuration} class methods.
 *
 * <p>May be declared at the class level or as a meta-annotation.
 *
 * <p>If XML or other non-{@code @Configuration} bean definition resources need to be
 * imported, use the {@link ImportResource @ImportResource} annotation instead.
 *
 * @author Chris Beams
 * @author Juergen Hoeller
 * @since 3.0
 * @see Configuration
 * @see ImportSelector
 * @see ImportResource
 */
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Import {

  /**
   * {@link Configuration}, {@link ImportSelector}, {@link ImportBeanDefinitionRegistrar}
   * or regular component classes to import.
   */
  Class<?>[] value();

}

2. ImportSelector

因为 @Import 的实现有很多时候需要借助 ImportSelector 接口， 所以我们再看下这个接口的描述, 总结下来就是需要通过这个接口的实现去决定要引入哪些 @Configuration。 它如果实现了以下四个Aware 接口， 那么spring保证会在调用它之前先调用Aware接口的方法。

至于为什么要保证调用Aware， 我个人觉得应该是你可以通过这些Aware去感知系统里边所有的环境变量， 从而决定你具体的选择逻辑。

/**
 * Interface to be implemented by types that determine which @{@link Configuration}
 * class(es) should be imported based on a given selection criteria, usually one or more
 * annotation attributes.
 *
 * <p>An {@link ImportSelector} may implement any of the following
 * {@link org.springframework.beans.factory.Aware Aware} interfaces, and their respective
 * methods will be called prior to {@link #selectImports}:
 * <ul>
 * <li>{@link org.springframework.context.EnvironmentAware EnvironmentAware}</li>
 * <li>{@link org.springframework.beans.factory.BeanFactoryAware BeanFactoryAware}</li>
 * <li>{@link org.springframework.beans.factory.BeanClassLoaderAware BeanClassLoaderAware}</li>
 * <li>{@link org.springframework.context.ResourceLoaderAware ResourceLoaderAware}</li>
 * </ul>
 *
 * <p>ImportSelectors are usually processed in the same way as regular {@code @Import}
 * annotations, however, it is also possible to defer selection of imports until all
 * {@code @Configuration} classes have been processed (see {@link DeferredImportSelector}
 * for details).
 *
 * @author Chris Beams
 * @since 3.1
 * @see DeferredImportSelector
 * @see Import
 * @see ImportBeanDefinitionRegistrar
 * @see Configuration
 */
public interface ImportSelector {

  /**
   * Select and return the names of which class(es) should be imported based on
   * the {@link AnnotationMetadata} of the importing @{@link Configuration} class.
   */
  String[] selectImports(AnnotationMetadata importingClassMetadata);

}

3. Springboot 对@Import注解的处理过程

Springboot对注解的处理都发生在AbstractApplicationContext -> refresh() -> invokeBeanFactoryPostProcessors(beanFactory) -> ConfigurationClassPostProcessor -> postProcessBeanDefinitionRegistry()方法中。

(稍微说下也免得我自己忘了， springboot初始化的普通context(非web) 是AnnotationConfigApplicationContext, 在初始化的时候会初始化两个工具类， AnnotatedBeanDefinitionReader 和 ClassPathBeanDefinitionScanner 分别用来从 annotation driven 的配置和xml的配置中读取beanDefinition并向context注册, 那么在初始化 AnnotatedBeanDefinitionReader 的时候， 会向BeanFactory注册一个ConfigurationClassPostProcessor 用来处理所有的基于annotation的bean, 这个ConfigurationClassPostProcessor 是 BeanFactoryPostProcessor 的一个实现，springboot会保证在  invokeBeanFactoryPostProcessors(beanFactory) 方法中调用注册到它上边的所有的BeanFactoryPostProcessor)

如下代码显示是通过 ConfigurationClassParser 类来转换的

// Parse each @Configuration class
    ConfigurationClassParser parser = new ConfigurationClassParser(
        this.metadataReaderFactory, this.problemReporter, this.environment,
        this.resourceLoader, this.componentScanBeanNameGenerator, registry);

那么在 ConfigurationClassParser -> processConfigurationClass() -> doProcessConfigurationClass() 方法中我们找到了 (这里边的流程还是很清楚的， 分别按次序处理了@PropertySource， @ComponentScan, @Import, @ImportResource, 在处理这些注解的时候是通过递归处理来保证所有的都被处理了)

// Process any @Import annotations
    processImports(configClass, sourceClass, getImports(sourceClass), true);

那接下来就看它到底是怎么做的 . 流程依然清晰 ：

　　首先， 判断如果被import的是 ImportSelector.class 接口的实现， 那么初始化这个被Import的类， 然后调用它的selectImports方法去获得所需要的引入的configuration， 然后递归处理

　　其次， 判断如果被import的是 ImportBeanDefinitionRegistrar 接口的实现， 那么初始化后将对当前对象的处理委托给这个ImportBeanDefinitionRegistrar (不是特别明白， 只是我的猜测）

　　最后， 将import引入的类作为一个正常的类来处理 ( 调用最外层的doProcessConfigurationClass())

所以， 从这里我们知道， 如果你引入的是一个正常的component， 那么会作为@Compoent或者@Configuration来处理， 这样在BeanFactory里边可以通过getBean拿到， 但如果你是 ImportSelector 或者 ImportBeanDefinitionRegistrar 接口的实现， 那么spring并不会将他们注册到beanFactory中，而只是调用他们的方法。

private void processImports(ConfigurationClass configClass, SourceClass currentSourceClass,
      Collection<SourceClass> importCandidates, boolean checkForCircularImports) {

    if (importCandidates.isEmpty()) {
      return;
    }

    if (checkForCircularImports && isChainedImportOnStack(configClass)) {
      this.problemReporter.error(new CircularImportProblem(configClass, this.importStack));
    }
    else {
      this.importStack.push(configClass);
      try {
        for (SourceClass candidate : importCandidates) {
          if (candidate.isAssignable(ImportSelector.class)) {
            // Candidate class is an ImportSelector -> delegate to it to determine imports
            Class<?> candidateClass = candidate.loadClass();
            ImportSelector selector = BeanUtils.instantiateClass(candidateClass, ImportSelector.class);
            ParserStrategyUtils.invokeAwareMethods(
                selector, this.environment, this.resourceLoader, this.registry);
            if (this.deferredImportSelectors != null && selector instanceof DeferredImportSelector) {
              this.deferredImportSelectors.add(
                  new DeferredImportSelectorHolder(configClass, (DeferredImportSelector) selector));
            }
            else {
              String[] importClassNames = selector.selectImports(currentSourceClass.getMetadata());
              Collection<SourceClass> importSourceClasses = asSourceClasses(importClassNames);
              processImports(configClass, currentSourceClass, importSourceClasses, false);
            }
          }
          else if (candidate.isAssignable(ImportBeanDefinitionRegistrar.class)) {
            // Candidate class is an ImportBeanDefinitionRegistrar ->
            // delegate to it to register additional bean definitions
            Class<?> candidateClass = candidate.loadClass();
            ImportBeanDefinitionRegistrar registrar =
                BeanUtils.instantiateClass(candidateClass, ImportBeanDefinitionRegistrar.class);
            ParserStrategyUtils.invokeAwareMethods(
                registrar, this.environment, this.resourceLoader, this.registry);
            configClass.addImportBeanDefinitionRegistrar(registrar, currentSourceClass.getMetadata());
          }
          else {
            // Candidate class not an ImportSelector or ImportBeanDefinitionRegistrar ->
            // process it as an @Configuration class
            this.importStack.registerImport(
                currentSourceClass.getMetadata(), candidate.getMetadata().getClassName());
            processConfigurationClass(candidate.asConfigClass(configClass));
          }
        }
      }
      catch (BeanDefinitionStoreException ex) {
        throw ex;
      }
      catch (Throwable ex) {
        throw new BeanDefinitionStoreException(
            "Failed to process import candidates for configuration class [" +
            configClass.getMetadata().getClassName() + "]", ex);
      }
      finally {
        this.importStack.pop();
      }
    }
  }

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。