Spring提供了几种方式来注册Bean，日常开发中使用最多的是ComponentScan。得益于ComponentScan，注册bean非常的简单，只需要在被注册的类上声明@Component或者@Service等注解即可。

除了ComponentScan，Spring还支持使用Configuration注解来注册Bean。在大型的项目中，模块化开发能极大地降低系统的复杂性，这时需要每个模块来定义本模块Bean注册情况，Configuration发挥着巨大的作用。

@Configuration
@ConditionalOnProperty(prefix = "module.wx-login", value = "enable", havingValue = "true")
@ComponentScan(basePackages = "com.lin.decorator.wxlogin")
public class WxLoginConfiguration {
}

每个模块定义了Configuration之后，需要将多个模块的Configuration组合。Spring提供了Import注解来实现多个Configuration组合。

@Import(WxLoginConfiguration.class)
public class Application {
    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
}

Spring官方文档中关于Import的描述如下：

Provides functionality equivalent to the element in Spring XML. Allows for importing @Configuration classes, ImportSelector and ImportBeanDefinitionRegistrar implementations, as well as regular component classes (as of 4.2; analogous to AnnotationConfigApplicationContext.register(java.lang.Class<?>...)).

@Bean definitions declared in imported @Configuration classes should be accessed by using @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 @Configuration class methods.

除了Configuration，Import还支持引入ImportSelector和ImportBeanDefinitionRegistrar。既然要全面了解Import机制，那另外两个也要一探究竟。

ImportSelector

Spring官方文档中，对ImportSelector的描述如下：

Interface to be implemented by types that determine which @Configuration class(es) should be imported based on a given selection criteria, usually one or more annotation attributes.

从字面上理解，ImportSelector可以根据注解里面的一个或多个属性来决定引入哪些Configuration。举个例子：

小伙伴都用过Transactional注解，Transactional注解生效的前提是EnableTransactionManagement生效。看过EnableTransactionManagement源代码的小伙伴应该都知道，它通过Import引入了一个ImportSelector。

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import(TransactionManagementConfigurationSelector.class)
public @interface EnableTransactionManagement {
	boolean proxyTargetClass() default false;
	AdviceMode mode() default AdviceMode.PROXY;
	int order() default Ordered.LOWEST_PRECEDENCE;
}

而TransactionManagementConfigurationSelector会根据注解里面的AdviceMode不同，来确定引入不同的Configuration。

	protected String[] selectImports(AdviceMode adviceMode) {
		switch (adviceMode) {
			case PROXY:
				return new String[] {AutoProxyRegistrar.class.getName(),
						ProxyTransactionManagementConfiguration.class.getName()};
			case ASPECTJ:
				return new String[] {determineTransactionAspectClass()};
			default:
				return null;
		}
	}

ImportBeanDefinitionRegistrar

Spring官方文档中，对ImportBeanDefinitionRegistrar的描述如下：

Interface to be implemented by types that register additional bean definitions when processing @Configuration classes. Useful when operating at the bean definition level (as opposed to @Bean method/instance level) is desired or necessary.

字面意思是，通过继承这个接口可以额外定义Bean。举个例子：

在使用Mybatis的时候，会使用到MapperScan这个注解，这个注解通过Import引入了ImportBeanDefinitionRegistrar，这也解释了为什么我们只在Interface上申明了一个Mapper，mybatis就帮我们生成好了Bean。

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(MapperScannerRegistrar.class)
@Repeatable(MapperScans.class)
public @interface MapperScan {
}

有小伙伴在编码工程中，并没有使用MapperScan，为什么也能正常使用呢？其实是Mybatis starter的功劳。在MybatisAutoConfiguration里面定义了ImportBeanDefinitionRegistrar，当MapperScan没有激活时，它就会生效。

 @org.springframework.context.annotation.Configuration
  @Import(AutoConfiguredMapperScannerRegistrar.class)
  @ConditionalOnMissingBean({ MapperFactoryBean.class, MapperScannerConfigurer.class })
  public static class MapperScannerRegistrarNotFoundConfiguration implements InitializingBean {

    @Override
    public void afterPropertiesSet() {
      logger.debug(
          "Not found configuration for registering mapper bean using @MapperScan, MapperFactoryBean and MapperScannerConfigurer.");
    }
  }

Import执行流程

了解了Import支持的三种不同类型的资源之后，接下来通过debug来看一下import的执行过程。通过设置断点，发现在ConfigurationClassParser类中，通过深度遍历来处理Import。

private void collectImports(SourceClass sourceClass, Set imports, Set visited)
			throws IOException {

		if (visited.add(sourceClass)) {
			for (SourceClass annotation : sourceClass.getAnnotations()) {
				String annName = annotation.getMetadata().getClassName();
				if (!annName.equals(Import.class.getName())) {
					collectImports(annotation, imports, visited);
				}
			}
			imports.addAll(sourceClass.getAnnotationAttributes(Import.class.getName(), "value"));
		}
	}

而上面介绍的ImportSelector，需要调用selectImports方法进行解析。

private void processImports(ConfigurationClass configClass, SourceClass currentSourceClass,
			Collection importCandidates, Predicate exclusionFilter,
			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 = ParserStrategyUtils.instantiateClass(candidateClass, ImportSelector.class,
								this.environment, this.resourceLoader, this.registry);
						Predicate selectorFilter = selector.getExclusionFilter();
						if (selectorFilter != null) {
							exclusionFilter = exclusionFilter.or(selectorFilter);
						}
						if (selector instanceof DeferredImportSelector) {
							this.deferredImportSelectorHandler.handle(configClass, (DeferredImportSelector) selector);
						}
						else {
							String[] importClassNames = selector.selectImports(currentSourceClass.getMetadata());
							Collection importSourceClasses = asSourceClasses(importClassNames, exclusionFilter);
							processImports(configClass, currentSourceClass, importSourceClasses, exclusionFilter, 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 =
								ParserStrategyUtils.instantiateClass(candidateClass, ImportBeanDefinitionRegistrar.class,
										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), exclusionFilter);
					}
				}
			}
			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();
			}
		}
	}

通过递归的方式，实现了资源的加载。

本文基于Spring 5.3.4，如有错误，欢迎指正。