@Import可以导入以下几种种类：
- 普通类
- 实现ImportSelector接口的类
- 实现DeferredImportSelector接口的类
- 实现ImportBeanDefinitionRegistrar接口的类
普通类
被导入的类会被容器注册成一个Bean，可以被依赖注入使用。【4.2 版本之前只可以导入配置类；4.2版本之后也可以导入普通类，导入的类会被当作配置类】
@Import注册一个类时，这个配置类不应该被@Component或者@Configuration注解标记。Spring中会将所有的bean class封装成一个ConfigurationClass，并且此后会判断被封装的bean class是否是由其他类导入的.
@Configuration
<strong>@Import(OtherBean.</strong><strong>class)
</strong>public class SpringConfig { }
public class OtherBean { }
ImportSelector实现类
实现类不会被注册成Bean，接口方法的返回值会被注册成Bean。【BeanName是全类名】
@Configuration
<strong>@Import(MyImportSelector.</strong><strong>class)
</strong>public class SpringConfig { }
public class MyImportSelector implements<strong> ImportSelector</strong> {
@Override
public String[] selectImports(AnnotationMetadata importingClassMetadata) {
return new String[]{OtherBean.class.getName()};
}
}
DeferredImportSelector实现类
DeferredImportSelector是ImportSelector的子接口, 所以它们的实现方式一样，只是Spring的处理方式不同。DeferredImportSelector和SpringBoot中自动导入配置文件的延迟导入有关。
@Configuration
<strong>@Import(MyDeferredImportSelector.</strong><strong>class)
</strong>public class SpringConfig { }
public class MyDeferredImportSelector implements<strong> DeferredImportSelector</strong> {
@Override
public String[] selectImports(AnnotationMetadata importingClassMetadata) {
return new String[]{OtherBean.class.getName()};
}
}
ImportBeanDefinitionRegistrar实现类
实现类不会被注册为bean，但是会回调其接口方法，由开发者通过Spring api手动向Spring容器注册bean。【类似于BeanFactoryPostRegister】
@Configuration
<strong>@Import(MyImportBeanDefinitionRegistrar.</strong><strong>class)
</strong>public class SpringConfig { }
public class MyImportBeanDefinitionRegistrar implements<strong> ImportBeanDefinitionRegistrar</strong> {
@Override
public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
BeanDefinition beanDefinition = new RootBeanDefinition();
String beanName = StringUtils.uncapitalize(OtherBean.class.getSimpleName());
beanDefinition.setBeanClassName(OtherBean.class.getName());
registry.registerBeanDefinition(beanName,beanDefinition);
}
}
附：@Import相关源码解析
加载解析@Import注解位于BeanFactoryPostProcessor处理的时候：
AbstractApplicationContext的refresh方法
->invokeBeanFactoryPostProcessors(beanFactory);
->PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());
->registryProcessor.postProcessBeanDefinitionRegistry(registry);
这里的registryProcessor,我们指ConfigurationClassPostProcessor
ConfigurationClassPostProcessor.postProcessBeanDefinitionRegistry(registry)
->processConfigBeanDefinitions(registry):
public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) {
//省略一些配置检查与设置的逻辑
//根据@Order注解，排序所有的@Configuration类
configCandidates.sort((bd1, bd2) -> {
int i1 = ConfigurationClassUtils.getOrder(bd1.getBeanDefinition());
int i2 = ConfigurationClassUtils.getOrder(bd2.getBeanDefinition());
return Integer.compare(i1, i2);
});
// 创建ConfigurationClassParser解析@Configuration类
ConfigurationClassParser parser = new ConfigurationClassParser(
this.metadataReaderFactory, this.problemReporter, this.environment,
this.resourceLoader, this.componentScanBeanNameGenerator, registry);
//剩余没有解析的@Configuration类
Set<BeanDefinitionHolder> candidates = new LinkedHashSet<>(configCandidates);
//已经解析的@Configuration类
Set<ConfigurationClass> alreadyParsed = new HashSet<>(configCandidates.size());
do {
//解析
parser.parse(candidates);
parser.validate();
Set<ConfigurationClass> configClasses = new LinkedHashSet<>(parser.getConfigurationClasses());
configClasses.removeAll(alreadyParsed);
// 生成类定义读取器读取类定义
if (this.reader == null) {
this.reader = new ConfigurationClassBeanDefinitionReader(
registry, this.sourceExtractor, this.resourceLoader, this.environment,
this.importBeanNameGenerator, parser.getImportRegistry());
}
this.reader.loadBeanDefinitions(configClasses);
alreadyParsed.addAll(configClasses);
candidates.clear();
if (registry.getBeanDefinitionCount() > candidateNames.length) {
//省略检查是否有其他需要加载的配置的逻辑
}
}
while (!candidates.isEmpty());
//省略后续清理逻辑
}
其中parser.parse(candidates)的逻辑主要由org.springframework.context.annotation.ConfigurationClassParser实现，功能是加载@Import注解还有即系@Import注解。reader.loadBeanDefinitions(configClasses);的逻辑主要由org.springframework.context.annotation.ConfigurationClassBeanDefinitionReader的loadBeanDefinitionsForConfigurationClass方法实现，功能是将上面解析的配置转换为BeanDefinition就是Bean定义。