java - @Import 注解的用例是什么？

Question

根据official doc :

Annotation Type Configuration

Indicates that a class declares one or more @Bean methods and may beprocessed by the Spring container to generate bean definitions...

@Configuration classes may be composed using the @Import annotation,not unlike the way that works in Spring XML. Because@Configuration objects are managed as Spring beans within thecontainer..

但我也可以在没有@Import 的情况下使用@Configuration 注释。我已经测试了下面列出的代码，它按预期工作。那么使用@Import 的目的是什么？

DispatcherServletInitializer

public class ApplicationInitializer extends AbstractAnnotationConfigDispatcherServletInitializer {

    @Override
    protected Class<?>[] getRootConfigClasses() {
        return new Class<?>[] { WebConfig.class };
    }

    @Override
    protected Class<?>[] getServletConfigClasses() {
        return null;
    }

    @Override
    protected String[] getServletMappings() {
        return new String[] { "/" };
    }
    
 }

WebMvcConfigurerAdapter

@Configuration
@EnableWebMvc
@ComponentScan(basePackages = { "package.name" })
// @Import(OptionalConfig.class) 
public class WebConfig extends WebMvcConfigurerAdapter {
    // ...
}

可选配置

@Configuration
public class OptionalConfig {
    
    @Bean(name = "myClass")
    public MyClass myClass() {
        return new MyClass();
    }
}

服务

@Service
public class MyServiceImpl implements MyService {
    
    @Autowired
    private MyClass myClass;    // yes, it works

    // ...
}

Answer 1

如果启用组件扫描，您可以在多个@Configuration 类中拆分bean 定义，而无需使用@Import。而且您不需要将所有这些都提供给应用程序上下文构造函数。

我认为 @Import 的主要目的是 如果您想避免组件扫描(从 Spring Framework 4.2 开始，根据引用)，为您提供一种简化多个配置注册的方法手册)。

Spring Reference Documentation about @Import usage: 中有一条注释

As of Spring Framework 4.2, @Import also supports references to regular component classes, analogous to the AnnotationConfigApplicationContext.register method. This is particularly useful if you’d like to avoid component scanning, using a few configuration classes as entry points for explicitly defining all your components.