Spring中@Import注解的作用和使用

分手后的思念是犯贱 2023-06-25 04:55 9阅读 0赞

@Import用来导入@Configuration注解的配置类、声明@Bean注解的bean方法、导入ImportSelector的实现类或导入ImportBeanDefinitionRegistrar的实现类。

## @Import注解的作用 ##

查看Import注解源码

/**
     * Indicates one or more {@link Configuration @Configuration} classes to import.
     *
     * <p>Provides functionality equivalent to the {@code <import/>} element in Spring XML.
     * Only supported for classes annotated with {@code @Configuration} or declaring at least
     * one {@link Bean @Bean} method, as well as {@link ImportSelector} and
     * {@link ImportBeanDefinitionRegistrar} implementations.
     *
     * <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 {@link ImportResource @ImportResource}
     *
     * @author Chris Beams
     * @since 3.0
     * @see Configuration
     * @see ImportSelector
     * @see ImportResource
     */
    @Target(ElementType.TYPE)
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    public @interface Import {
    
    	/**
    	 * The @{@link Configuration}, {@link ImportSelector} and/or
    	 * {@link ImportBeanDefinitionRegistrar} classes to import.
    	 */
    	Class<?>[] value();
    }

分析类注释得出结论：

1.  声明一个bean
2.  导入@Configuration注解的配置类
3.  导入ImportSelector的实现类
4.  导入ImportBeanDefinitionRegistrar的实现类

## @Import注解的使用 ##

1、声明一个bean

package com.example.demo.bean;
    
    public class TestBean1 {
    }
    
    package com.example.demo;
    
    import com.example.demo.bean.TestBean1;
    import org.springframework.context.annotation.Configuration;
    import org.springframework.context.annotation.Import;
    
    @Import({TestBean1.class})
    @Configuration
    public class AppConfig {
    }

2、导入@Configuration注解的配置类

package com.example.demo.bean;
    
    public class TestBean2 {
    }
    
    package com.example.demo.bean;
    
    import org.springframework.context.annotation.Bean;
    import org.springframework.context.annotation.Configuration;
    
    @Configuration
    public class TestConfig {
        @Bean
        public TestBean2 getTestBean2(){
            return new TestBean2();
        }
    }
    
    package com.example.demo;
    
    import com.example.demo.bean.TestBean1;
    import com.example.demo.bean.TestConfig;
    import org.springframework.context.annotation.Configuration;
    import org.springframework.context.annotation.Import;
    
    @Import({TestBean1.class,TestConfig.class})
    @Configuration
    public class AppConfig {
    }

3、导入ImportSelector的实现类

package com.example.demo.bean;
    
    public class TestBean3 {
    }
    
    package com.example.demo.bean;
    
    import org.springframework.context.annotation.ImportSelector;
    import org.springframework.core.type.AnnotationMetadata;
    
    public class TestImportSelector implements ImportSelector {
        @Override
        public String[] selectImports(AnnotationMetadata importingClassMetadata) {
            return new String[]{"com.example.demo.bean.TestBean3"};
        }
    }
    
    package com.example.demo;
    
    import com.example.demo.bean.TestBean1;
    import com.example.demo.bean.TestConfig;
    import com.example.demo.bean.TestImportSelector;
    import org.springframework.context.annotation.Configuration;
    import org.springframework.context.annotation.Import;
    
    @Import({TestBean1.class,TestConfig.class,TestImportSelector.class})
    @Configuration
    public class AppConfig {
    }

4、导入ImportBeanDefinitionRegistrar的实现类

package com.example.demo.bean;
    
    public class TestBean4 {
    }
    
    package com.example.demo.bean;
    
    import org.springframework.beans.factory.support.BeanDefinitionRegistry;
    import org.springframework.beans.factory.support.RootBeanDefinition;
    import org.springframework.context.annotation.ImportBeanDefinitionRegistrar;
    import org.springframework.core.type.AnnotationMetadata;
    
    public class TestImportBeanDefinitionRegistrar implements ImportBeanDefinitionRegistrar {
        @Override
        public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
            RootBeanDefinition rootBeanDefinition = new RootBeanDefinition(TestBean4.class);
            registry.registerBeanDefinition("TestBean4", rootBeanDefinition);
        }
    }
    
    package com.example.demo;
    
    import com.example.demo.bean.TestBean1;
    import com.example.demo.bean.TestConfig;
    import com.example.demo.bean.TestImportBeanDefinitionRegistrar;
    import com.example.demo.bean.TestImportSelector;
    import org.springframework.context.annotation.Configuration;
    import org.springframework.context.annotation.Import;
    
    @Import({TestBean1.class,TestConfig.class,TestImportSelector.class,TestImportBeanDefinitionRegistrar.class})
    @Configuration
    public class AppConfig {
    }

最后，我们来看下导出结果：

package com.example.demo;
    
    import org.junit.Test;
    import org.junit.runner.RunWith;
    import org.springframework.boot.test.context.SpringBootTest;
    import org.springframework.context.annotation.AnnotationConfigApplicationContext;
    import org.springframework.test.context.junit4.SpringRunner;
    
    import java.util.Arrays;
    
    @RunWith(SpringRunner.class)
    @SpringBootTest
    public class DemoApplicationTests {
    
        @Test
        public void test() {
            AnnotationConfigApplicationContext annotationConfigApplicationContext = new AnnotationConfigApplicationContext(AppConfig.class);
            String[] beanDefinitionNames = annotationConfigApplicationContext.getBeanDefinitionNames();
            System.out.println("--------------------------------------------------------");
            for (String beanDefinitionName: beanDefinitionNames) {
                System.out.println(beanDefinitionName);
            }
            System.out.println("--------------------------------------------------------");
        }
    
    }

打印结果如下：

--------------------------------------------------------
    org.springframework.context.annotation.internalConfigurationAnnotationProcessor
    org.springframework.context.annotation.internalAutowiredAnnotationProcessor
    org.springframework.context.annotation.internalRequiredAnnotationProcessor
    org.springframework.context.annotation.internalCommonAnnotationProcessor
    org.springframework.context.event.internalEventListenerProcessor
    org.springframework.context.event.internalEventListenerFactory
    appConfig
    com.example.demo.bean.TestBean1
    com.example.demo.bean.TestConfig
    getTestBean2
    com.example.demo.bean.TestBean3
    TestBean4
    --------------------------------------------------------

可以看出TestBean1,TestBean2,TestBean3,TestBean4通过不同的4种导入方法被导入SpringIOC容器中。

参考：

[https://segmentfault.com/a/1190000020267424?utm\_source=tag-newest][https_segmentfault.com_a_1190000020267424_utm_source_tag-newest]

[https://blog.csdn.net/weixin\_45283443/article/details/99306878][https_blog.csdn.net_weixin_45283443_article_details_99306878]

[https_segmentfault.com_a_1190000020267424_utm_source_tag-newest]: https://segmentfault.com/a/1190000020267424?utm_source=tag-newest
[https_blog.csdn.net_weixin_45283443_article_details_99306878]: https://blog.csdn.net/weixin_45283443/article/details/99306878