@Import用來匯入@Configuration註解的配置類、宣告@Bean註解的bean方法、匯入ImportSelector的實現類或匯入ImportBeanDefinitionRegistrar的實現類。

@Import註解的作用

檢視Import註解原始碼

/**
 * 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();
}

分析類註釋得出結論：

1. 宣告一個bean
2. 匯入@Configuration註解的配置類
3. 匯入ImportSelector的實現類
4. 匯入ImportBeanDefinitionRegistrar的實現類

@Import註解的使用

1. 宣告一個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. 匯入@Configuration註解的配置類

   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. 匯入ImportSelector的實現類

   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. 匯入ImportBeanDefinitionRegistrar的實現類

   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 {
   }
   

最後，我們來看下匯入結果：

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("--------------------------------------------------------");
    }

}

列印結果如下：

--------------------------------------------------------
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通過不同的4種匯入方法被匯入SpringIOC容器中。