Spring中@Import注解的作用和使用（转）

add by zhj: @Import的作用是创建Spring bean，具体有四种用法

1. 导入普通类，即将普通类变为Spring bean

2. 导入@Configuration，即将该注解生效，具体来说就是：将其注解的类成为Spring bean，该类中的@Bean注解的方法也变为Spring bean。注：在应用启动类上使用@ComponentScan也能让@Configuration生效

3. 导入ImportSelector的实现类。ImportSelector接口中定义了方法selectImports，它返回字符串数组，里面是类的全路径。使用@Import导入ImportSelector的实现类，就是将selectImports方法返回的类注册为Spring bean

4. 导入ImportBeanDefinitionRegistrar的实现类。ImportBeanDefinitionRegistrar接口中定义了方法registerBeanDefinitions，它的功能就是通过BeanDefinitionRegistry实例注册Spring bean。

原文：https://blog.csdn.net/panchao888888/article/details/82882279

@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，我们来看下导入结果：

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