Spring中@Import注解的作用和使用（轉）

add by zhj: @Import的作用是創建Spring bean，具體有四種用法

1. 導入普通類，即將普通類變為Spring bean

2. 導入@Configuration，即將該注解生效，具體來說就是：將其注解的類成為Spring bean，該類中的@Bean注解的方法也變為Spring bean。注：在應用啟動類上使用@ComponentScan也能讓@Configuration生效

3. 導入ImportSelector的實現類。ImportSelector接口中定義了方法selectImports，它返回字符串數組，里面是類的全路徑。使用@Import導入ImportSelector的實現類，就是將selectImports方法返回的類注冊為Spring bean

4. 導入ImportBeanDefinitionRegistrar的實現類。ImportBeanDefinitionRegistrar接口中定義了方法registerBeanDefinitions，它的功能就是通過BeanDefinitionRegistry實例注冊Spring bean。

 

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

@Import用來導入@Configuration注解的配置類、聲明@Bean注解的bean方法、導入ImportSelector的實現類或導入ImportBeanDefinitionRegistrar的實現類。

@Import注解的作用

查看Import注解源碼

/**
 * 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();
}

分析類注釋得出結論：

1. 聲明一個bean
2. 導入@Configuration注解的配置類
3. 導入ImportSelector的實現類
4. 導入ImportBeanDefinitionRegistrar的實現類

@Import注解的使用

1. 聲明一個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. 導入@Configuration注解的配置類

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. 導入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. 導入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 {
}

最后，我們來看下導入結果：

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

}

打印結果如下：

--------------------------------------------------------
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種導入方法被導入SpringIOC容器中