Three ways Spring uses @Import to inject beans

1. Preparation

1. Import spring-context dependencies

<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-context</artifactId>
    <version>5.2.22.RELEASE</version>
</dependency>

2. Create entity classes Red, Blue, Yellow

public class Red {

}
public class Blue {

}
public class Yellow {

}

Second, the use of annotations

1. Create a BeanConfig configuration class

import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Import;

//@Import(Red.class)
@Import({Red.class, Yellow.class, Blue.class})  
@Configuration
public class BeansConfig {

}

2. Start the test

public static void main(String[] args) {
    AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(BeansConfig.class);
    String[] names = context.getBeanDefinitionNames();
    for (String name:names) {
        System.out.println(name);
    }
    /**
     * @import注入bean的名称为：全类名
     * 包路径.Red
     * 包路径.Yellow
     * 包路径.Blue
     */
}

三、ImportSelector

1. Create ColorSelector

import org.springframework.context.annotation.ImportSelector;
import org.springframework.core.type.AnnotationMetadata;

public class ColorSelector implements ImportSelector {

    @Override
    public String[] selectImports(AnnotationMetadata importingClassMetadata) {
        // 这里可以通过AnnotationMetadata处理业务逻辑，为了演示我们就尽量简单着来了
        return new String[]{Red.class.getName(), Blue.class.getName(), Yellow.class.getName()};
    }

}

2. Configure to @Import

import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Import;

//@Import({Red.class, Yellow.class, Blue.class})
@Import(ColorSelector.class)
@Configuration
public class BeansConfig {

}

3. Re-test, the result is the same

四、ImportBeanDefinitionRegistrar

1. Create a Color Registrar

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 ColorRegistrar implements ImportBeanDefinitionRegistrar {

    @Override
    public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
        // 如果red没有注入，则手动注入
        if(!registry.containsBeanDefinition(Red.class.getName())){
            RootBeanDefinition red = new RootBeanDefinition(Red.class);
            registry.registerBeanDefinition("red",red);
        }
        // 如果blue没有注入，则手动注入
        if(!registry.containsBeanDefinition(Blue.class.getName())){
            RootBeanDefinition blue = new RootBeanDefinition(Blue.class);
            registry.registerBeanDefinition("blue",blue);
        }
        // 如果yellow没有注入，则手动注入
        if(!registry.containsBeanDefinition(Yellow.class.getName())){
            RootBeanDefinition yellow = new RootBeanDefinition(Yellow.class);
            registry.registerBeanDefinition("yellow",yellow);
        }
    }

}

2. Configure to @Import

import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Import;

//@Import(ColorSelector.class)
@Import(ColorRegistrar.class)
@Configuration
public class BeansConfig {

}

3. Test again, the result is the same

3. Summary and analysis

This chapter mainly learns three ways to inject Bean using @Import annotation. "Reference Materials"