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1. Component annotation

Note: The class that adds annotations needs to be in the same package path as the startup class. If it is in another package, it needs to be inStartup classorThe class with the above annotation under the package where the startup class is locatedAdded in@ComponentScanannotation.

For example:

Startup class:

package com.springboottest;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.ComponentScan;

@SpringBootApplication
@ComponentScan(basePackages = {"", ""})
public class SpringbootTestApplication {

    public static void main(String[] args) {
        SpringApplication.run(SpringbootTestApplication.class, args);
    }

}

Student class (not in the same package path as the startup class):

package com.springtest;

import lombok.Data;
import org.springframework.stereotype.Component;

import java.lang.reflect.Field;

@Data
@Component
public class Student {

    private String name;
    private String nickName;
}

2、@Component + @Bean

@Component
public class Test {

   @Bean
   public User user() {
        return new User();
   }
}

Note:@BeanAnnotation method, return valueUserAs injectedBeanObject.

3. @Import(PlaceHolderClass) Quickly import a component

@Configuration
@Import({ImportDemo.class, MyImportSelector.class, MyImportBeanDefinitionRegistrar.class})
public class ImportConfig{
    @Bean
    public User user(){
        return new User();
    }
}

//Custom logic returns the components that need to be imported
public class MyImportSelector implements ImportSelector {
 
    //The return value is an array of fully qualified names of components imported into the container
    @Override
    public String[] selectImports(AnnotationMetadata importingClassMetadata) {
        //All annotations of the current class
        Set<String> annotationTypes = importingClassMetadata.getAnnotationTypes();
        System.out.println("Annotation information of the current configuration class:"+annotationTypes);
        return new String[]{".user01",".user02"};
    }
}

public class MyImportBeanDefinitionRegistrar implements ImportBeanDefinitionRegistrar {
 
    /**
      * AnnotationMetadata: The annotation information of the current class
      * BeanDefinitionRegistry:BeanDefinition registration class;
      * Add all beans that need to be added to the container; call
      * Manual registration
      */
    @Override
    public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
 
        boolean definition = registry.containsBeanDefinition(".User01");
        boolean definition2 = registry.containsBeanDefinition(".User02");
        if(definition && definition2){
            //Create a BeanDefinition (Bean's description information object)
            RootBeanDefinition beanDefinition = new RootBeanDefinition(User03.class);
            //Register a bean and specify the bean name
            registry.registerBeanDefinition("User03", beanDefinition);
        }
    }
}

4. Use FactoryBean Injection provided by Spring

public class UserFactoryBean implements FactoryBean<User> {
    
    // Injected Bean
    @Override
    public User getObject() throws Exception {
        return new User();
    }

    // Injected Bean type
    @Override
    public Class<?> getObjectType() {
        return User.class;
    }
    
    // Is it a single case
    @Override
    public boolean isSingleton() {
        return true;
    }
}