浅谈Spring中@Import注解的作用和使用

@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

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@documented
public @interface import {

 
 Class[] value();
}

分析类注释得出结论：

• 声明一个bean
• 导入@Configuration注解的配置类
• 导入importSelector的实现类
• 导入importBeanDefinitionRegistrar的实现类

@import注解的使用

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 {
}

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 {
}

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 {
}

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。 

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