Spring注解之@Import用法

朴灿烈づ我的快乐病毒、 2023-10-08 08:56 10阅读 0赞

> 最近在看spring5.1以及mybatis-spring源码的时候，发现@MapperScan以及很多springboot的@EnableXXX注解，都用到了这个@Import注解。所以打算仔细研究下该注解的作用。
> 
> 顺便把类似的@ImportRescource也在这里总结一下。

## 一、@Import简介 ##

**简介：**在Spring3.0之前，我们的bean可以通过xml配置文件和扫描特定包下的类来将指定的bean注入到Spring容器中。Spring 3.0之后提供了JavaConfig的方式，也就是将Spring容器里bean的元信息以java代码的方式进行描述。即通过@Configuration与@Bean这两个注解配合使用来将原来配置在xml文件里的Bean通过java代码的方式进行描述，进而将bean注入到Spring容器中**。**

**根据下面的源码，我们可以简单的总结，主要有三种用法：**

*  功能类似XML配置的，用来导入配置类。
 *  可以导入带有@Configuration注解的配置类或实现了ImportSelector或portBeanDefinitionRegistrar。
 *  导入普通的POJO（Spring会将其注册成Spring Bean，导入POJO需要使用Spring 4.2以上）。

/*
     * Copyright 2002-2016 the original author or authors.
     *
     * Licensed under the Apache License, Version 2.0 (the "License");
     * you may not use this file except in compliance with the License.
     * You may obtain a copy of the License at
     *
     *      http://www.apache.org/licenses/LICENSE-2.0
     *
     * Unless required by applicable law or agreed to in writing, software
     * distributed under the License is distributed on an "AS IS" BASIS,
     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     * See the License for the specific language governing permissions and
     * limitations under the License.
     */
    
    package org.springframework.context.annotation;
    
    import java.lang.annotation.Documented;
    import java.lang.annotation.ElementType;
    import java.lang.annotation.Retention;
    import java.lang.annotation.RetentionPolicy;
    import java.lang.annotation.Target;
    
    /**
     * Indicates one or more {@link Configuration @Configuration} classes to import.
     *
     * <p>Provides functionality equivalent to the {@code <import/>} element in Spring XML.
     * Allows for importing {@code @Configuration} classes, {@link ImportSelector} and
     * {@link ImportBeanDefinitionRegistrar} implementations, as well as regular component
     * classes (as of 4.2; analogous to {@link AnnotationConfigApplicationContext#register}).
     *
     * <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 the {@link ImportResource @ImportResource} annotation instead.
     *
     * @author Chris Beams
     * @author Juergen Hoeller
     * @since 3.0
     * @see Configuration
     * @see ImportSelector
     * @see ImportResource
     */
    @Target(ElementType.TYPE)
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    public @interface Import {
    
    	/**
    	 * {@link Configuration}, {@link ImportSelector}, {@link ImportBeanDefinitionRegistrar}
    	 * or regular component classes to import.
    	 */
    	Class<?>[] value();
    
    }

## 二、@Import用法详解 ##

#### 1、ImportSelector ####

*  根据AnnotationMetadata（实质上是引入标注了@Import注解的类的注解信息），来判断是否引入哪些配置类，通过字符串数组的方式返回配置类的全限定名。

#### 2、DeferredImportSelector ####

*  ImportSelector的子接口，区别是，他会在所有的@Configuration类加载完成之后再加载返回的配置类。ImportSelector会在当前Configuration类加载之前去加载返回的配置类。
 *  可以使用@Order注解或者Ordered接口来指定DeferredImportSelector的加载顺序。
 *  并且提供了新的方法getImportGroup()用来跨DeferredImportSelector实现自定义Configuration的加载顺序。

#### 3、ImportBeanDefinitionRegistrar ####

*  根据AnnotationMetadata（实质上是引入标注了@Import注解的类的注解信息）, 来注册BeanDenfinition,通过BeanDefinitionRegistry实例的register可以注册BeanDefinition。

## 三、@Import代码测试 ##

package com.evan.config;
    
    import com.evan.service.JustinService;
    import org.springframework.context.annotation.Configuration;
    import org.springframework.context.annotation.Import;
    
    @Configuration
    @Import({MyImportSelector.class,MyImportBeanDefinitionRegistrar.class,OtherConfig.class, JustinService.class})
    public class MyConfig {
    
    }

package com.evan.config;
    
    import com.evan.service.TomService;
    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;
    //使用ImportBeanDefinitionRegistrar的方式,直接导入BeanDefinition
    public class MyImportBeanDefinitionRegistrar implements ImportBeanDefinitionRegistrar {
        @Override
        public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
            registry.registerBeanDefinition("TomService", new RootBeanDefinition(TomService.class));
        }
    }

package com.evan.config;
    
    import com.evan.service.EvanService;
    import com.evan.service.PersonService;
    import org.springframework.context.annotation.ImportSelector;
    import org.springframework.core.type.AnnotationMetadata;
    //使用ImportSelector导入，相当于@Bean
    public class MyImportSelector implements ImportSelector {
        @Override
        public String[] selectImports(AnnotationMetadata importingClassMetadata) {
            //返回String数组。
            return new String[]{EvanService.class.getName(), PersonService.class.getName()};
        }
    }

package com.evan.config;
    
    import com.evan.service.JayService;
    import org.springframework.context.annotation.Bean;
    import org.springframework.context.annotation.Configuration;
    //直接导入配置类
    @Configuration
    public class OtherConfig {
        @Bean
        public JayService jay(){
            return new JayService();
        }
    }

package com.evan.service;
    
    public class EvanService {
    }

package com.evan.service;
    
    public class JayService {
    }

package com.evan.service;
    
    public class JustinService {
    }

package com.evan.service;
    
    public class PersonService {
    }

package com.evan.service;
    
    public class TomService {
    }

**主测试类：**

package com.evan.test;
    
    import com.evan.config.MyConfig;
    import org.springframework.context.annotation.AnnotationConfigApplicationContext;
    
    public class Test {
        public static void main(String[] args) {
            AnnotationConfigApplicationContext ac = new AnnotationConfigApplicationContext();
            ac.register(MyConfig.class);
            ac.refresh();
            String[] beanDefinitionNames = ac.getBeanDefinitionNames();
            for (String beanDefinitionName : beanDefinitionNames) {
                System.out.println(beanDefinitionName);
            }
        }
    }

**运行结果：**

org.springframework.context.annotation.internalConfigurationAnnotationProcessor
    org.springframework.context.annotation.internalAutowiredAnnotationProcessor
    org.springframework.context.annotation.internalCommonAnnotationProcessor
    org.springframework.context.event.internalEventListenerProcessor
    org.springframework.context.event.internalEventListenerFactory
    myConfig
    com.evan.service.EvanService
    com.evan.service.PersonService
    com.evan.config.OtherConfig
    jay
    com.evan.service.JustinService
    TomService