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背景

软件开发是一门实践性科学，对大多数人来说，学习一种新技术不是一开始就去深究其原理，而是先从做出一个可工作的DEMO入手。但在我个人学习和工作经历中，每次学习新技术总是要花费或多或少的时间、检索不止一篇资料才能得出一个可工作的DEMO，这占用了我大量的时间精力。因此本文旨在通过一篇文章即能还原出可工作的、甚至可用于生产的DEMO，期望初学者能尽快地迈过0到1的这一步骤，并在此基础上不断深化对相关知识的理解。
为达以上目的，本文会将开发环境、工程目录结构、开发步骤及源码尽量全面地展现出来，文字描述能简则简，能用代码注释的绝不在正文中再啰嗦一遍，正文仅对必要且关键的信息做重点描述。

介绍

本文介绍开发Spring Boot应用如何获取已注入的Bean实例。

开发环境

分类	名称	版本
操作系统	Windows	Windows 11
JDK	Oracle JDK	21.0.1
IDE	IntelliJ IDEA	2023.3.4
构建工具	Apache Maven	3.9.6

开发步骤及源码

1> 创建Maven工程，添加依赖。

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"><modelVersion>4.0.0</modelVersion><parent><groupId>com.jiyongliang</groupId><artifactId>springboot3-bean</artifactId><version>0.0.1</version></parent><artifactId>springboot3-bean-obtain</artifactId><properties><java.version>21</java.version><maven.compiler.source>21</maven.compiler.source><maven.compiler.target>21</maven.compiler.target><project.build.sourceEncoding>UTF-8</project.build.sourceEncoding><spring-boot.version>3.2.2</spring-boot.version><lombok.version>1.18.30</lombok.version></properties><dependencyManagement><dependencies><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-dependencies</artifactId><version>${spring-boot.version}</version><type>pom</type><scope>import</scope></dependency></dependencies></dependencyManagement><dependencies><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter</artifactId></dependency><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-test</artifactId><scope>test</scope></dependency><dependency><groupId>org.projectlombok</groupId><artifactId>lombok</artifactId><version>${lombok.version}</version></dependency></dependencies><build><pluginManagement><plugins><plugin><groupId>org.springframework.boot</groupId><artifactId>spring-boot-maven-plugin</artifactId><version>${spring-boot.version}</version></plugin></plugins></pluginManagement><plugins><plugin><groupId>org.springframework.boot</groupId><artifactId>spring-boot-maven-plugin</artifactId></plugin></plugins></build>
</project>

2> 定义SpringBoot应用启动类。

package org.example.springboot;import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;@SpringBootApplication
public class SpringBoot3BeanObtainApplication {public static void main(String[] args) {SpringApplication.run(SpringBoot3BeanObtainApplication.class, args);}
}

3> 定义获取已注入Bean的核心类。

package org.example.springboot.manager;import lombok.Getter;
import lombok.extern.slf4j.Slf4j;
import org.springframework.beans.factory.NoSuchBeanDefinitionException;
import org.springframework.context.ApplicationContext;
import org.springframework.stereotype.Component;@Component
@Getter
@Slf4j
public class BeanManager {private final ApplicationContext applicationContext;public BeanManager(ApplicationContext applicationContext) {this.applicationContext = applicationContext;}/*** 通过Bean名称获取*/public Object getBean(String name) {try {return this.applicationContext.getBean(name);} catch (NoSuchBeanDefinitionException e) {log.error(e.getMessage());return null;}}/*** 通过Bean类型获取* 如果存在同一类型的多个Bean实例则会抛出NoSuchBeanDefinitionException，异常信息：expected single matching bean but found 2*/public <T> T getBean(Class<T> clazz) {try {return this.applicationContext.getBean(clazz);} catch (NoSuchBeanDefinitionException e) {log.error(e.getMessage());return null;}}/*** 通过Bean名称和类型获取*/public <T> T getBean(String name, Class<T> clazz) {try {return this.applicationContext.getBean(name, clazz);} catch (NoSuchBeanDefinitionException e) {log.error(e.getMessage());return null;}}
}

4> 定义测试Bean。

package org.example.springboot.bean;import lombok.AllArgsConstructor;
import lombok.Data;@AllArgsConstructor
@Data
public class CustomBean {private Integer id;private String name;
}

5> 配置注入Bean。

package org.example.springboot.config;import org.example.springboot.bean.CustomBean;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;@Configuration
public class CustomBeanConfig {@Bean("beanX")public CustomBean beanX() {return new CustomBean(1, "Bean-X");}@Bean("beanY")public CustomBean beanY() {return new CustomBean(2, "Bean-X");}
}

6> 创建单元测试。

package org.example.springboot.manager;import jakarta.annotation.Resource;
import org.assertj.core.api.Assertions;
import org.example.springboot.bean.CustomBean;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;@SpringBootTest
class BeanManagerTests {@AutowiredBeanManager beanManager;@Resource(name = "beanX")CustomBean beanX;@Resource(name = "beanY")CustomBean beanY;@Testvoid test() {System.out.println("Test get bean by name");Assertions.assertThat(beanManager.getBean("beanX")).isSameAs(beanX);Assertions.assertThat(beanManager.getBean("beanY")).isSameAs(beanY);System.out.println("Test get bean by type");Assertions.assertThat(beanManager.getBean(CustomBean.class)).isNull();System.out.println("Test get bean by name and type");Assertions.assertThat(beanManager.getBean("beanX", CustomBean.class)).isSameAs(beanX);Assertions.assertThat(beanManager.getBean("beanY", CustomBean.class)).isSameAs(beanY);}
}

7> 单元测试结果。

单元测试日志：

Test get bean by name
Test get bean by type
2024-03-08T20:31:34.845+08:00 ERROR 18872 --- [           main] o.e.springboot.manager.BeanManager       : No qualifying bean of type 'org.example.springboot.bean.CustomBean' available: expected single matching bean but found 2: beanX,beanY
Test get bean by name and type

从日志中可以看出，测试根据类型获取注入的Bean时抛出了异常，因为找到2个同样类型的Bean，无法判断应该返回哪个Bean，因此通过类型获取Bean时，有且仅有一个类型匹配的Bean才会正常返回。

工程目录结构

总结

测试用的Bean和配置注入放在 src/test 目录下是为了辅助测试。