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随着开发者的技术要求日益提高，学习新知识才能使我们在面试中胜任并赶上潮流，开发者往往会被问到一些框架的底层实现。与背诵面试题相比，亲自编写一遍能够更深入理解框架的工作原理。因此，我决定手写一个简易的Spring IOC框架，探索其依赖注入的实现原理，避免仅仅停留在理论层面。

项目结构

项目采用Maven进行管理，主要依赖包括DOM4J解析XML的库和Spring相关组件。项目结构设计如下：

• src/main/java/com/model：存放实体类
• src/main/java/com/service：存放服务类
• src/main/resources：存放Spring配置文件

pom.xml依赖管理

   
    
    
     4.0.0
    
    
    
     com
    
    
    
     SpringIOC
    
    
    
     1.0-SNAPSHOT
    
    
    
        
     
            
      
       dom4j
      
            
      
       dom4j
      
            
      
       1.6.1
      
        
     
    
    
   

UserEntity 实体类

package com.model;
public class UserEntity {
    private String userName;
    private String userId;
    public String getUserName() {
        return userName;
    }
    public void setUserName(String userName) {
        this.userName = userName;
    }
    public String getUserId() {
        return userId;
    }
    public void setUserId(String userId) {
        this.userId = userId;
    }
}

UserService 服务类

package com.service;
import java.text.SimpleDateFormat;
import java.util.Date;
public class UserService {
    private String info;
    public String getInfo() {
        return info;
    }
    public void setInfo(String info) {
        this.info = info;
    }
    public String getNowTime() {
        return new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").format(new Date());
    }
}

SpringIOC实现

package com;
import com.model.UserEntity;
import com.service.UserService;
import org.dom4j.Attribute;
import org.dom4j.Document;
import org.dom4j.DocumentException;
import org.dom4j.Element;
import org.dom4j.io.SAXReader;
import java.lang.reflect.Field;
import java.util.List;
public class SpringApplicationContext {
    private String xmlPath;
    public SpringApplicationContext(String xmlPath) throws DocumentException, ClassNotFoundException, IllegalAccessException, InstantiationException, NoSuchFieldException {
        this.xmlPath = xmlPath;
    }
    public 
   
     T getBean(String beanId) throws DocumentException, ClassNotFoundException, IllegalAccessException, InstantiationException, NoSuchFieldException {
        SAXReader saxReader = new SAXReader();
        Document xmlDom = saxReader.read(getClass().getClassLoader().getResourceAsStream(xmlPath));
        Element rootElement = xmlDom.getRootElement();
        List
    
      elements = rootElement.elements();
        for (Element bean : elements) {
            String className = bean.attributeValue("class");
            Class
      aClass = Class.forName(className);
            Object obj = aClass.newInstance();
            List
     
       fieldList = bean.elements();
            for (Element fieldItem : fieldList) {
                String columnName = fieldItem.attributeValue("name");
                String value = fieldItem.attributeValue("value");
                Field declaredField = aClass.getDeclaredField(columnName);
                declaredField.setAccessible(true);
                declaredField.set(obj, value);
            }
            String id = bean.attributeValue("id");
            if (id.equals(beanId)) {
                return (T) obj;
            }
        }
        throw new RuntimeException("id不存在");
    }
    public static void main(String[] args) throws ClassNotFoundException, InstantiationException, DocumentException, IllegalAccessException, NoSuchFieldException {
        SpringApplicationContext springApplicationContext = new SpringApplicationContext("application.xml");
        UserEntity user = springApplicationContext.getBean("user1");
        System.out.println("用户ID: " + user.getUserId() + ", 用户名: " + user.getUserName());
        UserService userService = springApplicationContext.getBean("userService");
        System.out.println("当前时间: " + userService.getNowTime());
    }
}
     
    
   

application.xml配置文件

测试结果

通过Main方法运行，输出结果如下：

用户ID: 0002, 用户名: 张三
当前时间: 2023-10-25 00:00:00

测试成功，IOC框架能够正确注入用户实例和服务类。