使用java反射机制+dom4j实现SpringIOC原理

SpringIOC的底层原理:

1、解析xml文件——使用dom4j

2、通过bean id查找到对象的xml节点，并获取该xml上class节点属性——获取根节点后遍历得到二级节点

3、通过java的反射机制初始化类

4、使用java的反射机制为私有属性赋值——遍历二级节点获得三级节点 API对应:List<Element> ele3 = element.elements();

applicationContext.xml

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop"
	xmlns:tx="http://www.springframework.org/schema/tx" xmlns:context="http://www.springframework.org/schema/context"
	xsi:schemaLocation="    
     http://www.springframework.org/schema/beans     
     http://www.springframework.org/schema/beans/spring-beans-3.0.xsd    
     http://www.springframework.org/schema/tx     
     http://www.springframework.org/schema/tx/spring-tx-3.0.xsd    
     http://www.springframework.org/schema/aop     
     http://www.springframework.org/schema/aop/spring-aop-3.0.xsd    
     http://www.springframework.org/schema/context    
     http://www.springframework.org/schema/context/spring-context-3.0.xsd">

	<bean id="user1" class="com.itmayiedu.entity.UserEntity">
		<property name="userId" value="001"></property>
		<property name="userName" value="张三"></property>
	</bean>
	<bean id="user2" class="com.itmayiedu.entity.UserEntity">
		<property name="userId" value="002"></property>
		<property name="userName" value="李四"></property>
	</bean>


</beans>   

UserEntity.java

package com.itmayiedu.entity;

public class UserEntity {
	private String userId;
	private String userName;

	 public UserEntity(){
		 System.out.println("无参构造函数....");
	 }
	
	public String getUserId() {

		return userId;
	}

	public void setUserId(String userId) {

		this.userId = userId;
	}

	public String getUserName() {

		return userName;
	}

	public void setUserName(String userName) {

		this.userName = userName;
	}

	@Override
	public String toString() {
		return "UserEntity [userId=" + userId + ", userName=" + userName + "]";
	}

}

springioc实现类

import java.lang.reflect.Field;
import java.util.List;

import org.apache.commons.lang3.StringUtils;
import org.dom4j.Document;
import org.dom4j.DocumentException;
import org.dom4j.Element;
import org.dom4j.io.SAXReader;

import com.itmayiedu.entity.UserEntity;

public class ClassPathXmlApplicationContext {
	private static String PATH;
	private static String ID;
	private static String CLASS;
	private static String NAME;
	private static String VALUE;
	
	
	private void init(){
		this.ID = "id";
		this.CLASS = "class";
		this.NAME = "name";
		this.VALUE = "value";
	}
	
	/**
	 * @param applicationContext.xml的路径
	 */
	public ClassPathXmlApplicationContext(String path) {
		init();
		this.PATH = path;
	}
	
	public ClassPathXmlApplicationContext() {}
	
	public Object getBean(String id) throws DocumentException, ClassNotFoundException, InstantiationException, IllegalAccessException, NoSuchFieldException, SecurityException{
		if(StringUtils.isBlank(id)){
			return null;
		}
		//获取saxRead对象
		SAXReader saxReader = new SAXReader();
		//获取当前路径
		Document doc = saxReader.read(this.getClass().getClassLoader().getResource(PATH));
		//获取根节点<beans>
		Element rootEle = doc.getRootElement();
		List<Element> elements = rootEle.elements();
		for(Element element : elements){
			//遍历出来的就是二级节点,寻找属性id
			String beanId = element.attributeValue(ID);
			if(beanId==null && !beanId.equals(id)){
				//跳过本次循环
				continue;
			}
			//找到该id，获取该id对应的class属性
			String beanClass = element.attributeValue(CLASS);
			if(beanClass == null) {
				throw new RuntimeException("没有找到该id="+id+"中对应的class");
			}
			//找到了class,则进行反射初始化操作
			Class<?> classForName = Class.forName(beanClass);
			//反射初始化对象
			Object instance = classForName.newInstance();
			//为property赋值,三级节点
			List<Element> ele3 = element.elements();
			for(Element ele : ele3){
				//<property></property>
				String name = ele.attributeValue(NAME);
				String value = ele.attributeValue(VALUE);
				Field field = classForName.getDeclaredField(name);
				field.setAccessible(true);
				field.set(instance, value);
			}
			return instance;
		}
		return null;
	}
	
	public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException, NoSuchFieldException, SecurityException, DocumentException {
		ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");
		UserEntity user = (UserEntity) context.getBean("user1");
		System.out.println(user);
	}
}

打印结果:

无参构造函数....
UserEntity [userId=001, userName=张三]