通过读取xml文件，利用反射动态加载类和方法，其实就是spring的注入机制
模拟，可以清晰的看出整个运行流程

1、配置文件 applicationContext.xml

<beans>
    <bean id="sd" class="dao.impl.IStudentDaoImpl"/>
    <bean id="studentService" class="service.StudentService">
        <property name="studentDao" ref="sd"/>
    
    </bean>
</beans>

　　property标签中name 为 StudentService.java的属性

2、dao

package dao;
public interface StudentDao {
    public  void save();
}

package dao.impl;

import dao.StudentDao;public class IStudentDaoImpl implements StudentDao {
    public void save() {
        
        System.out.println("IStudentDaoImpl.save()");
    }
}

package dao.impl;

import dao.StudentDao;

public class StudentDaoImpl implements StudentDao{
    public void save() {
        System.out.println("StudentDaoImpl.save()");
    }
}

4、service

package service;

import dao.StudentDao;

public class StudentService {
    private StudentDao studentDao;

    public StudentDao getStudentDao() {
        return studentDao;
    }

    public void setStudentDao(StudentDao studentDao) {
        this.studentDao = studentDao;
    }
    public void save(){
        this.studentDao.save();
    }
}

package util;


import java.lang.reflect.Method;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import org.jdom2.Document;
import org.jdom2.Element;
import org.jdom2.input.SAXBuilder;

public class ClassPathXmlApplicationContext {
    static Map<String,Object> map=new HashMap<String,Object>();
    public  void SaxXml() throws Exception{
        SAXBuilder sb=new SAXBuilder();
        Document doc=sb.build(this.getClass().getClassLoader().getResourceAsStream("applicationContext.xml"));
        Element root=doc.getRootElement();
        List<Element> list=root.getChildren("bean");
        for(int i=0;i<list.size();i++){
            Element ele=list.get(i);
            String id=ele.getAttributeValue("id");
            String obj=ele.getAttributeValue("class");
            Object o=Class.forName(obj).newInstance();
            map.put(id, o);
            
            for(Element elePro:(List<Element>)ele.getChildren("property")){
                String name=elePro.getAttributeValue("name");
                String bean=elePro.getAttributeValue("ref");
                String methodName="set"+name.substring(0, 1).toUpperCase()+name.substring(1);
                System.out.println("MethodName:"+methodName);
                Method m=o.getClass().getMethod(methodName, map.get(bean).getClass().getInterfaces()[0]);
                m.invoke(o, map.get(bean));
            }
        }
    }
    public static  Object get(String id){
        return map.get(id);
    }
}

package service;

import static org.junit.Assert.*;

import org.junit.Test;

import util.ClassPathXmlApplicationContext;

public class StudentServiceTest {

    @Test
    public void testSave() throws Exception {
        ClassPathXmlApplicationContext cpac=new ClassPathXmlApplicationContext();
        cpac.SaxXml();
        StudentService ss=(StudentService)ClassPathXmlApplicationContext.get("studentService");
        ss.save();
    }

}