尚学堂Spring视频教程（一）：模拟Spring

  Spring简单的说就是作为控制反转的容器，看这篇文章前需要先搞懂“控制反转和依赖注入“这个设计模式

  我们先来模拟Spring，实现用户添加的功能，新建WEB项目”Spring_0100_AbstractOrientedProgramming“，新建包”com.bjsxt.model“，创建一个User实体

package com.bjsxt.model;

public class User {
    private String username;
    private String password;
    public String getUsername() {
        return username;
    }
    public void setUsername(String username) {
        this.username = username;
    }
    public String getPassword() {
        return password;
    }
    public void setPassword(String password) {
        this.password = password;
    }
}

  新建包”com.bjsxt.dao“，创建操作用户的接口"UserDAO"，针对接口编程的好处是，接口的实现类可以无限拓展

package com.bjsxt.dao;
import com.bjsxt.model.User;


public interface UserDAO {
    public void save(User user);
}

  新建包”com.bjsxt.dao.impl“，创建一个接口的实现类”UserDAOImpl“，用一个输出模拟保存成功

package com.bjsxt.dao.impl;

import com.bjsxt.dao.UserDAO;
import com.bjsxt.model.User;


public class UserDAOImpl implements UserDAO {

    public void save(User user) {
        //Hibernate
        //JDBC
        //XML
        //NetWork
        System.out.println("user saved!");
    }

}

  新建包”com.bjsxt.service“，新建类”UserService“，有一个”UserDAO“类型的属性，并且有对应的getter和setter方法，还有一个add方法调用UserDAO的save方法实现用户添加

package com.bjsxt.service;
import com.bjsxt.dao.UserDAO;
import com.bjsxt.dao.impl.UserDAOImpl;
import com.bjsxt.model.User;



public class UserService {
    private UserDAO userDAO;
    
    public void add(User user) {
        userDAO.save(user);
    }
    public UserDAO getUserDAO() {
        return userDAO; 
    }
    public void setUserDAO(UserDAO userDAO) {
        this.userDAO = userDAO;
    }
}

  按照传统方式，我们应该是在UserService中自己实例化UserDAO属性，但是现在这一切都交给Spring来完成，它会帮我们实例化，并且通过UserServcie的setter方法给UserDAO属性赋值。我们需要将对象写到配置文件里，然后Spring会读取配置文件，在运行时动态生成对象供我们调用

<beans>
    <bean id="u" class="com.bjsxt.dao.impl.UserDAOImpl" />
    <bean id="userService" class="com.bjsxt.service.UserService" >
        <property name="userDAO" bean="u"/>
    </bean>
    
</beans>

  新建包”com.bjsxt.spring“，创建模拟Spring的接口”BeanFactory“和"ClassPathXmlApplicationContext"

package com.bjsxt.spring;

public interface BeanFactory {
    public Object getBean(String id);
}

package com.bjsxt.spring;


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

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

public class ClassPathXmlApplicationContext implements BeanFactory {
    
    private Map<String , Object> beans = new HashMap<String, Object>();
    
    
    //IOC Inverse of Control DI Dependency Injection
    public ClassPathXmlApplicationContext() throws Exception {
        SAXBuilder sb=new SAXBuilder();
        
        Document doc=sb.build(this.getClass().getClassLoader().getResourceAsStream("beans.xml")); //�����ĵ�����
        Element root=doc.getRootElement(); //��ȡ��Ԫ��HD
        List list=root.getChildren("bean");//ȡ����Ϊdisk������Ԫ��
        for(int i=0;i<list.size();i++){
           Element element=(Element)list.get(i);
           String id=element.getAttributeValue("id");
           String clazz=element.getAttributeValue("class");
           Object o = Class.forName(clazz).newInstance();
           System.out.println(id);
           System.out.println(clazz);
           beans.put(id, o);
           
           for(Element propertyElement : (List<Element>)element.getChildren("property")) {
               String name = propertyElement.getAttributeValue("name"); //userDAO
               String bean = propertyElement.getAttributeValue("bean"); //u
               Object beanObject = beans.get(bean);//UserDAOImpl instance
               
               String methodName = "set" + name.substring(0, 1).toUpperCase() + name.substring(1);
               System.out.println("method name = " + methodName);
               
               Method m = o.getClass().getMethod(methodName, beanObject.getClass().getInterfaces()[0]);
               m.invoke(o, beanObject);
           }
           
           
        }  
      
    }



    public Object getBean(String id) {
        return beans.get(id);
    }

}

  接下来用JUnit做单元测试，新建源文件夹"Test"存放测试代码，新建包”com.bjsxt.service“，创建测试类”UserServiceTest“

package com.bjsxt.service;
import org.junit.Test;

import com.bjsxt.dao.UserDAO;
import com.bjsxt.model.User;
import com.bjsxt.spring.BeanFactory;
import com.bjsxt.spring.ClassPathXmlApplicationContext;


public class UserServiceTest {

    @Test
    public void testAdd() throws Exception {
        BeanFactory applicationContext = new ClassPathXmlApplicationContext();
        UserService service = (UserService)applicationContext.getBean("userService");
        User u = new User();
        u.setUsername("zhangsan");
        u.setPassword("zhangsan");
        service.add(u);
    }

}

  运行，效果如图

  看到了吗，UserService中的add方法需要调用UserDAO的实例方法save，但是并没有实例化UserDAO属性，而是交给了Spring来完成

  具体项目文件如图