11、spring原理模拟

说明：Spring 的bean容器相当于一个Map<String, Object> beanMap，key为id，value为该class对应的实例对象。故模拟时，把所有的bean都放在一个map中
1、自定义一个简单的 beans.xml 模拟spring的beans.xml

<beans>
    <bean id="u" class="com.wxx.dao.impl.UserDaoImpl" />
    <bean id="userService" class="com.wxx.service.UserSerivce">
        <property name="userDao" bean="u" />
    </bean>
</beans>

2、BeanFactory模拟spring里的BeanFactory

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

3、重点：模拟ClassPathXmlApplicationContext对xml文件处理的过程，通过反射实例化一个对象放到map中，对于他的property属性，通过反射调用set方法，给他装配相应的属性。

package com.wxx.spring;

import java.io.IOException;
import java.lang.reflect.InvocationTargetException;
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.JDOMException;
import org.jdom2.input.SAXBuilder;

public class ClassPathXmlApplicationContext2 implements BeanFactory {

    // bean容器
    private Map<String, Object> beanMap = new HashMap<String, Object>();
    
    // 构造方法
    public ClassPathXmlApplicationContext2() throws JDOMException, IOException, ClassNotFoundException, InstantiationException, IllegalAccessException, NoSuchMethodException, SecurityException, IllegalArgumentException, InvocationTargetException {
        SAXBuilder builder = new SAXBuilder();
        Document document = builder.build(this.getClass().getResourceAsStream("/beans.xml"));
        Element rootElement = document.getRootElement();
        List<Element> beanList = rootElement.getChildren();
        for(Element element : beanList) {
            String id = element.getAttributeValue("id");
            String classStr = element.getAttributeValue("class");
            // 根据类名实例化一个对象
            Class clazz = Class.forName(classStr);
            Object instance = clazz.newInstance();
            // 到第二个循环时：key="userService", value=userService实例对象
            beanMap.put(id, instance);
            
            // 获取当前 bean的子元素
            List<Element> beanPropertyList = element.getChildren();
            if(beanPropertyList != null && beanPropertyList.size() != 0) {
                for(Element element2 : beanPropertyList) {
                    String nameStr = element2.getAttributeValue("name");  // "userDao"
                    String beanStr = element2.getAttributeValue("bean");   // "u"
                    // 从beanMap中获取已经存在的bean
                    Object beanObj = beanMap.get(beanStr);   // userDao
                    //根据name获取set方法
                    String setMethodStr = "set" + nameStr.substring(0, 1).toUpperCase() + nameStr.substring(1);
                    System.out.println("拼装的set方法：" + setMethodStr);
                    Method method = instance.getClass().getMethod(setMethodStr, beanObj.getClass().getInterfaces()[0]);
                    // 调用userService的 setUserDao方法，传递参数为 userDao
                    method.invoke(instance, beanObj);
                }
            }
        }
    }
    
    @Override
    public Object getBean(String name) {
        return beanMap.get(name);
    }
}

4、User，UserService，UserDao等省略
5、测试

package com.wxx.service;

import java.io.IOException;
import java.lang.reflect.InvocationTargetException;
import org.jdom2.JDOMException;
import org.junit.Test;
import com.wxx.model.User;
import com.wxx.spring.BeanFactory;
import com.wxx.spring.ClassPathXmlApplicationContext2;

public class UserSerivceTest {

    @Test
    public void testAdd() throws InstantiationException, IllegalAccessException, ClassNotFoundException, NoSuchMethodException, SecurityException, IllegalArgumentException, InvocationTargetException, JDOMException, IOException {
        BeanFactory factory = new ClassPathXmlApplicationContext2();
        // 获取userservice，此时，已经自动装配 userDao完成
        UserSerivce service = new UserSerivce();
        User u = new User();
        // 调用service的add方法
        service.add(u);
    }
}