模拟 Spring 的 bean 加载过程

一天不学习就感觉浑身痒痒，忽然想写一篇东西来记录自己学习的点点滴滴，哈哈，学习之路还很长，不要轻易说放弃。

下面通过工厂模式简单的对 spring 的 bean 加载过程进行模拟：

改造前：

dao 层

public class UserDaoImpl implements IUserDao {
    public void saveUser() {
        System.out.println("保存成功");
    }
}

service 层

public class UserServiceImpl implements IUserService {
    private UserDaoImpl userDao=new UserDaoImpl();
    public void saveUser() {
        userDao.saveUser();
    }
}

client

public class Client {
    public static void main(String[] args) {
        UserServiceImpl service = new UserServiceImpl();
        service.saveUser();
    }
}

改造后：

bean.xml

<?xml version="1.0" encoding="UTF-8"?>
<beans>

    <bean id="userDao" class="com.gary.dao.impl.UserDaoImpl"></bean>
    <bean id="userService" class="com.gary.service.impl.UserServiceImpl"></bean>

</beans>

BeanFactory

public class BeanFactory {
    /**
     *  定义一个map容器，存入要创建的对象名(key)与实例对象(value)
     */
    private static Map<String, Object> beans;
    private static SAXReader reader;
    static {
        try {
            beans = new HashMap<String, Object>();
            reader = new SAXReader();
            //从xml获取数据
            Document document = reader.read(BeanFactory.class.getClassLoader().getResourceAsStream("bean.xml"));
            //获取根元素,如<beans>
            Element root = document.getRootElement();
            //获取子元素，如<bean>
            List<Element> elements = root.elements();
            String key = null;
            String beanPath = null;
            Object value = null;
            for (Element element : elements) {
                //拿到xml id的值
                key = element.attributeValue("id");
                //拿到xml class的值
                beanPath = element.attributeValue("class");
                //通过反射实例化对象
                value = Class.forName(beanPath).newInstance();
                //把对象存入容器
                beans.put(key, value);
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    /**
     * 创建对象
     * @param beanName
     * @return
     */
    public static Object getBean(String beanName) {
        return beans.get(beanName);
    }
}

dao 层

public class UserDaoImpl implements IUserDao {
    public void saveUser() {
        System.out.println("保存成功");
    }
}

service 层

public class UserServiceImpl implements IUserService {
    private IUserDao userDao = (IUserDao) BeanFactory.getBean("userDao");
    public void saveUser() {
        userDao.saveUser();
    }
}

client

public class Client {
    public static void main(String[] args) {
        IUserService service = (IUserService) BeanFactory.getBean("userService");
        service.saveUser();
    }
}