Mybatis学习--9.Web项目中使用Mybatis

在WEB项目中使用MyBatis(以银行转账业务为例)

使用maven新建项目，在pom文件中将打包方式改为war，并重载(reload)项目。在pom文件中引入相关依赖
编写mybatis配置文件及logback配置文件：mybatis-config.xml,XxxMapper.xml,logback.xml
右键项目，找到openMouduleSettings。双击红色路径，这一步是在项目中引入web框架
在webapp目录下新建一个html文件(当作欢迎页使用)，编写前端代码。项目逻辑：从前往后
编写后端代码
采用三层架构
建包：web(表现层)、service(业务层)、dao(可持久层)、util(工具包)、exceptions(异常包)、pojo(javaBean)

在web包下创建Servlet类，接收前端用户提交的数据，并调用相关业务方法。于此同时，编写pojo类

package com.deng.bank.web;

import com.deng.bank.exceptions.moneyNotEnoughException;
import com.deng.bank.exceptions.transferException;
import com.deng.bank.service.AccountService;
import com.deng.bank.service.impl.AccountServiceIm;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

@WebServlet("/transfer")
public class AccountServlet extends HttpServlet {
    public void doPost(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
        //get data from page
        String fromActno = request.getParameter("fromActno");
        String toActno = request.getParameter("toActno");
        Double money = Double.parseDouble(request.getParameter("money"));

        AccountService accountService = new AccountServiceIm();
        try {
            accountService.transfer(fromActno,toActno,money);
            response.sendRedirect(request.getContextPath() + "/Success.html");
        } catch (moneyNotEnoughException e) {
            response.sendRedirect(request.getContextPath() + "/Error1.html");
        } catch (transferException e) {
            response.sendRedirect(request.getContextPath() + "/Error2.html");
        }catch(Exception e){
            response.sendRedirect(request.getContextPath() + "/Error2.html");
        }
    }
}

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* 层与层之间通过接口交互。在service包和dao包下建立接口，并创建相应的子类实现该接口

  * ![](../../../笔记/MyBatis/9.在WEB中应用MyBatis.assets/image-20231109104834245.png)

* 业务层调用dao层接口获取数据库连接，进行相应业务逻辑代码的编写

  * ```java
    package com.deng.bank.service.impl;
    
    import com.deng.bank.dao.AccountDao;
    import com.deng.bank.dao.impl.AccountDaoIm;
    import com.deng.bank.exceptions.moneyNotEnoughException;
    import com.deng.bank.exceptions.transferException;
    import com.deng.bank.pojo.Account;
    import com.deng.bank.service.AccountService;
    import com.deng.bank.utils.SqlSessionUtil;
    import org.apache.ibatis.session.SqlSession;
    
    public class AccountServiceIm implements AccountService {
        private AccountDao accountDao = new AccountDaoIm();
    
        public void transfer(String fromActno,String toActno,Double money) throws moneyNotEnoughException, transferException {
            SqlSession sqlSession = SqlSessionUtil.getSqlSession();
            Account fromAct = accountDao.select(fromActno);
            if(fromAct.getBalance() < money){
                throw new moneyNotEnoughException("余额不足");
            }else{
                Account toAct = (Account)accountDao.select(toActno);
                fromAct.setMoney(fromAct.getBalance() - money);
                toAct.setMoney(toAct.getBalance() + money);
                int count = accountDao.update(fromAct);
                String str = null;
                str.toString();
                count += accountDao.update(toAct);
                if(count != 2){
                    throw new transferException("转账失败，未知错误");
                }
    			
                sqlSession.commit();
            }
            SqlSessionUtil.close();
        }
    }

如果我们在dao层进行事务管理，那么当业务层25-26行出现类似情况的异常时，本来应该终止转账，但由于我们将事务设置在dao层，那么fromAct账户的金额会发生改变，而toAct账户的金额却不会改变，相当于有一笔钱不翼而飞，这对银行来说是致命的错误。因此我们需要将事务管理设置在业务层。我们之前的做法是：每执行一次数据库CRUD操作就需要创建一个SqlSession对象，这显然无法做到原子性。

因此我们引入ThreadLocal对象，将SqlSession对象和当前线程绑定，从而实现事务的原子性

package com.deng.bank.utils;

import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

import java.io.IOException;

public class SqlSessionUtil {
    private static SqlSessionFactory sqlSessionFactory;

    static{
        try {
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(Resources.getResourceAsStream("mybatis-config.xml"));
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private static ThreadLocal<SqlSession> local = new ThreadLocal<>();

    public static SqlSession getSqlSession(){
        SqlSession sqlSession = local.get();
        if(sqlSession == null){
            sqlSessionFactory.openSession();
        }
        return sqlSession;
    }

    public static void close(){
        SqlSession sqlSession = local.get();
        if (sqlSession!=null) {
            sqlSession.close();
            local.remove();
        }
    }
}