1、SQL语句

DROP TABLE if EXISTS tb_user;

-- 创建用户表
CREATE TABLE tb_user(
	id int primary key auto_increment,
	username varchar(20) unique,
	password varchar(32)
);

-- 添加数据
INSERT INTO tb_user(username,password) values('zhangsan','123'),('lisi','234');

SELECT * FROM tb_user;

表格：

2、RegisterServlet:

package web;

import mapper.UserMapper;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import pojo.User;
import util.SqlSessionFactoryUtil;

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;
import java.io.IOException;
import java.io.PrintWriter;

@WebServlet("/registerServlet")
public class RegisterServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String username = request.getParameter("username");
        String password = request.getParameter("password");

        User user = new User();
        user.setUsername(username);
        user.setPassword(password);

        SqlSessionFactory sqlSessionFactory = SqlSessionFactoryUtil.getSqlSessionFactory();
        SqlSession sqlSession = sqlSessionFactory.openSession();
        UserMapper userMapper = sqlSession.getMapper(UserMapper.class);

        User user1 = userMapper.selectByUserName(username);
        response.setContentType("text/html;charset=utf-8");
        PrintWriter writer = response.getWriter();
        if (user1==null){
            userMapper.add(user);
            sqlSession.commit();
            sqlSession.close();
        }else {
            writer.write("用户已存在");
        }
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        this.doGet(request, response);
    }
}

3、插入数据失败，id仍自增

4、使用事务回滚

添加语句：

sqlSession.rollback();

即：