MyBatis_HelloWorld

MyBatis是目前使用最多的ORM框架之一，今天学习了MyBatis基本配置，使用MyBatis读取数据库内容并直接映射为对象

MyBatis简化了DAO层与DB层间操作

下面通过最简单的一个实例了解MyBatis

一、构建项目

1.建立JAVA项目结构如下，导入MySQL、MyBatis jar包：

2.Mybatis数据库如下：

建表SQL语句：

SET FOREIGN_KEY_CHECKS=0;

-- ----------------------------
-- Table structure for tbl_employee
-- ----------------------------
CREATE TABLE `tbl_employee` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `last_name` varchar(255) DEFAULT NULL,
  `gender` char(1) DEFAULT NULL,
  `email` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of tbl_employee
-- ----------------------------
INSERT INTO `tbl_employee` VALUES ('1', 'tom', '0', 'tom@atguigu.com');

3.编写与关系数据库对应的JavaBean：

package com.atguigu.mybatis.bean;

public class Employee {

    private Integer id;
    private String lastName;
    private String gender;
    private String email;
    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    public String getLastName() {
        return lastName;
    }
    public void setLastName(String lastName) {
        this.lastName = lastName;
    }
    public String getGender() {
        return gender;
    }
    public void setGender(String gender) {
        this.gender = gender;
    }
    public String getEmail() {
        return email;
    }
    public void setEmail(String email) {
        this.email = email;
    }
    
    @Override
    public String toString() {
        return "Employee [id=" + id + ", lastName=" + lastName + ", gender=" + gender + ", email=" + email + "]";
    }
}

二、配置MyBatis

1.在conf/下建立全局配置文件mybatis-config.xml：

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE configuration
 PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
 "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/mybatis"/>
                <property name="username" value="root"/>
                <property name="password" value="root"/>
            </dataSource>
        </environment>
    </environments>
    
    <!-- 配置SQL语句映射文件 -->
    <mappers>
        <mapper resource="EmployeeMapper.xml"/>
    </mappers>
</configuration>

2.建立SQL语句映射文件EmployeeMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
 PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
 "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<!-- namespace可以任意填写，只是为了便于区分 -->
<mapper namespace="com.atguigu.mybatis.EmployeeMapper">

    <!-- id是sql唯一标识，resultType相当于返回类型 -->
    <select id="selectEmp" resultType="com.atguigu.mybatis.bean.Employee">
    
        <!-- 查询结果字段与Employee属性保持一致才可以正确映射，所以这里使用别名lastName -->
        select id,last_name lastName,gender,email from tbl_employee where id = #{id}
    </select>
</mapper>

三、建立单元测试文件

package com.atguigu.mybatis.test;

import java.io.IOException;
import java.io.InputStream;

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 org.junit.jupiter.api.Test;

import com.atguigu.mybatis.bean.Employee;

class MybatisTest {

    @Test
    void test() throws IOException {
        
        /**
         * 使用mybatis-config.xml获取sqlSession工厂sqlSessionFactory
         */
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
    
        /**
         * 通过sqlSessionFactory获取SqlSession
         */
        SqlSession openSession = sqlSessionFactory.openSession();
        try {
            Employee employee = openSession.selectOne("com.atguigu.mybatis.EmployeeMapper.selectEmp", 1);
            System.out.println(employee);
        }finally {
            openSession.close();    //必须关闭
        }

    }

}

四、执行结果

五、本次学习中遇到的问题：

1.运行测试文件时提示找不到mybatis-config.xml文件：

java.io.IOException: Could not find resource mybatis-config.xml
    at org.apache.ibatis.io.Resources.getResourceAsStream(Resources.java:114)
    at org.apache.ibatis.io.Resources.getResourceAsStream(Resources.java:100)
    at com.atguigu.mybatis.test.MybatisTest.test(MybatisTest.java:23)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
　……

解决办法：

新建conf文件夹时选择Source Folder而不是Folder（参考https://ask.csdn.net/questions/252045下面xl1007的评论）