MyBatis笔记(一)

MyBatis学习随笔

第一个MyBatis项目

• 创建项目
• 导入maven依赖,根据需要选择性添加mysql/oracle,spring,spring-mybatis等依赖,这里就不一一列出了

<dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.4.1</version>
        </dependency>

• 创建mybatis配置文件

<?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>
    <!--加载外部配置文件-->
    <properties resource="mysql8.properties"></properties>
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <!--
                    这里的value可以使用常量
                    同时可以使用${}在配置文件中读取
                    后期整合spring就不需在此配置
                -->
                <property name="driver" value="${driverClassName}"/>
                <property name="url" value="${url}"/>
                <property name="username" value="${username}"/>
                <property name="password" value="${password}"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <!--这里拷贝加载的mapper文件的全路径-->
        <mapper resource="com/offcn/mapper/CustomerMapper.xml"/>
    </mappers>
</configuration>

MyBatis基本配置文件

• 创建mapper的配置文件

<?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">
<mapper namespace="com.offcn.mapper.CustomerMapper">
    <!--根据cust_id查询客户-->
    <select id="selectCustomerById" parameterType="Int" resultType="com.offcn.bean.Customer">
      SELECT * FROM `customer` WHERE cust_id  = #{cust_id}
    </select>
</mapper>

Mapper.xml

• 创建mapper.xml对应的接口,该接口名要和mapper文件名完全一致

• package com.offcn.mapper;
  
  import com.offcn.bean.Customer;
  import org.springframework.stereotype.Repository;
  
  @Repository
  public interface CustomerMapper {
      public Customer selectCustomerById(Integer id);
  }

• Bean对象代码

• package com.offcn.bean;
  
  import lombok.Getter;
  import lombok.Setter;
  import lombok.ToString;
  import org.springframework.stereotype.Component;
  
  /**
   * @author 张瑞丰
   * @description 客户表bean
   * @date 2019/4/19
   */
  @Getter@Setter@ToString@Component
  public class Customer {
      private Integer custId;
      private String custName;
      private String custProfession;
      private String custPhone;
      private String email;
  }

  测试类代码

•     @Test
      public void test() throws IOException {
          //创建SqlSessionFactoryBuilder
          SqlSessionFactoryBuilder sqlSessionFactoryBuilder = new SqlSessionFactoryBuilder();
          //读取配置文件
          InputStream resourceAsStream = Resources.getResourceAsStream("mybatis.xml");
          //获取session工厂
          SqlSessionFactory sessionFactory = sqlSessionFactoryBuilder.build(resourceAsStream);
          //获取会话
          SqlSession sqlSession = sessionFactory.openSession();
          //执行sql,并得到返回值
          Customer result = (Customer)sqlSession.selectOne("selectCustomerById", 2);
          //获取结果
          System.out.println(result);
      }

  输出结果:Customer(custId=null, custName=null, custProfession=null, custPhone=null, email=libai@163.com)

• 只有email正常输出,其余字段都未正常输出,原因:表中字段除email外,名为cust_xx而bean则为custXx,因列名和字段名不一致导致,解决方案如下:
  • 　　在mapper中创建resultMap将表中字段和bean的属性进行映射,同时要修改select的ResultMap为其id,一定不能打错,打错会报一万个错误
  • 　　在mybatis全局配置文件中开启驼峰转下划线
  • 在select语句中给列名起别名的形式

  • <?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">
    <mapper namespace="com.offcn.mapper.CustomerMapper">
        <!--映射表字段和列名 type为bean的全类名-->
        <resultMap id="BaseResultMap" type="com.offcn.bean.Customer">
            <id column="cust_id" property="custId"></id>
            <result column="cust_name" property="custName"></result>
            <result column="cust_profession" property="custProfession"></result>
            <result column="cust_phone" property="custPhone"></result>
            <result column="email" property="email"></result>
        </resultMap>
        <!--根据cust_id查询客户-->
        <select id="selectCustomerById" parameterType="Int" resultMap="BaseResultMap">
          SELECT * FROM `customer` WHERE cust_id  = #{cust_id}
        </select>
    </mapper>