MyBatis的ORM！！！

首先你要明白为什么使用ORM：我们看一个示例，我们发现我们要声明的pojo类中的属性名和数据库中的字段名不一致，这时就需要我们使用MyBatis的ORM。

首先还是准备工作：

??? 1.创建Maven工程，还没有配置Maven的和还不会的去看这里啦：maven的下载安装与配置环境变量！！！（全网最详细）-CSDN博客

Role.java:???? (pojo类)

/*
 * Copyright (c) 2020, 2023,  All rights reserved.
 *
 */
package com.by.pojo;

/**
 * <p>Project: mybatis - Role</p>
 * <p>Powered by scl On 2023-12-20 14:36:04</p>
 * <p>描述：<p>
 *
 * @author 孙臣龙 [1846080280@qq.com]
 * @version 1.0
 * @since 17
 */
public class Role {
    private int id;
    private String roleName;
    private String roleDesc;

    @Override
    public String toString() {
        return "Role{" +
                "id=" + id +
                ", roleName='" + roleName + '\'' +
                ", roleDesc='" + roleDesc + '\'' +
                '}';
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getRoleName() {
        return roleName;
    }

    public void setRoleName(String roleName) {
        this.roleName = roleName;
    }

    public String getRoleDesc() {
        return roleDesc;
    }

    public void setRoleDesc(String roleDesc) {
        this.roleDesc = roleDesc;
    }
}

?2.导入依赖到pom.xml文件中

<dependencies>
            <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.4.5</version>
            </dependency>
            <dependency>
                <groupId>mysql</groupId>
                <artifactId>mysql-connector-java</artifactId>
                <version>5.1.47</version>
            </dependency>
            <dependency>
                <groupId>log4j</groupId>
                <artifactId>log4j</artifactId>
                <version>1.2.17</version>
            </dependency>
            <dependency>
                <groupId>junit</groupId>
                <artifactId>junit</artifactId>
                <version>4.12</version>
            </dependency>
        </dependencies>
        <build>
            <!-- 如果不添加此节点src/main/java目录下的所有配置文件都会被漏掉。 -->
            <resources>
                <resource>
                    <directory>src/main/java</directory>
                    <includes>
                        <include>**/*.xml</include>
                    </includes>
                </resource>
                <resource>
                    <directory>src/main/resources</directory>
                </resource>
            </resources>
        </build>

3.在resources中创建log4j.properites和mybatis-config.xml

log4j.properites:

log4j.properites:

    # Global logging configuration
    log4j.rootLogger=DEBUG, stdout
    # Console output...
    log4j.appender.stdout=org.apache.log4j.ConsoleAppender
    log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
    log4j.appender.stdout.layout.ConversionPattern=%5p [%t] - %m%n

?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>
        <!--使用dev环境-->
        <environments default="dev">
            <!--dev环境-->
            <environment id="dev">
                <transactionManager type="JDBC"></transactionManager>
                <!--使用连接池中的数据源-->
                <dataSource type="POOLED">
                    <property name="driver" value="com.mysql.jdbc.Driver"/>
                    <property name="url" value="jdbc:mysql://127.0.0.1:3305/mybatis?characterEncoding=UTF-8"/>
                    <property name="username" value="root"/>
                    <property name="password" value=""/>
                </dataSource>
            </environment>
        </environments>
        <!-- 扫描映射文件 -->
        <mappers>
            <mapper resource="com/by/mapper/UserMapper.xml"/>
        </mappers>
    </configuration>

4.创建RolerMapper接口和RoleMapper.xml文件

RolerMapper.java:

package com.by.mapper;

import com.by.pojo.Role;

import java.util.List;

/**
 * <p>Project: mybatis - RoleMapper</p>
 * <p>Powered by scl On 2023-12-20 14:37:16</p>
 * <p>描述：<p>
 *
 * @author 孙臣龙 [1846080280@qq.com]
 * @version 1.0
 * @since 17
 */
public interface RoleMapper {
  
}

?RoleMapper.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">
<mapper namespace="com.by.mapper.RoleMapper">

  
</mapper>

5.创建测试类MyBatisTest

/*
 * Copyright (c) 2020, 2023,  All rights reserved.
 *
 */
package com.by;

import com.by.mapper.RoleMapper;
import com.by.pojo.Role;
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.After;
import org.junit.Before;
import org.junit.Test;

import java.io.IOException;
import java.io.InputStream;
import java.util.List;


/**
 * <p>Project: mybatis - MyBatisTest</p>
 * <p>Powered by scl On 2023-12-18 11:44:53</p>
 * <p>描述：<p>
 *
 * @author 孙臣龙 [1846080280@qq.com]
 * @version 1.0
 * @since 17
 */
public class MyBatisTestRole {

    private InputStream inputStream;
    private SqlSession sqlSession;

    @Before
    public void init() throws IOException {
        // 加载配置文件
        String resource = "mybatis-config.xml";
        inputStream = Resources.getResourceAsStream(resource);

        // 创建sqlSessionFActory
        SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

        // 获得数据的绘画实例
        sqlSession = sessionFactory.openSession();
    }

    


    @After
    public void close() throws IOException {
        inputStream.close();
        sqlSession.close();
    }

}

? 准备工作完成后，开始功能的开发：

一.ORM的使用：通过查询数据库时，为字段起别名的方式实现实体类属性名和数据库的字段名不一致问题

RoleMapper.java:

package com.by.mapper;

import com.by.pojo.Role;

import java.util.List;

public interface RoleMapper {
    List<Role> findAll();
  
}

RoleMapper.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">
<mapper namespace="com.by.mapper.RoleMapper">

    <select id="findAll" resultType="com.by.pojo.Role">
        SELECT id ,ROLE_NAME roleName,ROLE_DESC roleDesc FROM role
    </select>
   
</mapper>

测试类：

/*
 * Copyright (c) 2020, 2023,  All rights reserved.
 *
 */
package com.by;

import com.by.mapper.RoleMapper;
import com.by.pojo.Role;
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.After;
import org.junit.Before;
import org.junit.Test;

import java.io.IOException;
import java.io.InputStream;
import java.util.List;


/**
 * <p>Project: mybatis - MyBatisTest</p>
 * <p>Powered by scl On 2023-12-18 11:44:53</p>
 * <p>描述：<p>
 *
 * @author 孙臣龙 [1846080280@qq.com]
 * @version 1.0
 * @since 17
 */
public class MyBatisTestRole {

    private InputStream inputStream;
    private SqlSession sqlSession;

    @Before
    public void init() throws IOException {
        // 加载配置文件
        String resource = "mybatis-config.xml";
        inputStream = Resources.getResourceAsStream(resource);

        // 创建sqlSessionFActory
        SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

        // 获得数据的绘画实例
        sqlSession = sessionFactory.openSession();
    }

    @Test
    public void findAll(){
        RoleMapper roleMapper = sqlSession.getMapper(RoleMapper.class);
        List<Role> roleList = roleMapper.findAll();
        for (Role role : roleList) {
            System.out.println(role);
        }
    }
   

    @After
    public void close() throws IOException {
        inputStream.close();
        sqlSession.close();
    }

}

二.结果映射的方式实现实体类属性名和数据库的字段名不一致问题

RoleMapper.java:

package com.by.mapper;

import com.by.pojo.Role;

import java.util.List;

/**
 * <p>Project: mybatis - RoleMapper</p>
 * <p>Powered by scl On 2023-12-20 14:37:16</p>
 * <p>描述：<p>
 *
 * @author 孙臣龙 [1846080280@qq.com]
 * @version 1.0
 * @since 17
 */
public interface RoleMapper {
  
    List<Role> findAll2();
}

RoleMapper.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">
<mapper namespace="com.by.mapper.RoleMapper">

    <resultMap id="findAll2resultMap" type="com.by.pojo.Role">
        <id column="id" property="id"></id>
        <result column="role_name" property="roleName"></result>
        <result column="role_desc" property="roleDesc"></result>
    </resultMap>
    <select id="findAll2" resultMap="findAll2resultMap">
        select * from role
    </select>
</mapper>

测试类：

/*
 * Copyright (c) 2020, 2023,  All rights reserved.
 *
 */
package com.by;

import com.by.mapper.RoleMapper;
import com.by.pojo.Role;
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.After;
import org.junit.Before;
import org.junit.Test;

import java.io.IOException;
import java.io.InputStream;
import java.util.List;


/**
 * <p>Project: mybatis - MyBatisTest</p>
 * <p>Powered by scl On 2023-12-18 11:44:53</p>
 * <p>描述：<p>
 *
 * @author 孙臣龙 [1846080280@qq.com]
 * @version 1.0
 * @since 17
 */
public class MyBatisTestRole {

    private InputStream inputStream;
    private SqlSession sqlSession;

    @Before
    public void init() throws IOException {
        // 加载配置文件
        String resource = "mybatis-config.xml";
        inputStream = Resources.getResourceAsStream(resource);

        // 创建sqlSessionFActory
        SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

        // 获得数据的绘画实例
        sqlSession = sessionFactory.openSession();
    }

  
    @Test
    public void findAll2(){
        RoleMapper roleMapper = sqlSession.getMapper(RoleMapper.class);
        List<Role> roleList = roleMapper.findAll2();
        for (Role role : roleList) {
            System.out.println(role);
        }
    }

    @After
    public void close() throws IOException {
        inputStream.close();
        sqlSession.close();
    }

}