1.首先使用得到是springboot继承环境，创建mybatis的配置文件，在yml文件中添加mybatis的一些配置信息

mybatis:
        typeAliasesPackage:com.ruoyi.**.domain
        mapperLocations: classpath*:mapper/**/*Mapper.xml
      #加载全局配置文件
        configLocation:classpath:mybatis/mybatis-config.xml

mybatis的配置文件

<?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>
    <!-- 全局参数 -->
    <settings>
        <!-- 使全局的映射器启用或禁用缓存 -->
        <setting name="cacheEnabled"             value="true"   />
        <!-- 允许JDBC 支持自动生成主键 -->
        <setting name="useGeneratedKeys"         value="true"   />
        <!-- 配置默认的执行器.SIMPLE就是普通执行器;REUSE执行器会重用预处理语句(prepared statements);BATCH执行器将重用语句并执行批量更新 -->
        <setting name="defaultExecutorType"      value="SIMPLE" />
		<!-- 指定 MyBatis 所用日志的具体实现 -->
        <setting name="logImpl"                  value="SLF4J"  />
        <!-- 使用驼峰命名法转换字段 -->
		<!-- <setting name="mapUnderscoreToCamelCase" value="true"/> -->
	</settings>
	
</configuration>

2.xml文件中的一些细节

例如这个mapper接口中的方法：
public List<DjDeclaration> selectDjDeclarationList(DjDeclaration djDeclaration);

mapper文件表头

<?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.ruoyi.dangjian.mapper.DjDeclarationMapper">
//namespace是对应mapper接口的位置

通用的操作可以写到<sql />中，一般查询所有字段不会使用*，具体写出所有的字段

  <sql id="selectDjDeclarationVo">
        select declaration_id, title, content, date, 
send_time, major_class, student_name, zan, is_hot, sort,
comment_num, status, remark, created_by, 
created_time, updated_by, update_time from dj_declaration
  </sql>

上边接口方法的select<> ,其中<include refid="selectDjDeclarationVo"/>就是引用上边的sql通用语句，resultMap还是resultType一定要注意清楚，返回多个字段一般创建一个resultmap，返回一对多的数据（eg：用户对应多个角色，在mybatis-plus中使用@TableField(exist = false)注释的字段@TableField(exist = false)

private List<SysMenu> menus;

）也需要使用resultmap，等会穿插进去。

<select id="selectDjDeclarationList" parameterType="DjDeclaration" resultMap="DjDeclarationResult">
        <include refid="selectDjDeclarationVo"/>
        <where>  
            <if test="title != null  and title != ''"> and title = #{title}</if>
            <if test="content != null  and content != ''"> and content = #{content}</if>
            <if test="date != null  and date != ''"> and date = #{date}</if>
            <if test="sendTime != null "> and send_time = #{sendTime}</if>
            <if test="majorClass != null  and majorClass != ''"> and major_class = #{majorClass}</if>
            <if test="studentName != null  and studentName != ''"> and student_name like concat('%', #{studentName}, '%')</if>
            <if test="zan != null "> and zan = #{zan}</if>
            <if test="isHot != null  and isHot != ''"> and is_hot = #{isHot}</if>
            <if test="sort != null "> and sort = #{sort}</if>
            <if test="commentNum != null "> and comment_num = #{commentNum}</if>
            <if test="status != null "> and status = #{status}</if>
            <if test="createdBy != null "> and created_by = #{createdBy}</if>
            <if test="createdTime != null "> and created_time = #{createdTime}</if>
            <if test="updatedBy != null "> and updated_by = #{updatedBy}</if>
        </where>
    </select>

注意这里的<where> 和 <if test>的标签的使用，虽然这里是list全部列表的方法的xml文件，但是参数有个对象实例，如果是直接查询就是对象实例为空where标签都不会执行，如果是进行模糊查询列表后边的就会根据where和if的组合查询出来。

eg：还有很多多种操作使用同一个方法的案例：增加和修改使用同一个controller方法，如果添加时sysUser就为null，修改时sysuser就是前端传回来的数据

    /**
     * 编辑
     */
    @RequestMapping("/form")
    public String form(Model model, Integer id, SysUser sysUser) {
        //获取所有的角色
        List<SysRole> roleList = sysRoleService.list();
        if (id != null) {
            sysUser = sysUserService.findUserById(id);
        }
        model.addAttribute("roleList", roleList);
        System.out.println(sysUser);
        model.addAttribute("sysUser", sysUser);
        return "sys/userForm";
    }

接下来继续写resultmap，<id>对应前边写的resultmap名称，<result>中的property对应中的字段名称，column对应数据库中的查询字段。

<resultMap type="DjDeclaration" id="DjDeclarationResult">
        <result property="declarationId"    column="declaration_id"    />
        <result property="title"    column="title"    />
        <result property="content"    column="content"    />
        <result property="date"    column="date"    />
        <result property="sendTime"    column="send_time"    />
        <result property="majorClass"    column="major_class"    />
        <result property="studentName"    column="student_name"    />
        <result property="zan"    column="zan"    />
        <result property="isHot"    column="is_hot"    />
        <result property="sort"    column="sort"    />
        <result property="commentNum"    column="comment_num"    />
        <result property="status"    column="status"    />
        <result property="remark"    column="remark"    />
        <result property="createdBy"    column="created_by"    />
        <result property="createdTime"    column="created_time"    />
        <result property="updatedBy"    column="updated_by"    />
        <result property="updateTime"    column="update_time"    />
    </resultMap>

前边讲到mybatis-plus中使用@TableField(exist = false)注释数据库中不存在的中间字段，在xml文件中查询的时候，需要在<resultmap> 中添加<collection>标签对应一对多

<rseultMap id="userMap" type="SysUser">
<id property="id" column="id">
<result peoperty="username" column="username">
<collection peoperty="roles" ofType="SysUser">
    <id property="id" column="role_id">
      <result property="name" column="role_name">
</collection>

添加操作，id为mapper文件的方法名称，

useGeneratedKeys="true" keyProperty="declarationId"这里是主键declarationId自增，插入时不用写id值

<trim>标签中prefix"("是在下边if判断的所有标签前边加上（  

suffix=")" 是在所有的结束后加上） ，suffixoverrides=","是去掉，换上）

<insert id="insertDjDeclaration" parameterType="DjDeclaration" useGeneratedKeys="true" keyProperty="declarationId">
        insert into dj_declaration
        <trim prefix="(" suffix=")" suffixOverrides=",">
            <if test="title != null">title,</if>
            <if test="content != null">content,</if>
            <if test="date != null">date,</if>，
            <if test="sendTime != null">send_time,</if>
            <if test="majorClass != null">major_class,</if>
            <if test="studentName != null">student_name,</if>
            <if test="zan != null">zan,</if>
            <if test="isHot != null">is_hot,</if>
            <if test="sort != null">sort,</if>
            <if test="commentNum != null">comment_num,</if>
            <if test="status != null">status,</if>
            <if test="remark != null">remark,</if>
            <if test="createdBy != null">created_by,</if>
            <if test="createdTime != null">created_time,</if>
            <if test="updatedBy != null">updated_by,</if>
            <if test="updateTime != null">update_time,</if>
         </trim>
        <trim prefix="values (" suffix=")" suffixOverrides=",">
            <if test="title != null">#{title},</if>
            <if test="content != null">#{content},</if>
            <if test="date != null">#{date},</if>
            <if test="sendTime != null">#{sendTime},</if>
            <if test="majorClass != null">#{majorClass},</if>
            <if test="studentName != null">#{studentName},</if>
            <if test="zan != null">#{zan},</if>
            <if test="isHot != null">#{isHot},</if>
            <if test="sort != null">#{sort},</if>
            <if test="commentNum != null">#{commentNum},</if>
            <if test="status != null">#{status},</if>
            <if test="remark != null">#{remark},</if>
            <if test="createdBy != null">#{createdBy},</if>
            <if test="createdTime != null">#{createdTime},</if>
            <if test="updatedBy != null">#{updatedBy},</if>
            <if test="updateTime != null">#{updateTime},</if>
         </trim>
    </insert>

删除操作，in配合<foreach>实现删除语句

<delete id="deleteDjDeclarationByDeclarationIds" parameterType="String">
        delete from dj_declaration where declaration_id in 
        <foreach item="declarationId" collection="array" open="(" separator="," close=")">
            #{decla rationId}
        </foreach>
    </delete>