一 resultMap标签之一对一映射

1.准备阶段

User.java文件

public class User {

    private Integer user_id;
    private String account;
    private String password;
    private String user_name;
    private Integer status;
    private Date login_time;
    private String ip;
    private Integer fk_role_id;
    //关联对象
    private Role role;
        
        //省略getter和setter方法
}

Role.java 文件

public class Role {

    private Integer role_id;
    private String role_name;
    private String role_key;
    private Integer status;

        //省略getter和setter方法
}

2.传统方式

• UserMapper.xml映射文件

<mapper namespace="com.shxt.model.User">
    <resultMap type="com.shxt.model.User" id="BaseResultMapper">
        <id column="user_id" property="user_id"/>
        <result column="account" property="account"/>
        <result column="password" property="password"/>
        <result column="user_name" property="user_name"/>
        <result column="status" property="status"/>
        <result column="login_time" property="login_time"/>
        <result column="ip" property="ip"/>
        <result column="fk_role_id" property="fk_role_id"/>
    </resultMap>
    <sql id="sys_user_columns">
        user_id,account,password,user_name,status,login_time,ip,fk_role_id
    </sql>    
    <select id="load" parameterType="_int" resultMap="BaseResultMapper">
        SELECT
            <include refid="sys_user_columns"/>
        FROM
            sys_user
        WHERE
            user_id=#{user_id}
    </select>
</mapper>

• UserDao接口和UserDaoImpl实现类

UserDao.java接口代码

public interface UserDao {
    User getUserByPK(int user_id);
}

UserDaoImpl.java实现类

public class UserDaoImpl implements UserDao {
    @Override
    public User getUserByPK( int user_id ) {
        SqlSession sqlSession = null;
        try {
            sqlSession = MyBatisUtils.getSqlSession();
            return sqlSession.selectOne(User.class.getName()+".load", user_id);
        } finally {
            MyBatisUtils.closeSqlSession(sqlSession);
        }
    }
}

• RoleMapper.xml映射文件

<mapper namespace="com.shxt.model.Role">
    <resultMap type="com.shxt.model.Role" id="BaseResultMapper">
        <id column="role_id" property="role_id"/>
        <result column="role_name" property="role_name"/>
        <result column="role_key" property="role_key"/>
        <result column="status" property="status"/>
    </resultMap>
    <sql id="sys_role_columns">
        role_id,role_name,role_key,status
    </sql>
    <select id="get" parameterType="_int" resultMap="BaseResultMapper">
        SELECT
            <include refid="sys_role_columns"/>
        FROM
            sys_role
        WHERE role_id=#{role_id}
    </select>
</mapper>

• RoleDao.java文件和RoleDaoImpl.java实现类

RoleDao.java接口

public interface RoleDao {
    Role getRoleByPK(int role_id);
}

RoleDaoImpl.java实现类

public class RoleDaoImpl implements RoleDao {
    @Override
    public Role getRoleByPK( int role_id ) {
        SqlSession sqlSession = null;
        try {
            sqlSession = MyBatisUtils.getSqlSession();

            return sqlSession.selectOne(Role.class.getName()+".get", role_id);
        } finally {
            MyBatisUtils.closeSqlSession(sqlSession);
        }
    }
}

• Java代码测试

    @Test
    public void 获取用户信息(){
        UserDao userDao = new UserDaoImpl();//接口回调
        User user = userDao.getUserByPK(-999);
        //获取对应外键信息
        if(user.getFk_role_id()!=null){
            RoleDao roleDao = new RoleDaoImpl();
            //查询角色对应的信息
            Role role = roleDao.getRoleByPK(user.getFk_role_id());
            //建立关系
            user.setRole(role);
        }
        System.out.println(user);
    }

• 图解说明

  
  一对一传统方式

3.resultMap标签之select方式

其实就是替换了

//获取对应外键信息
if(user.getFk_role_id()!=null){
  RoleDao roleDao = new RoleDaoImpl();
   //查询角色对应的信息
   Role role = roleDao.getRoleByPK(user.getFk_role_id());
   //建立关系
   user.setRole(role);
}

这段代码

• 映射文件

     <resultMap type="com.shxt.model.User" id="BaseResultMapper">
        <id column="user_id" property="user_id"/>
        <result column="account" property="account"/>
        <result column="password" property="password"/>
        <result column="user_name" property="user_name"/>
        <result column="status" property="status"/>
        <result column="login_time" property="login_time"/>
        <result column="ip" property="ip"/>
        <result column="fk_role_id" property="fk_role_id"/>
    </resultMap>

 <resultMap type="com.shxt.model.User" id="SimpleResultMapper"  extends="BaseResultMapper">
        <!-- association 用来映射关联对象  -->
        <association property="role" javaType="com.shxt.model.Role"
        column="fk_role_id"
        select="com.shxt.model.Role.get"
        />
    </resultMap>

    <sql id="sys_user_columns">
        user_id,account,password,user_name,status,login_time,ip,fk_role_id
    </sql>
    <!-- 使用了另一个ID resultMap="SimpleResultMapper"  -->
    <select id="load" parameterType="_int" resultMap="SimpleResultMapper">
        SELECT
            <include refid="sys_user_columns"/>
        FROM
            sys_user
        WHERE
            user_id=#{user_id}
    </select>

• Java测试代码

    @Test
    public void SELECT方式(){
        UserDao userDao = new UserDaoImpl();//接口回调
        User user = userDao.getUserByPK(-999);
        System.out.println(user);
    }

• 图解说明

  
  select方式说明
  结果集继承

4.resultMap标签之resultMap属性方式

• UserDao接口和UserDaoImpl实现类
  UserDao.java代码

public interface UserDao {
    User getUserLeftJoin(int user_id);
}

UserDaoImpl.java代码

public class UserDaoImpl implements UserDao {
    @Override
    public User getUserLeftJoin( int user_id ) {
        SqlSession sqlSession = null;
        try {
            sqlSession = MyBatisUtils.getSqlSession();

            return sqlSession.selectOne(User.class.getName()+".getUserLeftJoin", user_id);
        } finally {
            MyBatisUtils.closeSqlSession(sqlSession);
        }
    }
}

• 映射文件

<mapper namespace="com.shxt.model.User">

    <sql id="sys_user_columns_alias">
        ${alias}.user_id,${alias}.account,${alias}.password,
        ${alias}.user_name,${alias}.status,${alias}.login_time,
        ${alias}.ip,${alias}.fk_role_id
    </sql>
    

     <resultMap type="com.shxt.model.User" id="BaseResultMapper">
        <id column="user_id" property="user_id"/>
        <result column="account" property="account"/>
        <result column="password" property="password"/>
        <result column="user_name" property="user_name"/>
        <result column="status" property="status"/>
        <result column="login_time" property="login_time"/>
        <result column="ip" property="ip"/>
        <result column="fk_role_id" property="fk_role_id"/>
    </resultMap>
    <resultMap type="com.shxt.model.User" 
        id="JoinResultMapper" 
        extends="BaseResultMapper">
         <association property="role" javaType="com.shxt.model.Role"
            resultMap="com.shxt.model.Role.BaseResultMapper"
        >
            <!-- <id column="role_id" property="role_id"/>
            <result column="role_name" property="role_name"/>
            <result column="role_key" property="role_key"/>
            <result column="status" property="status"/> -->
        </association>
    
    </resultMap>
    <select id="getUserLeftJoin" parameterType="_int" 
        resultMap="JoinResultMapper">
        SELECT
            <include refid="sys_user_columns_alias">
                <property name="alias" value="u"/>
            </include>
            , 
            <!-- 关键如何找到那些字段 -->
            <include refid="com.shxt.model.Role.sys_role_columns_alias">
                <property name="alias" value="r"/>
            </include>
        FROM
            sys_user u
        LEFT JOIN sys_role r ON u.fk_role_id = r.role_id
        WHERE
            u.user_id =#{user_id}
    </select>

• Java测试代码

    @Test
    public void 连接查询_结果集处理(){
        UserDao userDao = new UserDaoImpl();//接口回调
        User user = userDao.getUserLeftJoin(-999);
        System.out.println(user);
    }

• 图解说明

  
  推荐说明

二 多对一映射测试

• UserDao接口和UserDaoImpl实现类
  UserDao.java代码

public interface UserDao {
    List<User> list01();
    List<User> list02();

}

UserDaoImpl.java代码

public class UserDaoImpl implements UserDao {
    @Override
    public List<User> list01() {
        SqlSession sqlSession = null;
        try {
            sqlSession = MyBatisUtils.getSqlSession();

            return sqlSession.selectList(User.class.getName()+".list01");
        } finally {
            MyBatisUtils.closeSqlSession(sqlSession);
        }
    }

    @Override
    public List<User> list02() {
        SqlSession sqlSession = null;
        try {
            sqlSession = MyBatisUtils.getSqlSession();

            return sqlSession.selectList(User.class.getName()+".list02");
        } finally {
            MyBatisUtils.closeSqlSession(sqlSession);
        }
    }

}

• 映射文件

    <select id="list01" resultMap="SimpleResultMapper">
        SELECT
            <include refid="sys_user_columns"/>
        FROM
            sys_user
    </select>
    <select id="list02"  resultMap="JoinResultMapper">
        SELECT
            <include refid="sys_user_columns_alias">
                <property name="alias" value="u"/>
            </include>
            , 
            <include refid="com.shxt.model.Role.sys_role_columns_alias">
                <property name="alias" value="r"/>
            </include>
        FROM
            sys_user u
        LEFT JOIN sys_role r ON u.fk_role_id = r.role_id
    </select>

• 图解说明

  
  多对一测试