mybatis入门篇：Mybatis高级查询

1、ResultMap的association与collection

association与collection功能类似，区别是一对一与一对多，这里以association为例。

首先说明一下需求：通过员工ID获取员工信息，同时获取员工的角色，涉及到了员工信息表、角色表、还有二者的关联表。最简单的做法是写一个SQL语句，语句里写了三个表，互相关联进行查询，但这种方式存在问题：1、SQL语句不易维护 2、复用性不强 3、我只想获取用户的信息，不用角色信息时，查询了多余的信息，徒增消耗。

解决办法：1、对每个实体都定义一个基础的ResultMap，根据需要继承基础ResultMap再添加自定义的属性 2、在ResultMap中使用association或者collection进行关联查询 3、使用y延迟加载，需要角色信息时再去查询。

实际操作过程：

新建实体类User和Role

package com.forest.owl.entity;

import java.util.Date;

public class Role {
    private Long id;

    private String roleName;

    private Integer enabled;

    private Long createBy;

    private Date createTime;

    private Long updateBy;

    private Date updateTime;

    public Long getId() {
        return id;
    }

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

    public String getRoleName() {
        return roleName;
    }

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

    public Integer getEnabled() {
        return enabled;
    }

    public void setEnabled(Integer enabled) {
        this.enabled = enabled;
    }

    public Long getCreateBy() {
        return createBy;
    }

    public void setCreateBy(Long createBy) {
        this.createBy = createBy;
    }

    public Date getCreateTime() {
        return createTime;
    }

    public void setCreateTime(Date createTime) {
        this.createTime = createTime;
    }

    public Long getUpdateBy() {
        return updateBy;
    }

    public void setUpdateBy(Long updateBy) {
        this.updateBy = updateBy;
    }

    public Date getUpdateTime() {
        return updateTime;
    }

    public void setUpdateTime(Date updateTime) {
        this.updateTime = updateTime;
    }
}

package com.forest.owl.entity;

import java.util.Date;
import java.util.List;

public class User {
    private Long id;

    private String userName;

    private String userPassword;

    private String userPhone;

    private String userEmail;

    private Date createTime;

    private Date updateTime;

    private byte[] headImg;

    private Role role;

    public Long getId() {
        return id;
    }

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

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName) {
        this.userName = userName;
    }

    public String getUserPassword() {
        return userPassword;
    }

    public void setUserPassword(String userPassword) {
        this.userPassword = userPassword;
    }

    public String getUserPhone() {
        return userPhone;
    }

    public void setUserPhone(String userPhone) {
        this.userPhone = userPhone;
    }

    public String getUserEmail() {
        return userEmail;
    }

    public void setUserEmail(String userEmail) {
        this.userEmail = userEmail;
    }

    public Date getCreateTime() {
        return createTime;
    }

    public void setCreateTime(Date createTime) {
        this.createTime = createTime;
    }

    public Date getUpdateTime() {
        return updateTime;
    }

    public void setUpdateTime(Date updateTime) {
        this.updateTime = updateTime;
    }

    public byte[] getHeadImg() {
        return headImg;
    }

    public void setHeadImg(byte[] headImg) {
        this.headImg = headImg;
    }

    public Role getRole() { return role; }

    public void setRole(Role role) { this.role = role; }
}

然后编写Mapper.xml，首先先编写BaseResultMap

UserMapper.xml

  <resultMap id="BaseResultMap" type="com.forest.owl.entity.User">
    <id column="id" jdbcType="BIGINT" property="id" />
    <result column="user_name" jdbcType="VARCHAR" property="userName" />
    <result column="user_password" jdbcType="VARCHAR" property="userPassword" />
    <result column="user_phone" jdbcType="VARCHAR" property="userPhone" />
    <result column="user_email" jdbcType="VARCHAR" property="userEmail" />
    <result column="create_time" jdbcType="TIMESTAMP" property="createTime" />
    <result column="update_time" jdbcType="TIMESTAMP" property="updateTime" />
    <result column="head_img" jdbcType="LONGVARBINARY" property="headImg" />
  </resultMap>

RoleMapper.xml

  <resultMap id="BaseResultMap" type="com.forest.owl.entity.Role">
    <id column="id" jdbcType="BIGINT" property="id" />
    <result column="role_name" jdbcType="VARCHAR" property="roleName" />
    <result column="enabled" jdbcType="INTEGER" property="enabled" />
    <result column="create_by" jdbcType="BIGINT" property="createBy" />
    <result column="create_time" jdbcType="TIMESTAMP" property="createTime" />
    <result column="update_by" jdbcType="BIGINT" property="updateBy" />
    <result column="update_time" jdbcType="TIMESTAMP" property="updateTime" />
  </resultMap>

编写RoleMapper.xml获取Role的接口和XML

package com.forest.owl.mapper;

import com.forest.owl.entity.Role;

public interface RoleMapper {
    Role selectByPrimaryKey(Long id);
}

RoleMapper.xml

  <select id="selectByPrimaryKey" resultMap="BaseResultMap">
    select id, role_name, enabled, create_by, create_time, update_by, update_time
    from role
    where id = #{id,jdbcType=BIGINT}
  </select>

然后编写UserMapper.xml

  <resultMap id="UserAndRole" extends="BaseResultMap" type="com.forest.owl.entity.User">
    <association property="role" fetchType="lazy" column="{id=role_id}" select="com.forest.owl.mapper.RoleMapper.selectByPrimaryKey"/>
  </resultMap>

  <select id="selectUsersById" resultMap="UserAndRole">
    SELECT u.*, ur.role_id
    FROM user u
    INNER JOIN user_role ur on ur.user_id=u.id
    WHERE u.id=#{id}
  </select>

可以看到，UserMapper的SQL语句中少了role表，这样SQL语句就简洁多了。需要注意，association中的fetchType=“lazy”选项，当执行getRole()的时候，才回去查询角色信息加载到查询结果中。如果用户还是需要在触发某个方法时加载全部数据，则可以配置mybatis的配置选项lazyLoadTriggerMethods，按需加载。

对于3.4.1 及之前版本的mybatis用户，还需要配置mybatis-config.xml的一个选项：

<setting name="aggressiveLazyLoading" value="false"/>

测试：

    @Test
    public void UserMapperTest(){
        SqlSession sqlSession = getSqlSession();
        UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
        List<User> userList = userMapper.selectUsersById(();
        System.out.println(userList.size());
        System.out.println("--------getRole---------");
        System.).getRole());
    }

执行结果

collection的代码

User实体类

package com.forest.owl.entity;

import java.util.Date;
import java.util.List;

public class User {
    private Long id;

    private String userName;

    private String userPassword;

    private String userPhone;

    private String userEmail;

    private Date createTime;

    private Date updateTime;

    private byte[] headImg;

    private List<Role> roleList;

    public Long getId() {
        return id;
    }

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

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName) {
        this.userName = userName;
    }

    public String getUserPassword() {
        return userPassword;
    }

    public void setUserPassword(String userPassword) {
        this.userPassword = userPassword;
    }

    public String getUserPhone() {
        return userPhone;
    }

    public void setUserPhone(String userPhone) {
        this.userPhone = userPhone;
    }

    public String getUserEmail() {
        return userEmail;
    }

    public void setUserEmail(String userEmail) {
        this.userEmail = userEmail;
    }

    public Date getCreateTime() {
        return createTime;
    }

    public void setCreateTime(Date createTime) {
        this.createTime = createTime;
    }

    public Date getUpdateTime() {
        return updateTime;
    }

    public void setUpdateTime(Date updateTime) {
        this.updateTime = updateTime;
    }

    public byte[] getHeadImg() {
        return headImg;
    }

    public void setHeadImg(byte[] headImg) {
        this.headImg = headImg;
    }

    public List<Role> getRoleList() { return roleList; }

    public void setRoleList(List<Role> roleList) { this.roleList = roleList; }
}

UserMapper.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.forest.owl.mapper.UserMapper">
  <resultMap id="BaseResultMap" type="com.forest.owl.entity.User">
    <id column="id" jdbcType="BIGINT" property="id" />
    <result column="user_name" jdbcType="VARCHAR" property="userName" />
    <result column="user_password" jdbcType="VARCHAR" property="userPassword" />
    <result column="user_phone" jdbcType="VARCHAR" property="userPhone" />
    <result column="user_email" jdbcType="VARCHAR" property="userEmail" />
    <result column="create_time" jdbcType="TIMESTAMP" property="createTime" />
    <result column="update_time" jdbcType="TIMESTAMP" property="updateTime" />
    <result column="head_img" jdbcType="LONGVARBINARY" property="headImg" />
  </resultMap>

  <resultMap id="UserAndRole" extends="BaseResultMap" type="com.forest.owl.entity.User">
    <collection property="roleList" columnPrefix="role_" fetchType="lazy"
                resultMap="com.forest.owl.mapper.RoleMapper.BaseResultMap">
      <id property="id" column="id"/>
      <result column="role_name" property="roleName" />
      <result column="enabled" property="enabled" />
      <result column="create_by" property="createBy" />
      <result column="create_time" property="createTime" />
      <result column="update_by" property="updateBy" />
      <result column="update_time" property="updateTime" />
    </collection>
  </resultMap>

  <select id="selectUsersById" resultMap="UserAndRole">
    SELECT u.*,
    r.id role_id,
    r.role_name role_role_name,
    r.enabled role_enabled,
    r.create_by role_create_by,
    r.create_time role_create_time,
    r.update_by role_update_by,
    r.update_time role_update_time
    FROM user u
    INNER JOIN user_role ur on ur.user_id=u.id
    INNER JOIN role r on r.id=ur.role_id
    WHERE u.id=#{id}
  </select>
</mapper>

UserMapper接口

package com.forest.owl.mapper;

import com.forest.owl.entity.User;

public interface UserMapper {
    User selectUsersById(Long id);
}

测试：

    @Test
    public void UserMapperTest(){
        SqlSession sqlSession = getSqlSession();
        UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
        User user = userMapper.selectUsersById(();
        System.).getRoleName());
        System.).getRoleName());
    }