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Mybatis开发要点:resultType和resultMap的区别?

小沙坨 2021-10-04 阅读 58

Mybatis返回Xml返回值有resultType和resultMap,我们一般都该如何选择呢?

一、resultType

1.1 resultType介绍

当使用resultType做SQL语句返回结果类型处理时,对于SQL语句查询出的字段在相应的pojo中必须有和它相同的字段对应,而resultType中的内容就是pojo在本项目中的位置。

1.2 映射规则

  • 基本类型 :resultType=基本类型

  • List类型: resultType=List中元素的类型

  • Map类型 单条记录:resultType =map 多条记录:resultType =Map中value的类型

1.3 自动映射注意事项

  • 前提:SQL列名和JavaBean的属性是一致的;

  • 使用resultType,如用简写需要配置typeAliases (别名);

  • 如果列名和JavaBean不一致,但列名符合单词下划线分割,Java是驼峰命名法,则mapUnderscoreToCamelCase可设置为true;

1.4 代码演示

(1)t_user_test.sql准备

CREATE TABLE `t_user_test` (
  `id` int(20) NOT NULL AUTO_INCREMENT,
  `user_name` varchar(60) DEFAULT NULL COMMENT '用户名称',
  `real_name` varchar(60) DEFAULT NULL COMMENT '真实名称',
  `sex` tinyint(3) DEFAULT NULL COMMENT '姓名',
  `mobile` varchar(20) DEFAULT NULL COMMENT '电话',
  `email` varchar(60) DEFAULT NULL COMMENT '邮箱',
  `note` varchar(200) DEFAULT NULL COMMENT '备注',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=142 DEFAULT CHARSET=utf8;

(2)实体类


package com.enjoylearning.mybatis.entity;

import java.io.Serializable;
import java.util.List;

import org.apache.ibatis.annotations.Param;

import com.mysql.jdbc.Blob;

public class TUser implements Serializable{

    private Integer id;

    private String userName;

    private String realName;

    private Byte sex;

    private String mobile;

    private String email;

    private String note;

    private TPosition position;

    private List<TJobHistory> jobs ;

    private List<HealthReport> healthReports;

    private List<TRole> roles;

    @Override
    public String toString() {
        String positionId=  (position == null ? "" : String.valueOf(position.getId()));
        return "TUser [id=" + id + ", userName=" + userName + ", realName="
                + realName + ", sex=" + sex + ", mobile=" + mobile + ", email="
                + email + ", note=" + note + ", positionId=" + positionId + "]";
    }

    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    public String getUserName() {
        return userName;
    }
    public void setUserName(String userName) {
        this.userName = userName;
    }
    public String getRealName() {
        return realName;
    }
    public void setRealName(String realName) {
        this.realName = realName;
    }

    public Byte getSex() {
        return sex;
    }

    public void setSex(Byte sex) {
        this.sex = sex;
    }

    public String getMobile() {
        return mobile;
    }

    public void setMobile(String mobile) {
        this.mobile = mobile;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public String getNote() {
        return note;
    }

    public void setNote(String note) {
        this.note = note;
    }

    public TPosition getPosition() {
        return position;
    }

    public void setPosition(TPosition position) {
        this.position = position;
    }

    public List<TJobHistory> getJobs() {
        return jobs;
    }

    public void setJobs(List<TJobHistory> jobs) {
        this.jobs = jobs;
    }

    public List<HealthReport> getHealthReports() {
        return healthReports;
    }

    public void setHealthReports(List<HealthReport> healthReports) {
        this.healthReports = healthReports;
    }

    public List<TRole> getRoles() {
        return roles;
    }

    public void setRoles(List<TRole> roles) {
        this.roles = roles;
    }

}

(3)Mapper接口类

public interface TUserTestMapper {

    TUser selectByPrimaryKey(Integer id);
    List<TUser> selectAll();

}

(4)Mapper 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.mybatis.mapper.TUserTestMapper">

    <select id="selectByPrimaryKey" resultType="TUser">
        select
        id, user_name, real_name, sex, mobile, email, note
        from t_user_test
        where id = #{id,jdbcType=INTEGER}
    </select>

    <select id="selectAll" resultType="TUser">
        select
        id, user_name, real_name, sex, mobile, email, note
        from t_user_test
    </select>

</mapper>

(5)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>

    <properties resource="db.properties"/>

    <settings>
        <!-- 设置自动驼峰转换        -->
        <setting name="mapUnderscoreToCamelCase" value="true" />

        <!-- 开启懒加载 -->      
         <!-- 当启用时,有延迟加载属性的对象在被调用时将会完全加载任意属性。否则,每种属性将会按需要加载。默认:true -->
      <setting name="aggressiveLazyLoading" value="false" />

    </settings>
    <!-- 别名定义 -->
    <typeAliases>
        <package name="com.enjoylearning.mybatis.entity" />
    </typeAliases>

    <plugins>
        <plugin interceptor="com.enjoylearning.mybatis.Interceptors.ThresholdInterceptor"> 
            <property name="threshold" value="10"/>
        </plugin>

         <plugin interceptor="com.github.pagehelper.PageInterceptor">
            <property name="pageSizeZero" value="true" />
        </plugin>
    </plugins>

    <!--配置environment环境 -->
    <environments default="development">
        <!-- 环境配置1,每个SqlSessionFactory对应一个环境 -->
        <environment id="development">
            <transactionManager type="JDBC" />
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.jdbc.Driver" />
                <property name="url" value="jdbc:mysql://ip:port/test?useUnicode=true" />
                <property name="username" value="root" />
                <property name="password" value="123456" />
            </dataSource>
        </environment>
    </environments>

    <!-- 映射文件,mapper的配置文件 -->
    <mappers>
        <!--直接映射到相应的mapper文件 -->
        <mapper resource="sqlmapper/TUserTestMapper.xml" />
    </mappers>

</configuration>

(6)启动测试类

public class MybatisDemo2 {

    private SqlSessionFactory sqlSessionFactory;

    @Before
    public void init() throws IOException {
        //--------------------第一阶段---------------------------
        // 1.读取mybatis配置文件创SqlSessionFactory
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        // 1.读取mybatis配置文件创SqlSessionFactory
        sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        inputStream.close();
    }

    @Test
    //知识点:resultType
    public void testAutoMapping() throws IOException {
        // 2.获取sqlSession   
        SqlSession sqlSession = sqlSessionFactory.openSession();
        // 3.获取对应mapper
        TUserTestMapper mapper = sqlSession.getMapper(TUserTestMapper.class);
        // 4.执行查询语句并返回多条数据

        List<TUser> users = mapper.selectAll();
        for (TUser tUser : users) {
            System.out.println(tUser);
        }

    }

}

(7)执行结果


sql语句:“com.mysql.jdbc.JDBC4PreparedStatement@654f0d9c: select
        id, user_name, real_name, sex, mobile, email, note
        from t_user_test”执行时间为:35毫秒,已经超过阈值!
TUser [id=1, userName=zhangsan, realName=张三, sex=1, mobile=186995587411, email=zhangsan@qq.com, note=zhangsan的备注, positionId=]
TUser [id=2, userName=lisi, realName=李四, sex=1, mobile=18677885200, email=lisi@qq.com, note=lisi的备注, positionId=]
TUser [id=3, userName=wangwu, realName=王五, sex=2, mobile=18695988747, email=xxoo@163.com, note=wangwu's note, positionId=]

resultType当返基本类型的时候建议选择,当返回POJO类的时候由于需要完全和数据库字段进行对应,存在不灵活、问题排查难等问题。

二、resultMap

2.1 resultMap 介绍

resultMap 元素是 MyBatis 中最重要最强大的元素。它可以让你从 90% 的 JDBC ResultSets 数据提取代码中解放出来,在对复杂语句进行联合映射的时候,它很可能可以代替数千行的同等功能的代码。ResultMap 的设计思想是,简单的语句不需要明确的结果映射,而复杂一点的语句只需要描述它们的关系就行了。

2.2 resultMap属性

2.3 使用场景

  • 字段有自定义的转化规则

  • 复杂的多表查询

2.4 resultMap子元素属性

  • id –一个 ID 结果;标记出作为 ID 的结果可以帮助提高整体性能,一对多的查询中用于结果集合并;

  • result – 注入到字段或 JavaBean 属性的普通结果

  • association – 一个复杂类型的关联;许多结果将包装成这种类型。关联可以指定为一个 resultMap 元素,或者引用一个

  • collection – 一个复杂类型的集合

2.5 代码演示

实体类,配置文件同上

(1)mapper接口


public interface TUserMapper {

    List<TUser> selectTestResultMap();

}

(2)Mapper.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.mybatis.mapper.TUserMapper">

    <resultMap id="UserResultMap" type="TUser" autoMapping="true">
        <id column="id" property="id" />
        <result column="userName" property="userName"/>
        <result column="realName" property="realName" />
        <result column="sex" property="sex" />
        <result column="mobile" property="mobile" />
        <result column="email" property="email" />
        <result column="note" property="note" />
        <association property="position" javaType="TPosition" columnPrefix="post_">
            <id column="id" property="id"/>
            <result column="name" property="postName"/>
            <result column="note" property="note"/>
        </association>
    </resultMap>

    <select  id="selectTestResultMap" resultMap="UserResultMap" >
        select
            a.id,
            userName,
            realName,
            sex,
            mobile,
            email,
            a.note,
            b.id  post_id,
            b.post_name,
            b.note post_note
        from t_user a,
            t_position b
        where a.position_id = b.id

    </select>

</mapper>

(3)启动测试

public class MybatisDemo2 {

    private SqlSessionFactory sqlSessionFactory;

    @Before
    public void init() throws IOException {
        //--------------------第一阶段---------------------------
        // 1.读取mybatis配置文件创SqlSessionFactory
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        // 1.读取mybatis配置文件创SqlSessionFactory
        sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        inputStream.close();
    }

    @Test
    public void testResultMap() throws IOException {
        //--------------------第二阶段---------------------------
        // 2.获取sqlSession   
        SqlSession sqlSession = sqlSessionFactory.openSession();
        // 3.获取对应mapper
        TUserMapper mapper = sqlSession.getMapper(TUserMapper.class);

        //--------------------第三阶段---------------------------

        // 4.执行查询语句并返回单条数据
        List<TUser> users = mapper.selectTestResultMap();
        for (TUser tUser : users) {
            System.out.println(tUser.getUserName());
            System.out.println(tUser.getPosition().getPostName());
        }
    }
}

(4)执行结果


sql语句:“com.mysql.jdbc.JDBC4PreparedStatement@19bb07ed: select
            a.id,
            userName,
            realName,
            sex,
            mobile,
            email,
            a.note,
            b.id  post_id,
            b.post_name,
            b.note post_note
        from t_user a,
            t_position b
        where a.position_id = b.id”执行时间为:52毫秒,已经超过阈值!
zhangsan
总经理
lisi
零时工
wangwu
总经理

三、结论

当返回对象为基础类型时建议走resultType,当返回对象为POJO时,强制走resultMap。同时可以参考阿里巴巴JAVA开发手册中的5.4.3节,返回要解耦,不讷讷更直接使用resultClass。

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