mybatis中實(shí)現(xiàn)讓返回值與bean中字段相匹配

更新時間：2020年10月01日 11:48:43 作者：smile-yan

這篇文章主要介紹了mybatis中實(shí)現(xiàn)讓返回值與bean中字段相匹配，具有很好的參考價值，希望對大家有所幫助。一起跟隨小編過來看看吧

1. 編寫目的

這個介紹的與那些修改mybatis.xml文件的方法不一樣，目的也不一樣。

當(dāng)我們需要查詢的數(shù)據(jù)跟entity的bean完全不匹配的時候（比如說需要統(tǒng)計的時候），我們不可能寫多個dao層的查詢接口，然后一個一個map到xml中去。

我們可以專門寫一個類，根據(jù)自己的需要把統(tǒng)計的屬性都寫到里面去，然后一次性查詢，就可以獲得所有需要統(tǒng)計的數(shù)據(jù)。

2. 重要方法

專門編寫一個實(shí)體類。實(shí)體類中包含的都是自己需要統(tǒng)計的屬性。

編寫dao層的接口方法的返回值就是這個實(shí)體類。

在映射的map.xml中編寫查詢時，使用as …的方法與實(shí)體類中的屬性一一對應(yīng)。

3. 具體案例

需求：需要統(tǒng)計日記表中某個用戶的以下屬性：

總共編寫日記數(shù)目

刪除日記數(shù)目

心情開心篇數(shù)

心情一般篇數(shù)

心情差篇數(shù)

心情極差篇數(shù)

晴朗天氣篇數(shù)

陰天篇數(shù)

雨天篇數(shù)

…

實(shí)體類的編寫

package cn.ailanglang.diary.util;

public class StatisticBean {
  private Integer sum;
  private Integer mood0;
  private Integer mood1;
  private Integer mood2;
  private Integer mood3;
  private Integer weather0;
  private Integer weather1;
  private Integer weather2;
  private Integer weather3;
  private Integer weather4;
  private Integer weather5;
  private Integer weather6;
  private Integer weather7;
  
  private Integer unknow_weather;
  private Integer unknow_mood;

  public Integer getSum() {
    return sum;
  }

  public void setSum(Integer sum) {
    this.sum = sum;
  }

  public Integer getWeather0() {
    return weather0;
  }

  public void setWeather0(Integer weather0) {
    this.weather0 = weather0;
  }

  public Integer getWeather1() {
    return weather1;
  }

  public void setWeather1(Integer weather1) {
    this.weather1 = weather1;
  }

  public Integer getWeather2() {
    return weather2;
  }

  public void setWeather2(Integer weather2) {
    this.weather2 = weather2;
  }

  public Integer getWeather3() {
    return weather3;
  }

  public void setWeather3(Integer weather3) {
    this.weather3 = weather3;
  }

  public Integer getWeather4() {
    return weather4;
  }

  public void setWeather4(Integer weather4) {
    this.weather4 = weather4;
  }

  public Integer getWeather5() {
    return weather5;
  }

  public void setWeather5(Integer weather5) {
    this.weather5 = weather5;
  }

  public Integer getWeather6() {
    return weather6;
  }

  public void setWeather6(Integer weather6) {
    this.weather6 = weather6;
  }

  public Integer getWeather7() {
    return weather7;
  }

  public void setWeather7(Integer weather7) {
    this.weather7 = weather7;
  }

  public Integer getUnknow_weather() {
    return unknow_weather;
  }

  public void setUnknow_weather(Integer unknow_weather) {
    this.unknow_weather = unknow_weather;
  }

  public Integer getUnknow_mood() {
    return unknow_mood;
  }

  public void setUnknow_mood(Integer unknow_mood) {
    this.unknow_mood = unknow_mood;
  }

  public Integer getMood0() {
    return mood0;
  }

  public void setMood0(Integer mood0) {
    this.mood0 = mood0;
  }

  public Integer getMood1() {
    return mood1;
  }

  public void setMood1(Integer mood1) {
    this.mood1 = mood1;
  }

  public Integer getMood2() {
    return mood2;
  }

  public void setMood2(Integer mood2) {
    this.mood2 = mood2;
  }

  public Integer getMood3() {
    return mood3;
  }

  public void setMood3(Integer mood3) {
    this.mood3 = mood3;
  }
}

dao層接口方法

  /**
   * 統(tǒng)計
   * @param userid
   * @return
   */
  StatisticBean countMood(Long userid);

mapper.xml的編寫

重點(diǎn)注意as …

<select id="countMood" parameterType="java.lang.Long" resultType="cn.smileyan.diary.util.StatisticBean">
    select
     count(diary.pk_diaryid) as sum,
     sum(case when diary.mood='0' then 1 else 0 end) as mood0,
     sum(case when diary.mood='1' then 1 else 0 end) as mood1,
     sum(case when diary.mood='2' then 1 else 0 end) as mood2,
     sum(case when diary.mood='3' then 1 else 0 end) as mood3,
     sum(case when diary.weather='0' then 1 else 0 end) as weather0,
     sum(case when diary.weather='1' then 1 else 0 end) as weather1,
     sum(case when diary.weather='2' then 1 else 0 end) as weather2,
     sum(case when diary.weather='3' then 1 else 0 end) as weather3,
     sum(case when diary.weather='4' then 1 else 0 end) as weather4,
     sum(case when diary.weather='5' then 1 else 0 end) as weather5,
     sum(case when diary.weather='6' then 1 else 0 end) as weather6,
     sum(case when diary.weather='7' then 1 else 0 end) as weather7
    from user_diary,diary
    where diary.pk_diaryid = user_diary.fk_diaryid
    and user_diary.fk_userid = #{userid};
  </select>

編寫service層就不再介紹了。

測試類方法

 @Test
  public void test6() {
    StatisticBean statisticBean = diaryService.countMood((long) 25);
    System.out.println("sum=="+statisticBean.getSum());
    System.out.println("mood(0) == "+statisticBean.getMood0());
    System.out.println("mood(1) == "+statisticBean.getMood1());
    System.out.println("mood(2) == "+statisticBean.getMood2());
    System.out.println("mood(3) == "+statisticBean.getMood3());
    System.out.println("weather(0) == "+statisticBean.getWeather0());
    System.out.println("weather(1) == "+statisticBean.getWeather1());
    System.out.println("weather(2) == "+statisticBean.getWeather2());
    System.out.println("weather(3) == "+statisticBean.getWeather3());
    System.out.println("weather(4) == "+statisticBean.getWeather4());
    System.out.println("weather(5) == "+statisticBean.getWeather5());
    System.out.println("weather(6) == "+statisticBean.getWeather6());
    System.out.println("weather(7) == "+statisticBean.getWeather7());
  }

可以成功輸出數(shù)據(jù)庫中的數(shù)據(jù)，完成了我們的目的——統(tǒng)計。

4. 總結(jié)

重點(diǎn)了解一下as 的用法，as后面跟著的是實(shí)體類的屬性名，當(dāng)然xml文件中一定要指明returnType是那個實(shí)體類，注意要寫詳細(xì)的class地址。

然后就是理解一下 sum(case when diary.mood=‘3' then 1 else 0 end) as mood3,中sum和case when的用法。

補(bǔ)充知識：mybatis 學(xué)習(xí)總結(jié)3 表字段與javabean字段的映射

有時候我們封裝的javabean與庫表的字段并不能一一對應(yīng)，我們需要做一些必要的配置以保證數(shù)據(jù)能夠正確的獲取。

總的來說，解決庫表與javabean字段不統(tǒng)一的方法有以下幾種。

1.駝峰轉(zhuǎn)換

我們在封裝實(shí)體類的時候，通常將屬性命令為駝峰形式，例如

userName

而庫表的命名則遵循全小寫，多個單詞間使用 '_' 下劃線連接的方式，例如

user_name

這種情況我們可以使用mybatis的駝峰轉(zhuǎn)換策略在mybatis 的config.xml配置文件中添加如下代碼：

 <!--設(shè)置集標(biāo)簽-->
  <settings>
    <!--設(shè)置標(biāo)簽 === 駝峰轉(zhuǎn)換-->
    <setting name="mapUnderscoreToCamelCase" value="true"/>
  </settings>

添加如下策略后，針對查詢結(jié)果集中的字段出現(xiàn)上例中的沖突時則能夠得到解決。

2.sql語句中的 AS 關(guān)鍵字 — 起別名

我們在數(shù)據(jù)庫建一個course表

create table course (
id BIGINT KEY AUTO_INCREMENT,
course_name VARCHAR(30) NOT NULL,
grade SMALLINT NOT NULL,
teacher_id BIGINT NOT NULL,
add_time TIMESTAMP DEFAULT now(),
mod_time TIMESTAMP DEFAULT now()
)

插入幾條數(shù)據(jù)

INSERT INTO course (course_name,grade,teacher_id) VALUES ('高等數(shù)學(xué)',1,1);
INSERT INTO course (course_name,grade,teacher_id) VALUES ('微積分',2,2);
INSERT INTO course (course_name,grade,teacher_id) VALUES ('希臘文學(xué)史',3,3);

建立實(shí)體類

@Alias("courseBO")
public class CourseBO{

private Long courseId; //數(shù)據(jù)庫-- id
private String courseName; //數(shù)據(jù)庫 -- course_name
private Integer courseGrade; //數(shù)據(jù)庫 -- grade
private Long courseTeacherId; //數(shù)據(jù)庫 -- teacher_id
private Date addTime; //數(shù)據(jù)庫 -- add_time
private Date modTime; //數(shù)據(jù)庫 -- mod_time

/**
* 以下為 get set 方法 以及 toString 方法
*/
}

可以簡單的發(fā)現(xiàn)幾個庫表與實(shí)體字段的不同

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="mapping.CourseMapper">
  <select id="getCourseByPriMaryKey" resultType="Model.CourseBO">
    SELECT * FROM course WHERE id = #{id}
  </select>
</mapper>

mapper接口

public interface CourseMapper extends BaseMapper {
  CourseBO getCourseByPriMaryKey(@Param("id") Long id);
}

config.xml 注冊mapper

<!--映射注冊 加載映射文件-->
  <mappers>
    <!--第一種即第三種方法使用resource屬性注冊mapper,路徑為xml文件-->
    <mapper resource="resource/mapper/UserMapper.xml" />
    <!--第二種方式即注解方式使用class屬性注冊mapper,路徑為接口class文件-->
    <!--<mapper class="mapping.UserMapper" />-->
    <mapper resource="resource/mapper/CourseMapper.xml" />
  </mappers>

測試方法

public class FieldMappingDemo {
  public static void main(String[] args) throws IOException {
    String resource = "resource/common/mybatis-config.xml";
    InputStream in = Resources.getResourceAsStream(resource);
    SqlSession sqlSession= new SqlSessionFactoryBuilder().build(in).openSession();
    CourseMapper courseMapper = sqlSession.getMapper(CourseMapper.class);
    CourseBO courseBO = courseMapper.getCourseByPriMaryKey(1L);
    System.out.println(courseBO);
    sqlSession.close();
  }
}

執(zhí)行結(jié)果

可以看到由于主鍵映射的失敗，整個結(jié)果對象返回為null

我們現(xiàn)在在sql語句中加入字段別名，同時設(shè)置駝峰轉(zhuǎn)換策略

<select id="getCourseByPriMaryKey" resultType="Model.CourseBO">
    SELECT
    id courseId,
    course_name,
    grade courseGrade,
    teacher_id courseTeacherId,
    add_time,
    mod_time
    FROM course WHERE id = #{id}
</select>

  <!--設(shè)置集標(biāo)簽-->
  <settings>
    <!--設(shè)置標(biāo)簽 === 駝峰轉(zhuǎn)換-->
    <setting name="mapUnderscoreToCamelCase" value="true"/>
  </settings>

再次執(zhí)行，結(jié)果：

可以看到數(shù)據(jù)正確的獲取到了。

3.resultMap建立字段映射

我們也可以使用resultMap做字段映射

 <!--id:唯一標(biāo)識 type:JavaBean實(shí)體-->
  <resultMap id="SqlMap" type="Model.CourseBO">
    <!--id標(biāo)簽為主鍵標(biāo)簽，resultMap中必須存在一個id標(biāo)簽-->
    <id column="id" jdbcType="BIGINT" property="courseId" javaType="Long" />
    <!--result標(biāo)簽為屬性標(biāo)簽，
    column屬性指定表字段名，jdbcType為表字段數(shù)據(jù)類型
    property為實(shí)體屬性名，javaType為實(shí)體屬性數(shù)據(jù)類型-->
    <result column="course_name" jdbcType="VARCHAR" property="courseName" javaType="String"/>
    <result column="grade" jdbcType="SMALLINT" property="courseGrade" javaType="Integer"/>
    <result column="teacher_id" jdbcType="BIGINT" property="courseTeacherId" javaType="Long"/>
    <result column="add_time" jdbcType="TIMESTAMP" property="addTime" javaType="Date"/>
    <result column="mod_time" jdbcType="TIMESTAMP" property="modTime" javaType="Date"/>
  </resultMap>

 <!--select標(biāo)簽的 resultMap 指定resultMap標(biāo)簽中的id 值-->
 <!--去掉所有的別名 -->
  <select id="getCourseByPriMaryKey" resultMap="SqlMap">
    SELECT
    *
    FROM course WHERE id = #{id}
  </select>

執(zhí)行結(jié)果