基于Mybaits映射的一点心得(分享)

2019-11-26 11:00:16

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以前一直使用Hibernate，基本上没用过Mybatis，工作中需要做映射关系，简单的了解下Mybatis的映射。

两者相差不多都支持一对一，一对多，多对多，本章简单介绍一对一的使用以及注意点。

建表语句：

CREATE TABLE `bloc` (   `id` int(11) NOT NULL auto_increment,   `name` varchar(255) collate utf8_bin default NULL,   `company_id` int(11) default NULL,   `intro` varchar(255) collate utf8_bin default NULL,   PRIMARY KEY (`id`)  ) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8 COLLATE=utf8_bin;    INSERT INTO`bloc` (`id`, `name`, `company_id`, `intro`) VALUES ('1', '宏伟集团', '1', '跨国集团');

CREATE TABLE `company` (   `id` int(11) NOT NULL,   `name` varchar(255) collate utf8_bin default NULL,   `intro` varchar(255) collate utf8_bin default NULL,   PRIMARY KEY (`id`)  ) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin;  INSERT INTO company (`id`, `name`, `intro`) VALUES ('1', '', NULL);

形式一：子查询

JAVA代码：SqlSessionHelper.java

package com.demo.mybatis;  import java.io.IOException; import java.io.Reader;  import org.apache.ibatis.io.Resources; import org.apache.ibatis.session.SqlSessionFactory; import org.apache.ibatis.session.SqlSessionFactoryBuilder;  public class SqlSessionHelper {    public static SqlSessionFactory getSessionFactory() throws IOException{   SqlSessionFactory sessionFactory = null;   Reader reader = Resources.getResourceAsReader("configuration.xml");   try{    sessionFactory = new SqlSessionFactoryBuilder().build(reader);;   }catch(Exception ex){    ex.printStackTrace();   }   return sessionFactory;  } }

Test.java:

package com.demo.mybatis;  import java.util.List;  import org.apache.ibatis.session.SqlSession;  import mapper.BlocMapper; import model.Bloc;  public class Test {   /**   * @param args   */ public static void main(String[] args) {      try{    SqlSession sqlSession = SqlSessionHelper.getSessionFactory().openSession();    BlocMapper blocMapper = sqlSession.getMapper(BlocMapper.class);    List<Bloc> blocs = blocMapper.getBlocList("1");    for (Bloc bloc : blocs) {     System.out.println("companyName = "bloc.getCompany().getName());    }   }catch(Exception ex){    System.out.println(ex.getMessage());   }   }  }

mapper:

package mapper;  import java.util.List;  import model.Bloc;  public interface BlocMapper {   public List<Bloc> getBlocList(String name); }

package mapper;  public interface CompanyMapper {  }

model:

package model;  public class Bloc {   private Integer id;    private String name;    private String intro;    private Company company;   public Integer getId() {   return id;  }   public void setId(Integer id) {   this.id = id;  }   public String getName() {   return name;  }   public void setName(String name) {   this.name = name;  }   public String getIntro() {   return intro;  }   public void setIntro(String intro) {   this.intro = intro;  }   public Company getCompany() {   return company;  }   public void setCompany(Company company) {   this.company = company;  }     }

package model;  public class Company {   private Integer id;    private String name;    private Integer intro;   public Integer getId() {   return id;  }   public void setId(Integer id) {   this.id = id;  }   public String getName() {   return name;  }   public void setName(String name) {   this.name = name;  }   public Integer getIntro() {   return intro;  }   public void setIntro(Integer intro) {   this.intro = intro;  }     }

映射配置如下：（如果是一堆多的话“brandObject”改成实体List属性association 改成collection ）

<?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="mapper.CompanyMapper">    <resultMap id="BaseResultMap" type="model.Company" >   <id column="id" property="id" jdbcType="INTEGER" />   <result column="name" property="name" jdbcType="VARCHAR" />   <result column="intro" property="intro" jdbcType="VARCHAR" />  </resultMap>  <select id = "getCompanyInfo" parameterType="Integer" resultMap="BaseResultMap">   select * from company where id = #{id}  </select> </mapper>

<mapper namespace="mapper.BlocMapper"> <resultMap id="BaseResultMap" type="model.Bloc" >  <id column="id" property="id" jdbcType="INTEGER" />  <result column="name" property="name" jdbcType="VARCHAR" />  <result column="intro" property="intro" jdbcType="VARCHAR" />  <association column="company_id" property="company" select="mapper.CompanyMapper.getCompanyInfo">  </association> </resultMap>  <select id="getBlocList" parameterType="String" resultMap="BaseResultMap">  select * from bloc where name = #{name} </select></mapper>

column：表中的字段 property：实体当中的字段名 select：引入的另一个xxxMapper.xml的getCompanyInfo方法

这样当查询用的映射时检测到有select就会执行你引入的另一个Mapper的查询方法，查询条件是company_id= 查询方法的参数

运行结果

companyName =

形式二：关联查询

映射配置实体测试类一样:

<?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="mapper.BlocMapper">  <resultMap id="BaseResultMap" type="model.Bloc" >   <id column="id" property="id" jdbcType="INTEGER" />   <result column="name" property="name" jdbcType="VARCHAR" />   <result column="intro" property="intro" jdbcType="VARCHAR" />   <!-- 查询会有赋值紊乱问题 -->   <association column="company_id" property="company" resultMap = "mapper.CompanyMapper.BaseResultMap">   </association>   <!-- <association column="company_id" property="company" select="mapper.CompanyMapper.getCompanyInfo">   </association> -->  </resultMap>    <select id="getBlocList" parameterType="String" resultMap="BaseResultMap">   <!-- select * from bloc where name = #{name} -->   <!-- 查询会有赋值紊乱问题 -->   select * from bloc b left join company c on b.company_id = c.id where b.name = #{name}  </select> </mapper>

column：表中的字段 property：实体当中的字段名 resultMap :引入另一个Mapper的映射

值得注意的是：因为是嵌套映射，所以形式二在两个实体字段名一样的情况下会引发字段赋值的紊乱，例如两个实体都有name 当第一个实体name有值，第二个实体name没有值的时候，查询出来的结果是两个实体name都有值，且都是一样的为第一个实体的name值

运行结果为