基于Mybatis映射的一点心得(分享)

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

companyName = 宏伟集团

显然运行结果不是我们想要的结果

以上简单的Demo希望能帮助初学Mybatis童鞋！！

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