关于Mybaits映射一点心得

本人的第一篇手写博客 -日期2017-11-06 

在写博客之前一直使用手写笔记！！！

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！！

最后祝大家工作愉快