mybatis多对一查询-优快云博客

本文链接：https://blog.youkuaiyun.com/puhaitao1987/article/details/23446905

本文介绍了一个使用MyBatis进行关联查询的例子，通过Blog和Person两个表的联合查询来展示如何在一个实体中获取另一个相关实体的数据。具体实现包括定义DAO接口、XML映射文件及对应的POJO类。

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1. BlogDao.java

package pht.mybatis.dao;

import java.util.List;

import pht.mybatis.pojo.Blog;

public interface BlogDao {

	public List getBlog(int id);//查询多个数据时候，返回类型应该是List
	public void addBlog(Blog b);
}

2. BlogDao.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="pht.mybatis.dao.BlogDao">
<resultMap type="Blog" id="blog">
<association property="user" select="getUser" column="user" javaType="Person"></association>
</resultMap>
  <select id="getBlog" resultMap="blog">
    select * from blog where user = #{bid}
  </select>
  <select id="getUser" resultType="Person">
    select * from person where personid=#{bid}
  </select>
</mapper>

3.pojo

package pht.mybatis.pojo;

public class Blog {

	private int bid;
	private String title;
	private Person user; //这里体现了实体类的关系
	public int getBid() {
		return bid;
	}
	public void setBid(int bid) {
		this.bid = bid;
	}
	public String getTitle() {
		return title;
	}
	public void setTitle(String title) {
		this.title = title;
	}
	public Person getUser() {
		return user;
	}
	public void setUser(Person user) {
		this.user = user;
	}
}

4. 测试类

  
package pht.mybatis.test;

import java.io.IOException;
import java.io.Reader;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Properties;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

import pht.mybatis.dao.*;
import pht.mybatis.pojo.*;
import pht.mybatis.sessionfactory.Sessionfactory;

/**
 * myBatis数据库连接测试
 * 
 */
public class MyBatisTest {
    
	private static SqlSession sqlSession;
	public static void main(String[] args) {
		sqlSession=Sessionfactory.SQLSESSIONFACTORY.getSessionFactory().openSession();
		MyBatisTest test=new MyBatisTest();
		try{
		  BlogDao blogdao=sqlSession.getMapper(BlogDao.class);
		  List blogs=blogdao.getBlog(4);
		for(Iterator it=blogs.iterator();it.hasNext();){
			Blog blog=(Blog) it.next();
			System.out.println(blog.getTitle()+":"+blog.getUser().getName());
		}
		  
		}catch (Exception e) {
			// TODO: handle exception
		}finally{
			sqlSession.close();
		}
		
		
	}
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