Mybatis一对多双向关联

标签：mybatis

在弄Mybatis的一对多关联的时候，不知道有没有遇到这样的问题，本来数据库中对应有多条数据，然而关联出来却只有一条数据，比如数据库中有个班级表和学生表，数据库中一个班级中对应了多个学生，但使用Mybatis做一对多关联的时候，查询出来的却只有一条。如果出现这样的问题，那么就是两张数据表中的主键重名了，导致在关联查询时分不清到底是那一张表了，因此有关联的数据表时，主键id不要重名，一对多关联实现如下：

数据表：

CREATE TABLE tab_class (
	c_id INT PRIMARY KEY AUTO_INCREMENT,
	c_name VARCHAR(50)
)

CREATE TABLE tab_student(
	s_id INT PRIMARY KEY AUTO_INCREMENT,
	s_name VARCHAR(50),
	c_id INT
)

package com.tenghu.mybatis.model;

import java.util.List;

/**
 * 班级实体类
 * @author Arvin
 *
 */
public class Classes {
	private int cId;
	private String cName;
	private List<Student> studentList;
	public int getcId() {
		return cId;
	}
	public void setcId(int cId) {
		this.cId = cId;
	}
	public String getcName() {
		return cName;
	}
	public void setcName(String cName) {
		this.cName = cName;
	}
	public List<Student> getStudentList() {
		return studentList;
	}
	public void setStudentList(List<Student> studentList) {
		this.studentList = studentList;
	}
	
}

package com.tenghu.mybatis.model;
/**
 * 学生实体
 * @author Arvin
 *
 */
public class Student {
	private int sId;
	private String sName;
	private Classes classes;
	public int getsId() {
		return sId;
	}
	public void setsId(int sId) {
		this.sId = sId;
	}
	public String getsName() {
		return sName;
	}
	public void setsName(String sName) {
		this.sName = sName;
	}
	public Classes getClasses() {
		return classes;
	}
	public void setClasses(Classes classes) {
		this.classes = classes;
	}
	
}

映射文件：

<?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="com.tenghu.mybatis.model.xml.ClassesMapper">
	<resultMap type="Classes" id="getClasses">
		<id property="cId" column="c_id"/>
		<result property="cName" column="c_name"/>
		
		<!-- 映射到一个集合  表示多的一方-->
		<collection property="studentList" ofType="Student">
			<id property="sId" column="s_id"/>
			<result property="sName" column="s_name"/>
		</collection>
	</resultMap>
	
	<!-- 根据班级id查询 -->
	<select id="queryClassesById" resultMap="getClasses" parameterType="int">
		SELECT * FROM tab_class c,tab_student s WHERE c.c_id=s.c_id AND c.c_id=#{id}
	</select>
</mapper>

<?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="com.tenghu.mybatis.model.xml.StudentMapper">
	<resultMap type="Student" id="getStudent">
		<id property="sId" column="s_id"/>
		<result property="sName" column="s_name"/>
		
		<!-- 表示一的一方 -->
		<association property="classes" javaType="Classes">
			<id property="cId" column="c_id"/>
			<result property="cName" column="c_name"/>
		</association>
	</resultMap>
	
	<!-- 根据学生id查询 -->
	<select id="queryStudentById" parameterType="int" resultMap="getStudent">
		SELECT * FROM tab_class c,tab_student s WHERE c.c_id=s.c_id AND s.s_id=#{id}
	</select>
</mapper>

package com.tenghu.mybatis.util;

import java.io.IOException;
import java.io.InputStream;
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;

/**
 * Mybatis工具类
 * @author Arvin
 *
 */
public class MybatisUtil {
	private MybatisUtil(){}
	//声明SqlSessionFactory对象
	private static SqlSessionFactory sqlSessionFactory;
	
	static{
		//mybatis主配置文件
		String mybatisResource="mybatis-config.xml";
		//数据库属性文件
		String databaseProps="jdbc.properties";
		
		try {
			//获取mybatis住配置文件资源
			InputStream inputStream=Resources.getResourceAsStream(mybatisResource);
			//创建属性对象
			Properties props=new Properties();
			//加数据库属性配置文件资源
			props.load(Resources.getResourceAsStream(databaseProps));
			//创建SqlSession工厂对象
			sqlSessionFactory=new SqlSessionFactoryBuilder().build(inputStream, props);
		} catch (IOException e) {
			e.printStackTrace();
		}
		
	}
	
	/**
	 * 开启SqlSession,不自动提交
	 * @return
	 */
	public static SqlSession openSession(){
		return sqlSessionFactory.openSession();
	}
	
	/**
	 * 开启SqlSession，自动提交
	 * @param isAuto
	 * @return
	 */
	public static SqlSession openSessionAuto(){
		return sqlSessionFactory.openSession(true);
	}
	
	/**
	 * 关闭SqlSession对象
	 * @param sqlSession
	 */
	public static void closeSqlSession(SqlSession sqlSession){
		if(null!=sqlSession){
			sqlSession.close();
		}
	}
}

package com.tenghu.mybatis.test;

import org.apache.ibatis.session.SqlSession;
import org.junit.Test;

import com.tenghu.mybatis.model.Student;
import com.tenghu.mybatis.util.MybatisUtil;
/**
 * 学生实体测试类
 * @author Arvin
 *
 */
public class StudentTest {
	private String namespace="com.tenghu.mybatis.model.xml.StudentMapper.";
	
	@Test
	public void testQueryStudentById(){
		//获取SqlSession
		SqlSession sqlSession=MybatisUtil.openSession();
		try {
			//查询
			Student student=sqlSession.selectOne(namespace+"queryStudentById", 1);
			//输出
			System.out.println(student.getsName());
			System.out.println(student.getClasses().getcName());
		} catch (Exception e) {
			e.printStackTrace();
		}finally{
			//关闭SqlSession
			MybatisUtil.closeSqlSession(sqlSession);
		}
		
	}
}

package com.tenghu.mybatis.test;

import org.apache.ibatis.session.SqlSession;
import org.junit.Test;

import com.tenghu.mybatis.model.Classes;
import com.tenghu.mybatis.model.Student;
import com.tenghu.mybatis.util.MybatisUtil;

/**
 * 班级测试类
 * @author Arvin
 *
 */
public class ClassesTest {
	private String namespace="com.tenghu.mybatis.model.xml.ClassesMapper.";
	
	@Test
	public void testQueryClassById(){
		//获取SqlSession
		SqlSession sqlSession=MybatisUtil.openSession();
		try {
			//查询
			Classes classes=sqlSession.selectOne(namespace+"queryClassesById",1);
			//输出
			System.out.println(classes.getcName());
			for (Student student : classes.getStudentList()) {
				System.out.println(student.getsName());
			}
		} catch (Exception e) {
			e.printStackTrace();
		}finally{
			//关闭SqlSession
			MybatisUtil.closeSqlSession(sqlSession);
		}
	}
}

Mybatis一对多双向关联

标签：mybatis

原文地址：http://blog.csdn.net/u011109042/article/details/42503145