mybatis配置关联映射

在做一个Demo的时候，配置mybatis的关联映射，想分享一下。

数据库如下：

CREATE TABLE TB_CLAZZ(id INT PRIMARY KEY AUTO_INCREMENT,CODE VARCHAR (18));INSERT INTO TB_CLAZZ(CODE) VALUES('j1601');INSERT INTO TB_CLAZZ(CODE) VALUES('j1602');CREATE TABLE TB_STUDENT(id INT PRIMARY KEY AUTO_INCREMENT,NAME VARCHAR(18),sex CHAR(3),age INT,clazz_id INT,FOREIGN KEY(clazz_id) REFERENCES TB_CLAZZ(id));INSERT INTO TB_STUDENT(NAME,sex,age,clazz_id) VALUES('jack','男',22,1);INSERT INTO TB_STUDENT(NAME,sex,age,clazz_id) VALUES('rose','女',18,1);INSERT INTO TB_STUDENT(NAME,sex,age,clazz_id) VALUES('tom','男',25,1);INSERT INTO TB_STUDENT(NAME,sex,age,clazz_id) VALUES('mary','女',20,1);

就是这么个东西，先导包。如图：

然后配置mybatis的核心配置文件，sqlMapConfig.xml.

sqlMapConfig.xml:

<?xml version="1.0" encoding="UTF-8" ?>  <!DOCTYPE configuration    PUBLIC "-//mybatis.org//DTD Config 3.0//EN"    "http://mybatis.org/dtd/mybatis-3-config.dtd">  <configuration>  <properties resource="jdbc.properties"/><typeAliases><!-- <typeAlias type="com.liujie.pojo.User" alias="User"/>--><package name="com.liujie.pojo"/></typeAliases>    <!-- 对事务的管理和连接池的配置 -->      <environments default="development">          <environment id="development">              <transactionManager type="JDBC" />              <dataSource type="POOLED">                  <property name="driver" value="${jdbc.driver}" />                  <property name="url" value="jdbc:mysql://localhost:3306/mybatis2?characterEncoding=utf-8" />                  <property name="username" value="root" />                  <property name="password" value="123" />              </dataSource>          </environment>      </environments>            <!-- mapping 文件路径配置 -->      <mappers>        <mapper resource="com/liujie/mapper/UserMapper.xml" />      </mappers>  </configuration>

就这个文件有个小地方坑了我半个小时，因为始终报错could not find source ....../UserMapper.xml。是因为配置的路径是全名，把工程名还有src都带进去了，这里的时候可以按住control看能不能过去，不能就肯定路径有问题，在这里默认是classpath路径也就是src路径的。ps：我用的是eclipse，如果用的是IDEA，那么要注意，IDEA不会编译src目录的java的xml文件，所以要将IDEA maven项目中src源代码下的xml等资源文件编译进classes文件夹。

然后写pojo，写工具类。

SessionUtils.java：

package com.liujie.junit;import java.io.IOException;import java.io.InputStream;import org.apache.ibatis.io.Resources;import org.apache.ibatis.session.SqlSession;import org.apache.ibatis.session.SqlSessionFactory;import org.apache.ibatis.session.SqlSessionFactoryBuilder;public class SessionUtils {private static SqlSessionFactory sqlSessionFactory=null;static{try {InputStream in =Resources.getResourceAsStream("sqlMapConfig.xml");sqlSessionFactory=new SqlSessionFactoryBuilder().build(in);} catch (IOException e) {e.printStackTrace();}}public static SqlSession getSqlSession(){return sqlSessionFactory.openSession();}public static SqlSessionFactory getSqlSessionFactory(){return sqlSessionFactory;}}

Clazz.java:

package com.liujie.pojo;public class Clazz {private Integer id;private String code;public Integer getId() {return id;}public void setId(Integer id) {this.id = id;}public String getCode() {return code;}public void setCode(String code) {this.code = code;}}

Student.java:

package com.liujie.pojo;public class Student {private Integer id;private String name;private String sex;private String age;private Clazz clazz;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 getSex() {return sex;}public void setSex(String sex) {this.sex = sex;}public String getAge() {return age;}public void setAge(String age) {this.age = age;}public Clazz getClazz() {return clazz;}public void setClazz(Clazz clazz) {this.clazz = clazz;}@Overridepublic String toString() {return "Student [id=" + id + ", name=" + name + ", sex=" + sex + ", age=" + age + ", clazz=" + clazz + "]";}}

然后写UserMapper.xml:

<?xml version="1.0" encoding="UTF-8" ?><!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd"><mapper namespace="test"><resultMap type="com.liujie.pojo.Student" id="studentResultMap"><id property="name" column="name"/><result property="sex" column="sex"/><result property="age" column="age"/><association property="clazz" column="clazz_id" javaType="com.liujie.pojo.Clazz" select="selectClazzWithId"/></resultMap><select id="selectClazzWithId" resultType="Clazz">select * from TB_CLAZZ where id=#{id}</select><select id="selectStudent" resultMap="studentResultMap">select * from TB_STUDENT</select></mapper>

这里的association就是关联映射，还有一个是collection。里面的select属性表示执行一条查询语句，将查询到的数据封装property所代表的类型对象当中。上面的selectClazzWithId就是执行一台哦SQL语句，将学生的clazz_id作为参数查询对应的班级信息。