Mybatis多对一案例实现
1. 导入lombok
在进行测试之前，先把环境搭配好，把Lombok导入项目，Lombok jar包导入项目代码如下：
<dependencies>
<!-- https://mvnrepository.com/artifact/org.projectlombok/lombok -->
<dependency>
<groupId>org.projectlombok</groupId>
<artifactId>lombok</artifactId>
<version>1.18.10</version>
</dependency>
</dependencies>
具体代码可以查看网址：https://mvnrepository.com/artifact/org.projectlombok/lombok/1.18.20
2. 新建实体类Teacher、Student
import lombok.Data;
@Data
public class Student {
private int id;
private String name;
//多个学生需要关联一个老师！
private Teacher teacher;
}
import lombok.Data;
@Data
public class Teacher {
private int id;
private String name;
}
3. 建立Mapper接口
import com.mb.pojo.Student;
import java.util.List;
public interface StudentMapper {
//查询所有的学生信息，以及对应的老师的信息
public List<Student> getStudent();
public List<Student> getStudent2();
}
import com.mb.pojo.Teacher;
import org.apache.ibatis.annotations.Param;
import org.apache.ibatis.annotations.Select;
public interface TeacherMapper {
}
4.建立Mapper.xml文件
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<!--configuration核心配置文件-->
<mapper namespace="com.mb.mapper.StudentMapper"/>
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<!--configuration核心配置文件-->
<mapper namespace="com.mb.mapper.TeacherMapper"/>
5. 在核心配置文件mybatis-config.xml中注册mapper接口
<!--注册-->
<mappers>
<mapper class="com.mb.mapper.TeacherMapper"/>
<mapper class="com.mb.mapper.StudentMapper"/>
</mappers>
6. 开始测试
1. 编写工具类MybatisUtils
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 java.io.IOException;
import java.io.InputStream;
public class MybatisUtils {
private static SqlSessionFactory sqlSessionFactory;
static {
try {
String resource = "mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
} catch (IOException e) {
e.printStackTrace();
}
}
public static SqlSession getSqlSession() {
return sqlSessionFactory.openSession();
}
}
2. 在Mapper.xml文件编写查询语句
a. 按照查询嵌套处理
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<!--configuration核心配置文件-->
<mapper namespace="com.mb.mapper.StudentMapper">
<!--
思路：
1.查询所有的学生信息
2.根据查询出来的学生的tid，寻找对应的老师！
-->
<select id="getStudent" resultMap="StudentTeacher">
select * from mybatis.student;
</select>
<resultMap id="StudentTeacher" type="Student">
<!--
property对应数据库的名称
column对应实体类中的名称
-->
<result property="id" column="id"/>
<result property="name" column="name"/>
<association property="teacher" column="tid" javaType="Teacher" select="getTeacher"/>
</resultMap>
<!-- 查询老师的代码, 下面#{tid}对应colum中的tid,注#{}里的名字可以随便赋值，但一般和数据库的相同-->
<select id="getTeacher" resultType="Teacher">
select * from teacher where id = #{tid};
</select>
</mapper>
b.按照结果嵌套处理
<!--
方法二：按照结果嵌套处理
给数据库取别名
-->
<select id="getStudent2" resultMap="StudentTeacher2">
select s.id sid, s.name sname, t.name tname
from student s,
teacher t
where s.tid = t.id;
</select>
<!-- 映射-->
<resultMap id="StudentTeacher2" type="Student">
<result property="id" column="sid"/>
<result property="name" column="sname"/>
<association property="teacher" javaType="Teacher">
<result property="name" column="tname"/>
</association>
</resultMap>
3. 编写核心配置文件mybatis-config.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核心配置文件-->
<configuration>
<!--引入外部配置文件-->
<properties resource="db.properties"/>
<settings>
<!--标准的日志工厂实现-->
<setting name="logImpl" value="STDOUT_LOGGING"/>
</settings>
<!--给实体类取别名-->
<typeAliases>
<typeAlias type="com.mb.pojo.Teacher" alias="Teacher"/>
<typeAlias type="com.mb.pojo.Student" alias="Student"/>
</typeAliases>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC"/>
<dataSource type="POOLED">
<property name="driver" value="${driver}"/>
<property name="url" value="${url}"/>
<property name="username" value="${username}"/>
<property name="password" value="${password}"/>
</dataSource>
</environment>
</environments>
<!--注册-->
<mappers>
<mapper class="com.mb.mapper.TeacherMapper"/>
<mapper class="com.mb.mapper.StudentMapper"/>
</mappers>
</configuration>
4. 创建测试类MyTest开始测试
import com.mb.mapper.StudentMapper;
import com.mb.mapper.TeacherMapper;
import com.mb.pojo.Student;
import com.mb.pojo.Teacher;
import com.mb.utils.MybatisUtils;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;
import java.util.List;
public class MyTest {
@Test
public void getStudent() {
SqlSession sqlSession = MybatisUtils.getSqlSession();
StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
List<Student> studentList = mapper.getStudent();
for (Student student : studentList) {
System.out.println(student);
}
sqlSession.close();
}
@Test
public void getStudent2() {
SqlSession sqlSession = MybatisUtils.getSqlSession();
StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
List<Student> studentList2 = mapper.getStudent2();
for (Student student2 : studentList2) {
System.out.println(student2);
}
sqlSession.close();
}
}
测试成功！
7. 小结
- 关联- association(复杂类型)。
- 注意：
复杂类型需要在后面给tid它们赋值一个类型，使用JavaType来赋值
select：用来表示嵌套查询（数据库是列，而封装的是一个对象，
想要查询数据库中对应的tid赋值给teacher需要使用到嵌套查询。 - JavaType:用来指定实体类中对象属性的类型。