【MyBatis从入门到入土】6

11 关联、集合

在MySQL中，我们经常需要处理一对多的情况，使用MyBatis中的高级映射就能够很好地处理这种情况。

11.1 数据库中创建测试表

创建学生表student和教师表teacher，并插入测试数据，在这里学生和老师是多对一的关系：

CREATE TABLE `teacher` (
`id` INT(10) NOT NULL,
`name` VARCHAR(30) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=INNODB DEFAULT CHARSET=utf8INSERT INTO teacher(`id`, `name`) VALUES (1, '秦老师');CREATE TABLE `student` (
`id` INT(10) NOT NULL,
`name` VARCHAR(30) DEFAULT NULL,
`tid` INT(10) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `fktid` (`tid`),
CONSTRAINT `fktid` FOREIGN KEY (`tid`) REFERENCES `teacher` (`id`)
) ENGINE=INNODB DEFAULT CHARSET=utf8INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('1', '小明', '1');
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('2', '小红', '1');
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('3', '小张', '1');
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('4', '小李', '1');
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('5', '小王', '1');

11.2 创建新的模块

在resources下创建配置文件mybatis-config.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configurationPUBLIC "-//mybatis.org//DTD Config 3.0//EN""http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration><properties resource="db.properties" /><settings><setting name="logImpl" value="STDOUT_LOGGING"/></settings><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><mappers><mapper resource="com/cap/dao/TeacherMapping.xml" ></mapper><mapper resource="com/cap/dao/StudentMapping.xml" ></mapper></mappers></configuration>

这里需要引用到db.properties，里面存放连接数据库的相关信息

driver=com.mysql.cj.jdbc.Driver
url=jdbc:mysql://localhost:3306/mybatis
username=root
password=mysql

编写工具类，通过SqlSessionFactoryBuilder读取mybatis-config.xml配置文件后，构建SqlSessionFactory，再编写一个获取·SqlSession·的方法

public class MyBatisUtils {private static SqlSessionFactory sqlSessionFactory;static {try {String resource = "mybatis-config.xml";InputStream is = Resources.getResourceAsStream(resource);sqlSessionFactory = new SqlSessionFactoryBuilder().build(is);} catch (IOException e) {e.printStackTrace();}}public static SqlSession getSqlSession(){return sqlSessionFactory.openSession();}
}

创建实体类Student和Teacher

public class Student {private int id;private String name;//关联老师的idprivate Teacher teacher;
}

public class Teacher {private int id;private String name;
}

编写dao层的相关接口

public interface TeacherMapping {}

public interface StudentMapping {}

在resources下创建同样的层级包结构，编写相应的Mapping.xml文件

<?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="com.cap.dao.StudentMapping"></mapper>

<?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="com.cap.dao.TeacherMapping"></mapper>

整个的层级结构如下
6. 简单测试一下，利用注解在TeacherMapping接口编写一个通过id查询Teacher的方法

public interface TeacherMapping {@Select("SELECT * FROM teacher WHERE id = #{id}")Teacher getTeacherById(int id);
}

编写测试方法

public class Test {@org.junit.Testpublic void test(){SqlSession sqlSession = MyBatisUtils.getSqlSession();TeacherMapping mapper = sqlSession.getMapper(TeacherMapping.class);Teacher teacher = mapper.getTeacherById(1);System.out.println(teacher);sqlSession.close();}
}/**Teacher{id=1, name='秦老师'}*/

11.3 关联的嵌套 Select 查询

现在我们需要查询所有学生的信息，并且还要查询对应的老师的信息

public interface StudentMapping {/*** <p>获取学生的信息，以及对应的老师的信息</p>* @return*/List<Student> getStudent();
}

查询出来的结大概是这样子：
首先我们来讲解一下第一个思路——嵌套SELECT查询：
先查询所有学生的信息，再根据学生的tid，去寻找对应的老师

    <!--思路：1. 查询所有的学生信息2. 根据查询出来的学生的id，寻找对应的老师--><select id="getStudent" resultMap="StudentTeacher">SELECT * FROM student;</select><resultMap id="StudentTeacher" type="student"><result column="id" property="id" /><result column="name" property="name" /><!-- 复杂属性需要单独处理对象：association集合：collection--><association property="teacher" column="tid" javaType="teacher" select="getTeacher" /></resultMap><select id="getTeacher" resultType="teacher">SELECT * FROM teacher WHERE id = #{tid};</select>

resultMap结果集映射其他都能明白，关键在于如何处理Student中含有的Teacher对象，这里我们需要使用<association>标签来处理，它将查询得到的字段tid对应到Teacher对象上，再通过引用另一个SELECT查询，得到该tid所对应的Teacher对象。

注意type和javaType，他们用了别名student和teacher，分别代表实体类Student和Teacher，这个可以在mybatis-config.xml中配置

    <!-- 给pojo包取别名，默认为类名（首字母变小写，其他不变） --><typeAliases><package name="com.cap.pojo"/></typeAliases>

编写一个测试方法

    @org.junit.Testpublic void test1(){SqlSession sqlSession = MyBatisUtils.getSqlSession();StudentMapping mapper = sqlSession.getMapper(StudentMapping.class);List<Student> list = mapper.getStudent();for(Student student : list){System.out.println(student);}sqlSession.close();}

查询结果如下：

Student{id=1, name='小明', teacher=Teacher{id=1, name='秦老师'}}
Student{id=2, name='小红', teacher=Teacher{id=1, name='秦老师'}}
Student{id=3, name='小张', teacher=Teacher{id=1, name='秦老师'}}
Student{id=4, name='小李', teacher=Teacher{id=1, name='秦老师'}}
Student{id=5, name='小王', teacher=Teacher{id=1, name='秦老师'}}

11.4 关联的嵌套结果映射

    <resultMap id="StudentTeacher" type="student" ><result property="id" column="sid" /><result property="name" column="sname" /><association property="teacher" javaType="teacher"><result property="name" column="tname" /></association></resultMap>

测试方法得到：

Student{id=1, name='小明', teacher=Teacher{id=0, name='秦老师'}}
Student{id=2, name='小红', teacher=Teacher{id=0, name='秦老师'}}
Student{id=3, name='小张', teacher=Teacher{id=0, name='秦老师'}}
Student{id=4, name='小李', teacher=Teacher{id=0, name='秦老师'}}
Student{id=5, name='小王', teacher=Teacher{id=0, name='秦老师'}}

11.5 集合的嵌套结果映射

集合和关联非常相似，也是有嵌套 Select 查询和嵌套结果映射，这里只举嵌套结果映射的例子。
首先我们先修改实体类

public class Student {private int id;private String name;private int tid;}

public class Teacher {private int id;private String name;private List<Student> students;

在接口TeacherMapping中添加一个方法，用于查询指定id的Teacher

public interface TeacherMapping {Teacher getTeacherById(@Param("tid") int tid);
}

<mapper namespace="com.cap.dao.TeacherMapping"><!-- 按结果嵌套 --><select id="getTeacherById" resultMap="TeacherStudent">SELECTs.id   sid,s.name sname,t.name tname,t.id   tidFROM student s, teacher tWHERE s.tid = t.id AND t.id = #{tid};</select><resultMap id="TeacherStudent" type="teacher"><result property="id" column="tid" /><result property="name" column="tname" /><!--复杂的属性，我们需要单独处理对象：association集合：collectionjavaType : 指定属性的类型集合中的泛型信息，使用ofType获取--><collection property="students" ofType="com.cap.pojo.Student"><result property="id" column="sid" /><result property="name" column="sname" /><result property="tid" column="tid" /></collection></resultMap></mapper>

ofType：用于指定集合中的类型