mybatis级联，一对一

宽为限　紧用功　工夫到　滞塞通

mybatis中对象与对象之间的关系与hibernate不同，hibernate可以直接通过配置，一步到位将对象关联到对应的对象里。 mybatis需要实现对象关联主要分这几步骤： 1、对象里包含要关联的对象； 2、额外提供一个属性存关联对象的id； 3、xml文件里配置association(与被关联的Mapper接口里的查询方法对应)； 4、被关联的Mapper接口文件中提供对应的查询本对象方法(与association对应)。 mybatis中一对一级联在resultMap里是用 association属性配置的 在这里以学生（Student）和学生证（StudentSelfcard）之间的一对一关系来做案例，我们忽略次要的东西直接看主要的，这里Student里包含StudentSelfcard，然后我们也添加了一个studentSelfcardId属性，下面是Student类主要代码

... /** * 实际使用时的对象，mybatis通过studentSelfcardId关联 */ private StudentSelfcard studentSelfcard; public StudentSelfcard getStudentSelfcard() { return studentSelfcard; } public void setStudentSelfcard(StudentSelfcard studentSelfcard) { this.studentSelfcard = studentSelfcard; } /** * 额外提供的属性，mybatis关联用 */ private int studentSelfcardId; public int getStudentSelfcardId() { return studentSelfcardId; } public void setStudentSelfcardId(int studentSelfcardId) { this.studentSelfcardId = studentSelfcardId; } ...

这里先不说为什么要多一个额外属性，接下来我们看看studentMapper.xml文件里的配置

... <mapper namespace="com.learn.chapter4.mapper.StudentMapper"> <resultMap id="studentMap" type="com.learn.chapter4.po.Student"> <id property="id" column="id" /> <result property="cnname" column="cnname" /> <result property="sex" column="sex" jdbcType="INTEGER" javaType="com.learn.chapter4.enums.SexEnum" /> <result property="studentSelfcardId" column="student_selfcard_id" /> <!-- 本条语句会通过select的内容去它对应的Mapper.xml文件里去找到对应id的select方法，并得到方法返回的对象 --> <association property="studentSelfcard" column="id" select="com.learn.chapter4.mapper.StudentSelfcardMapper.findStudentSelfcardByStudentId" /> </resultMap> <select id="getStudent" parameterType="int" resultMap="studentMap"> select * from t_student where id = #{id} </select> <!-- 在StudentSelfcardMapper.xml配置中通过本条语句找到关联的Student对象 --> <select id="findStudentByStudentSelfcardId" parameterType="int" resultMap="studentMap"> select * from t_student where student_selfcard_id = #{studentSelfcardId} </select> </mapper>

ok，有疑问的话很正常，先存着。接下来我们看它关联的对象实体类及对应的xml配置文件，先是java文件

/** * 实际使用时的对象，mybatis通过studentId关联 */ private Student student; public Student getStudent() { return student; } public void setStudent(Student student) { this.student = student; } /** * 额外提供的属性，mybatis关联用 */ private int studentId; public int getStudentId() { return studentId; } public void setStudentId(int studentId) { this.studentId = studentId; }

嗯，还是一个样，然后看studentSelfcardMapper.xml 文件

... <mapper namespace="com.learn.chapter4.mapper.StudentSelfcardMapper"> <resultMap id="studentSelfcardMap" type="com.learn.chapter4.po.StudentSelfcard"> <id property="id" column="id" /> <result property="studentId" column="student_id" /> <result property="native_" column="native" /> <result property="issueDate" column="issue_date" /> <result property="endDate" column="end_date" /> <result property="note" column="note" /> <!-- 本条语句会通过select的内容去它对应的Mapper.xml文件里去找到对应id的select方法，并得到方法返回的对象 --> <association property="student" column="id" select="com.learn.chapter4.mapper.StudentMapper.findStudentByStudentSelfcardId" /> </resultMap> <select id="getStudentSelfcard" parameterType="int" resultMap="studentSelfcardMap"> select * from t_student_selfcard where id = #{id} </select> <!-- 在StudentMapper.xml配置中通过本条语句找到关联的StudentSelfcard对象 --> <select id="findStudentSelfcardByStudentId" parameterType="int" resultMap="studentSelfcardMap"> select * from t_student_selfcard where student_id = #{studentId} </select> </mapper>

到这里估计你已经发现，实体类之间它们除了各自的属性不一样之外，其关联关系配置的方式都是一样的。都有一个额外的要关联对象的id属性，那么它有什么用呢？

看图，很明显 StudentMapper.xml 

StudentSelfcardMapper.xml 

没错它们是专门为了找到对应关联对象的具体值而存在的，通过这个额外属性mybatis查到了关联的值，然后再将它封装成对象，放到主对象中。这里怎么查由我们自己定义，最后的功能hibernate一样，不过hibernate是不用自己定义查询方法和他们的具体关联，这点比较方便。 association 通过select值来找到对应Mapper的select。

我们再来看看Mapper接口文件

public interface StudentMapper { public Student getStudent(int id); /** * 关联专用 * @param selfcardId * @return */ public Student findStudentByStudentSelfcardId(int selfcardId); } public interface StudentSelfcardMapper { public StudentSelfcard getStudentSelfcard(int id); /** * 关联专用 * @param studentId * @return */ public StudentSelfcard findStudentSelfcardByStudentId(int studentId); }

最后我们看测试代码及测试效果

import org.apache.ibatis.session.SqlSession; import com.learn.chapter4.mapper.StudentMapper; import com.learn.chapter4.mapper.StudentSelfcardMapper; import com.learn.chapter4.po.Student; import com.learn.chapter4.po.StudentSelfcard; import com.learn.chapter4.util.SqlSessionFactoryUtil; /** * @Description: mybatis一对一关联测试 * @author zhougm * @date 2017年8月5日，下午10:57:06 * */ public class AssociationTest { public static void main(String[] args) { SqlSession sqlSession = SqlSessionFactoryUtil.openSqlSession(); // 学生类关联学生证类 StudentMapper studentMapper = sqlSession.getMapper(StudentMapper.class); Student student = studentMapper.getStudent(214840101); System.out.println(student.getStudentSelfcard().getStudentId()); // 学生证类关联学生类 StudentSelfcardMapper studentSelfcardMapper = sqlSession.getMapper(StudentSelfcardMapper.class); StudentSelfcard studentSelfcard = studentSelfcardMapper.getStudentSelfcard(1); System.out.println(studentSelfcard.getStudent().getStudentSelfcardId()); } }

运行结果 

数据库信息 t_student

t_student_selfcard 

ok! mybatis级联，一对一就这么简单