ResultMap结果映射解决复杂属性
之前我们提到了用resultMap解决数据表中字段名与bean属性名不一致的问题，这是resultMap的一种简单实现。下面我们来看如何利用ResultMap来解决更复杂的属性问题
场景：当我们需要联查两张表的时候，通常会在sql层面对两个表进行外键关联。那么设置了外键的从表对应的实体Bean中就需要定义一个对应主表的实例对象。
多对一关系处理
按照查询嵌套
示例：
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// 学生表 从表 public class Student { private int id; private String name; private Teacher teacher; // 定义主表对应bean实例 // 教师表 主表 public class Teacher { private int id; private String name; //teacher接口中定义方法 @Select ( "select name from teacher where id = #{tid}" ) Teacher getTeacherById( @Param ( "tid" ) int id); // student接口中定义方法 List<Student> getStudent(); |
sql：
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-- 可以直接利用多表联查sql 但是最终无法正确输出teacher类信息 select * from student s join teacher t on s.tid = t.id; -- 将上面的sql拆开成两句 先查询到学生信息 用学生的tid字段对应教师的id查询教师 select * from student; select * from teacher where id = #{tid} |
打印日志：
Setting autocommit to false on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@79da8dc5]
==> Preparing: select * from student;
==> Parameters:
<== Columns: id, name, tid
<== Row: 1, 小子三, 1
<== Row: 2, 小子四, 1
<== Row: 3, 小子五, 1
<== Row: 4, 小子六, 1
<== Row: 5, 小子七, 1
<== Row: 6, 小子八, 1
<== Total: 6
Student{id=1, name='小子三', teacher=null}
Student{id=2, name='小子四', teacher=null}
Student{id=3, name='小子五', teacher=null}
Student{id=4, name='小子六', teacher=null}
Student{id=5, name='小子七', teacher=null}
Student{id=6, name='小子八', teacher=null}
可以很清晰的看出sql一共只执行了一条，而根据学生表tid字段查询教师信息的sql根本就没有运行
下面我们对 select * from teacher where id = #{tid} 这句sql返回的结果配置，利用resultMap为其配置合理的结果集来接收查询到的结果
resultMap及sql配置如下：
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< resultMap id = "studentTeacher" type = "student" > <!-- 普通映射 --> < result column = "id" property = "id" /> < result column = "name" property = "name" /> <!-- 复杂映射 association对象 collection集合 --> < association property = "teacher" column = "tid" javaType = "teacher" select = "getTeacherById" /> <!-- student表中tid字段对应实体类student中的teacher属性 对应程序中的teacher类型 执行select语句 --> </ resultMap > |
最终打印结果如下：
Setting autocommit to false on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@79da8dc5]
// 执行sql1
==> Preparing: select * from student;
==> Parameters:
<== Columns: id, name, tid
<== Row: 1, 小子三, 1
// 执行sql1
====> Preparing: select * from teacher where id = ?
====> Parameters: 1(Integer)
<==== Columns: id, name
<==== Row: 1, 秦老师
<==== Total: 1
<== Row: 2, 小子四, 1
<== Row: 3, 小子五, 1
<== Row: 4, 小子六, 1
<== Row: 5, 小子七, 1
<== Row: 6, 小子八, 1
<== Total: 6
// 最终teacher也以对象的形式打印出来
Student{id=1, name='小子三', teacher=Teacher{id=1, name='秦老师'}}
Student{id=2, name='小子四', teacher=Teacher{id=1, name='秦老师'}}
Student{id=3, name='小子五', teacher=Teacher{id=1, name='秦老师'}}
Student{id=4, name='小子六', teacher=Teacher{id=1, name='秦老师'}}
Student{id=5, name='小子七', teacher=Teacher{id=1, name='秦老师'}}
Student{id=6, name='小子八', teacher=Teacher{id=1, name='秦老师'}}
按照结果嵌套
studentMapper.xml配置resultMap如下：
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<!-- 按照结果嵌套 --> < select id = "getStudent2" resultMap = "studentTeacher2" > select s.id sid,s.name sname,t.id tid,t.name tname from student s join teacher t on s.tid = t.id; </ select > < resultMap id = "studentTeacher2" type = "student" > < result column = "sid" property = "id" /> < result column = "sname" property = "name" /> < association property = "teacher" javaType = "teacher" > < result column = "tid" property = "id" /> < result property = "name" column = "tname" /> </ association > </ resultMap > |
如果采用结果嵌套配置，此时不论bean类属性是否是基本类型都需要用result进行映射，否则输出结果就会采用默认值
一对多关系处理
一对多关系处理，依旧以上述老师和学生为例。但是实体类需要修改，如下：
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public class Teacher { private int id; private String name; private List<Student> student; public class Student { private int id; private String name; private int tid; |
一个老师下对应多个学生，所以我们定义一个集合用于存储学生
下面尝试获取指定老师下的所有学生信息及该老师信息
按照结果嵌套
程序：
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// TeacherMapper // 指定老师下面的所有学生 Teacher getTeaById( @Param ( "teaId" ) int id); |
resultMap配置
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< select id = "getTeaById" resultMap = "TeaStu" > select s.id sid, s.name sname, t.name tname, t.id tpid,s.tid from student s ,teacher t where t.id = s.tid and t.id = #{teaId}; </ select > < resultMap id = "TeaStu" type = "Teacher" > < result column = "tpid" property = "id" /> < result column = "tname" property = "name" /> < collection property = "student" ofType = "student" > < result column = "sid" property = "id" /> < result column = "sname" property = "name" /> < result column = "tid" property = "tid" /> </ collection > </ resultMap > |
因为复杂属性的类型为集合，所以我们在配置resultMap结果集映射时不再使用association对象，换成collection集合。在配置collection与association不同的是将JavaType(Java类型)换成了OfType其他依旧不变
按照查询嵌套
TeacherMapper.xml配置resultMap
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< select id = "getTeaById" resultMap = "TeaStu2" > select id,name from teacher where id = #{teaId} </ select > < resultMap id = "TeaStu2" type = "teacher" > < result column = "id" property = "id" /> < result column = "name" property = "name" /> < collection property = "student" javaType = "ArrayList" ofType = "student" select = "getStuList" column = "id" /> </ resultMap > < select id = "getStuList" resultType = "student" > select id,name,tid from student where tid = #{tid} </ select > |
小结：个人感觉，按照查询嵌套虽然一定程度上简化了sql语句的编写，但是针对resultMap的配置极其复杂，如果使用次数多可能还好。建议使用按照结果嵌套，sql语句编写虽然复杂一些但是只要基础扎实都是有理可循的，而且sql的编写你可以在数据库中进行调试，选择最优sql，并且对于结果集的映射配置也比较简单，容易理解。
也可能是初学所以觉得按照子查询会有点难度，总之还是水平太低了
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原文链接：https://blog.csdn.net/yuqu1028/article/details/128883996
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