深入浅出Mybatis系列（八）---mapper映射文件配置之select、resultMap
上篇《深入浅出 Mybatis 系列（七）---mapper 映射文件配置之 insert、update、delete》介绍了 insert、update、delete 的用法，本篇将介绍 select、resultMap 的用法。select 无疑是我们最常用，也是最复杂的，mybatis 通过 resultMap 能帮助我们很好地进行高级映射。下面就开始看看 select 以及 resultMap 的用法：
先看 select 的配置吧：
<select <!-- 1. id （必须配置） id 是命名空间中的唯一标识符，可被用来代表这条语句。 一个命名空间（namespace） 对应一个 dao 接口, 这个 id 也应该对应 dao 里面的某个方法（相当于方法的实现），因此 id 应该与方法名一致 -->id="selectPerson" </span><span style="color: rgba(0, 128, 0, 1)"><!--</span><span style="color: rgba(0, 128, 0, 1)"> 2. parameterType （可选配置, 默认为mybatis自动选择处理） 将要传入语句的参数的完全限定类名或别名， 如果不配置，mybatis会通过ParameterHandler 根据参数类型默认选择合适的typeHandler进行处理 parameterType 主要指定参数类型，可以是int, short, long, string等类型，也可以是复杂类型（如对象） </span><span style="color: rgba(0, 128, 0, 1)">--></span><span style="color: rgba(0, 0, 0, 1)"> parameterType="int" </span><span style="color: rgba(0, 128, 0, 1)"><!--</span><span style="color: rgba(0, 128, 0, 1)"> 3. resultType (resultType 与 resultMap 二选一配置) resultType用以指定返回类型，指定的类型可以是基本类型，可以是java容器，也可以是javabean </span><span style="color: rgba(0, 128, 0, 1)">--></span><span style="color: rgba(0, 0, 0, 1)"> resultType="hashmap" </span><span style="color: rgba(0, 128, 0, 1)"><!--</span><span style="color: rgba(0, 128, 0, 1)"> 4. resultMap (resultType 与 resultMap 二选一配置) resultMap用于引用我们通过 resultMap标签定义的映射类型，这也是mybatis组件高级复杂映射的关键 </span><span style="color: rgba(0, 128, 0, 1)">--></span><span style="color: rgba(0, 0, 0, 1)"> resultMap="personResultMap" </span><span style="color: rgba(0, 128, 0, 1)"><!--</span><span style="color: rgba(0, 128, 0, 1)"> 5. flushCache (可选配置) 将其设置为 true，任何时候只要语句被调用，都会导致本地缓存和二级缓存都会被清空，默认值：false </span><span style="color: rgba(0, 128, 0, 1)">--></span><span style="color: rgba(0, 0, 0, 1)"> flushCache="false" </span><span style="color: rgba(0, 128, 0, 1)"><!--</span><span style="color: rgba(0, 128, 0, 1)"> 6. useCache (可选配置) 将其设置为 true，将会导致本条语句的结果被二级缓存，默认值：对 select 元素为 true </span><span style="color: rgba(0, 128, 0, 1)">--></span><span style="color: rgba(0, 0, 0, 1)"> useCache="true" </span><span style="color: rgba(0, 128, 0, 1)"><!--</span><span style="color: rgba(0, 128, 0, 1)"> 7. timeout (可选配置) 这个设置是在抛出异常之前，驱动程序等待数据库返回请求结果的秒数。默认值为 unset（依赖驱动）</span><span style="color: rgba(0, 128, 0, 1)">--></span><span style="color: rgba(0, 0, 0, 1)"> timeout="10000" </span><span style="color: rgba(0, 128, 0, 1)"><!--</span><span style="color: rgba(0, 128, 0, 1)"> 8. fetchSize (可选配置) 这是尝试影响驱动程序每次批量返回的结果行数和这个设置值相等。默认值为 unset（依赖驱动)</span><span style="color: rgba(0, 128, 0, 1)">--></span><span style="color: rgba(0, 0, 0, 1)"> fetchSize="256" </span><span style="color: rgba(0, 128, 0, 1)"><!--</span><span style="color: rgba(0, 128, 0, 1)"> 9. statementType (可选配置) STATEMENT，PREPARED 或 CALLABLE 的一个。这会让 MyBatis 分别使用 Statement，PreparedStatement 或 CallableStatement，默认值：PREPARED</span><span style="color: rgba(0, 128, 0, 1)">--></span><span style="color: rgba(0, 0, 0, 1)"> statementType="PREPARED" </span><span style="color: rgba(0, 128, 0, 1)"><!--</span><span style="color: rgba(0, 128, 0, 1)"> 10. resultSetType (可选配置) FORWARD_ONLY，SCROLL_SENSITIVE 或 SCROLL_INSENSITIVE 中的一个，默认值为 unset （依赖驱动）</span><span style="color: rgba(0, 128, 0, 1)">--></span><span style="color: rgba(0, 0, 0, 1)"> resultSetType="FORWARD_ONLY"></span></pre>
配置看起来总是这么多，不过实际常用的配置也就那么几个， 根据自己的需要吧，上面都已注明是否必须配置。
下面还是上个 demo 及时练练手吧：
------------------------------------------------------------------------ 下面是针对 select 的练手 demo---------------------------------------------------------------------------------------
数据库：新增两张表（t_course, t_student）
t_course:
t_student:
其中，1 个 student 可选择多个 course 进行学习。
我们还是拿上篇文章的 demo, 继续写：
增加后，项目目录如下所示：
Course.java:
package com.dy.entity;public class Course {
</span><span style="color: rgba(0, 0, 255, 1)">private</span> <span style="color: rgba(0, 0, 255, 1)">int</span><span style="color: rgba(0, 0, 0, 1)"> id; </span><span style="color: rgba(0, 0, 255, 1)">private</span><span style="color: rgba(0, 0, 0, 1)"> String name; </span><span style="color: rgba(0, 0, 255, 1)">private</span> <span style="color: rgba(0, 0, 255, 1)">int</span><span style="color: rgba(0, 0, 0, 1)"> deleteFlag; </span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">int</span><span style="color: rgba(0, 0, 0, 1)"> getId() { </span><span style="color: rgba(0, 0, 255, 1)">return</span><span style="color: rgba(0, 0, 0, 1)"> id; } </span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">void</span> setId(<span style="color: rgba(0, 0, 255, 1)">int</span><span style="color: rgba(0, 0, 0, 1)"> id) { </span><span style="color: rgba(0, 0, 255, 1)">this</span>.id =<span style="color: rgba(0, 0, 0, 1)"> id; } </span><span style="color: rgba(0, 0, 255, 1)">public</span><span style="color: rgba(0, 0, 0, 1)"> String getName() { </span><span style="color: rgba(0, 0, 255, 1)">return</span><span style="color: rgba(0, 0, 0, 1)"> name; } </span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">void</span><span style="color: rgba(0, 0, 0, 1)"> setName(String name) { </span><span style="color: rgba(0, 0, 255, 1)">this</span>.name =<span style="color: rgba(0, 0, 0, 1)"> name; } </span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">int</span><span style="color: rgba(0, 0, 0, 1)"> getDeleteFlag() { </span><span style="color: rgba(0, 0, 255, 1)">return</span><span style="color: rgba(0, 0, 0, 1)"> deleteFlag; } </span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">void</span> setDeleteFlag(<span style="color: rgba(0, 0, 255, 1)">int</span><span style="color: rgba(0, 0, 0, 1)"> deleteFlag) { </span><span style="color: rgba(0, 0, 255, 1)">this</span>.deleteFlag =<span style="color: rgba(0, 0, 0, 1)"> deleteFlag; }}
Student.java:
package com.dy.entity;import java.util.List;
public class Student {
</span><span style="color: rgba(0, 0, 255, 1)">private</span> <span style="color: rgba(0, 0, 255, 1)">int</span><span style="color: rgba(0, 0, 0, 1)"> id; </span><span style="color: rgba(0, 0, 255, 1)">private</span><span style="color: rgba(0, 0, 0, 1)"> String idCard; </span><span style="color: rgba(0, 0, 255, 1)">private</span><span style="color: rgba(0, 0, 0, 1)"> String name; </span><span style="color: rgba(0, 0, 255, 1)">private</span> List<Course><span style="color: rgba(0, 0, 0, 1)"> courseList; </span><span style="color: rgba(0, 0, 255, 1)">private</span> <span style="color: rgba(0, 0, 255, 1)">int</span><span style="color: rgba(0, 0, 0, 1)"> deleteFlag; </span><span style="color: rgba(0, 0, 255, 1)">public</span> Student(<span style="color: rgba(0, 0, 255, 1)">int</span> id, String idCard, String name, List<Course> courseList, <span style="color: rgba(0, 0, 255, 1)">int</span><span style="color: rgba(0, 0, 0, 1)"> deleteFlag) { </span><span style="color: rgba(0, 0, 255, 1)">this</span>.id =<span style="color: rgba(0, 0, 0, 1)"> id; </span><span style="color: rgba(0, 0, 255, 1)">this</span>.idCard =<span style="color: rgba(0, 0, 0, 1)"> idCard; </span><span style="color: rgba(0, 0, 255, 1)">this</span>.name =<span style="color: rgba(0, 0, 0, 1)"> name; </span><span style="color: rgba(0, 0, 255, 1)">this</span>.courseList =<span style="color: rgba(0, 0, 0, 1)"> courseList; </span><span style="color: rgba(0, 0, 255, 1)">this</span>.deleteFlag =<span style="color: rgba(0, 0, 0, 1)"> deleteFlag; } </span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">int</span><span style="color: rgba(0, 0, 0, 1)"> getId() { </span><span style="color: rgba(0, 0, 255, 1)">return</span><span style="color: rgba(0, 0, 0, 1)"> id; } </span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">void</span> setId(<span style="color: rgba(0, 0, 255, 1)">int</span><span style="color: rgba(0, 0, 0, 1)"> id) { </span><span style="color: rgba(0, 0, 255, 1)">this</span>.id =<span style="color: rgba(0, 0, 0, 1)"> id; } </span><span style="color: rgba(0, 0, 255, 1)">public</span><span style="color: rgba(0, 0, 0, 1)"> String getIdCard() { </span><span style="color: rgba(0, 0, 255, 1)">return</span><span style="color: rgba(0, 0, 0, 1)"> idCard; } </span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">void</span><span style="color: rgba(0, 0, 0, 1)"> setIdCard(String idCard) { </span><span style="color: rgba(0, 0, 255, 1)">this</span>.idCard =<span style="color: rgba(0, 0, 0, 1)"> idCard; } </span><span style="color: rgba(0, 0, 255, 1)">public</span><span style="color: rgba(0, 0, 0, 1)"> String getName() { </span><span style="color: rgba(0, 0, 255, 1)">return</span><span style="color: rgba(0, 0, 0, 1)"> name; } </span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">void</span><span style="color: rgba(0, 0, 0, 1)"> setName(String name) { </span><span style="color: rgba(0, 0, 255, 1)">this</span>.name =<span style="color: rgba(0, 0, 0, 1)"> name; } </span><span style="color: rgba(0, 0, 255, 1)">public</span> List<Course><span style="color: rgba(0, 0, 0, 1)"> getCourseList() { </span><span style="color: rgba(0, 0, 255, 1)">return</span><span style="color: rgba(0, 0, 0, 1)"> courseList; } </span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">void</span> setCourseList(List<Course><span style="color: rgba(0, 0, 0, 1)"> courseList) { </span><span style="color: rgba(0, 0, 255, 1)">this</span>.courseList =<span style="color: rgba(0, 0, 0, 1)"> courseList; } </span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">int</span><span style="color: rgba(0, 0, 0, 1)"> getDeleteFlag() { </span><span style="color: rgba(0, 0, 255, 1)">return</span><span style="color: rgba(0, 0, 0, 1)"> deleteFlag; } </span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">void</span> setDeleteFlag(<span style="color: rgba(0, 0, 255, 1)">int</span><span style="color: rgba(0, 0, 0, 1)"> deleteFlag) { </span><span style="color: rgba(0, 0, 255, 1)">this</span>.deleteFlag =<span style="color: rgba(0, 0, 0, 1)"> deleteFlag; }}
CourseDao.java:
package com.dy.dao;import com.dy.entity.Course;
public interface CourseDao {
</span><span style="color: rgba(0, 0, 255, 1)">public</span> Course findCourseById(<span style="color: rgba(0, 0, 255, 1)">int</span><span style="color: rgba(0, 0, 0, 1)"> courseId);}
StudentDao.java:
package com.dy.dao;import com.dy.entity.Student;
public interface StudentDao {
</span><span style="color: rgba(0, 0, 255, 1)">public</span><span style="color: rgba(0, 0, 0, 1)"> Student findStudentById(String idCard);}
courseDao.xml:
<mapper namespace="com.dy.dao.CourseDao"><span style="color: rgba(0, 128, 0, 1)"><!--</span><span style="color: rgba(0, 128, 0, 1)"> 1.此处直接将resultType 设置为course, 一看就知道我设置了别名吧，如果没有设置别名，那么resultType = com.dy.entity.Course。 2.可能细心的你会发现：Course.java中的属性名与数据库字段名不一致，下面，我就在sql语句中用了as, 使之匹配，当然方法不止一种， 在学习了resultMap之后，你能看到一种更直观优雅的方式去将javabean中的属性与数据库字段名保持一致 3.findCourseById 与CourseDao中findCourseById方法对应， 那么传入的参数名称以及类型也应该保持对应关系。 4.可以看到，在sql语句中，通过#{}表达式可以获取参数。 5.下面这条sql语句，实际上的形式是怎么样的？还记得之前说过，mybatis默认为preparedStatement吧，那么，用我们jdbc代码来看，它其实就是： select course_id as id, course_name as name, course_delete_flg as deleteFlag from t_course where course_id=? </span><span style="color: rgba(0, 128, 0, 1)">--></span> <span style="color: rgba(0, 0, 255, 1)"><</span><span style="color: rgba(128, 0, 0, 1)">select </span><span style="color: rgba(255, 0, 0, 1)">id</span><span style="color: rgba(0, 0, 255, 1)">="findCourseById"</span><span style="color: rgba(255, 0, 0, 1)"> resultType</span><span style="color: rgba(0, 0, 255, 1)">="course"</span> <span style="color: rgba(0, 0, 255, 1)">></span><span style="color: rgba(0, 0, 0, 1)"> select course_id as id, course_name as name, course_delete_flg as deleteFlag from t_course where course_id=#{courseId} </span><span style="color: rgba(0, 0, 255, 1)"></</span><span style="color: rgba(128, 0, 0, 1)">select</span><span style="color: rgba(0, 0, 255, 1)">></span></mapper>
CourseDaoTest.java:
package com.dy.dao;import java.io.IOException;
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 org.junit.Test;import com.dy.entity.Course;
public class CourseDaoTest {
@Test </span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">void</span><span style="color: rgba(0, 0, 0, 1)"> findCourseById() { SqlSessionFactory sqlSessionFactory </span>=<span style="color: rgba(0, 0, 0, 1)"> getSessionFactory(); SqlSession sqlSession </span>=<span style="color: rgba(0, 0, 0, 1)"> sqlSessionFactory.openSession(); CourseDao courseDao </span>= sqlSession.getMapper(CourseDao.<span style="color: rgba(0, 0, 255, 1)">class</span><span style="color: rgba(0, 0, 0, 1)">); Course course </span>= courseDao.findCourseById(1<span style="color: rgba(0, 0, 0, 1)">); } </span><span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)">Mybatis 通过SqlSessionFactory获取SqlSession, 然后才能通过SqlSession与数据库进行交互</span> <span style="color: rgba(0, 0, 255, 1)">private</span> <span style="color: rgba(0, 0, 255, 1)">static</span><span style="color: rgba(0, 0, 0, 1)"> SqlSessionFactory getSessionFactory() { SqlSessionFactory sessionFactory </span>= <span style="color: rgba(0, 0, 255, 1)">null</span><span style="color: rgba(0, 0, 0, 1)">; String resource </span>= "mybatis-conf.xml"<span style="color: rgba(0, 0, 0, 1)">; </span><span style="color: rgba(0, 0, 255, 1)">try</span><span style="color: rgba(0, 0, 0, 1)"> { sessionFactory </span>= <span style="color: rgba(0, 0, 255, 1)">new</span><span style="color: rgba(0, 0, 0, 1)"> SqlSessionFactoryBuilder().build(Resources .getResourceAsReader(resource)); } </span><span style="color: rgba(0, 0, 255, 1)">catch</span><span style="color: rgba(0, 0, 0, 1)"> (IOException e) { e.printStackTrace(); } </span><span style="color: rgba(0, 0, 255, 1)">return</span><span style="color: rgba(0, 0, 0, 1)"> sessionFactory; }}
上面的示例，我们针对 course, 简单演示了 select 的用法， 不过有个问题值得思考： 一个 student 可以对应多个 course, 那么，在 mybatis 中如何处理这种一对多， 甚至于多对多，一对一的关系呢？
这儿，就不得不提到 resultMap 这个东西， mybatis 的 resultMap 功能可谓十分强大，能够处理复杂的关系映射， 那么 resultMap 该怎么配置呢？ 别急，这就来了：
resultMap 的配置：
<!-- 1.type 对应类型，可以是 javabean, 也可以是其它 2.id 必须唯一， 用于标示这个 resultMap 的唯一性，在使用 resultMap 的时候，就是通过 id 指定 --> <resultMap type="" id=""><span style="color: rgba(0, 128, 0, 1)"><!--</span><span style="color: rgba(0, 128, 0, 1)"> id, 唯一性，注意啦，这个id用于标示这个javabean对象的唯一性， 不一定会是数据库的主键（不要把它理解为数据库对应表的主键） property属性对应javabean的属性名，column对应数据库表的列名 （这样，当javabean的属性与数据库对应表的列名不一致的时候，就能通过指定这个保持正常映射了） </span><span style="color: rgba(0, 128, 0, 1)">--></span> <span style="color: rgba(0, 0, 255, 1)"><</span><span style="color: rgba(128, 0, 0, 1)">id </span><span style="color: rgba(255, 0, 0, 1)">property</span><span style="color: rgba(0, 0, 255, 1)">=""</span><span style="color: rgba(255, 0, 0, 1)"> column</span><span style="color: rgba(0, 0, 255, 1)">=""</span><span style="color: rgba(0, 0, 255, 1)">/></span> <span style="color: rgba(0, 128, 0, 1)"><!--</span><span style="color: rgba(0, 128, 0, 1)"> result与id相比， 对应普通属性 </span><span style="color: rgba(0, 128, 0, 1)">--></span> <span style="color: rgba(0, 0, 255, 1)"><</span><span style="color: rgba(128, 0, 0, 1)">result </span><span style="color: rgba(255, 0, 0, 1)">property</span><span style="color: rgba(0, 0, 255, 1)">=""</span><span style="color: rgba(255, 0, 0, 1)"> column</span><span style="color: rgba(0, 0, 255, 1)">=""</span><span style="color: rgba(0, 0, 255, 1)">/></span> <span style="color: rgba(0, 128, 0, 1)"><!--</span><span style="color: rgba(0, 128, 0, 1)"> constructor对应javabean中的构造方法 </span><span style="color: rgba(0, 128, 0, 1)">--></span> <span style="color: rgba(0, 0, 255, 1)"><</span><span style="color: rgba(128, 0, 0, 1)">constructor</span><span style="color: rgba(0, 0, 255, 1)">></span> <span style="color: rgba(0, 128, 0, 1)"><!--</span><span style="color: rgba(0, 128, 0, 1)"> idArg 对应构造方法中的id参数 </span><span style="color: rgba(0, 128, 0, 1)">--></span> <span style="color: rgba(0, 0, 255, 1)"><</span><span style="color: rgba(128, 0, 0, 1)">idArg </span><span style="color: rgba(255, 0, 0, 1)">column</span><span style="color: rgba(0, 0, 255, 1)">=""</span><span style="color: rgba(0, 0, 255, 1)">/></span> <span style="color: rgba(0, 128, 0, 1)"><!--</span><span style="color: rgba(0, 128, 0, 1)"> arg 对应构造方法中的普通参数 </span><span style="color: rgba(0, 128, 0, 1)">--></span> <span style="color: rgba(0, 0, 255, 1)"><</span><span style="color: rgba(128, 0, 0, 1)">arg </span><span style="color: rgba(255, 0, 0, 1)">column</span><span style="color: rgba(0, 0, 255, 1)">=""</span><span style="color: rgba(0, 0, 255, 1)">/></span> <span style="color: rgba(0, 0, 255, 1)"></</span><span style="color: rgba(128, 0, 0, 1)">constructor</span><span style="color: rgba(0, 0, 255, 1)">></span> <span style="color: rgba(0, 128, 0, 1)"><!--</span><span style="color: rgba(0, 128, 0, 1)"> collection，对应javabean中容器类型, 是实现一对多的关键 property 为javabean中容器对应字段名 column 为体现在数据库中列名 ofType 就是指定javabean中容器指定的类型 </span><span style="color: rgba(0, 128, 0, 1)">--></span> <span style="color: rgba(0, 0, 255, 1)"><</span><span style="color: rgba(128, 0, 0, 1)">collection </span><span style="color: rgba(255, 0, 0, 1)">property</span><span style="color: rgba(0, 0, 255, 1)">=""</span><span style="color: rgba(255, 0, 0, 1)"> column</span><span style="color: rgba(0, 0, 255, 1)">=""</span><span style="color: rgba(255, 0, 0, 1)"> ofType</span><span style="color: rgba(0, 0, 255, 1)">=""</span><span style="color: rgba(0, 0, 255, 1)">></</span><span style="color: rgba(128, 0, 0, 1)">collection</span><span style="color: rgba(0, 0, 255, 1)">></span> <span style="color: rgba(0, 128, 0, 1)"><!--</span><span style="color: rgba(0, 128, 0, 1)"> association 为关联关系，是实现N对一的关键。 property 为javabean中容器对应字段名 column 为体现在数据库中列名 javaType 指定关联的类型 </span><span style="color: rgba(0, 128, 0, 1)">--></span> <span style="color: rgba(0, 0, 255, 1)"><</span><span style="color: rgba(128, 0, 0, 1)">association </span><span style="color: rgba(255, 0, 0, 1)">property</span><span style="color: rgba(0, 0, 255, 1)">=""</span><span style="color: rgba(255, 0, 0, 1)"> column</span><span style="color: rgba(0, 0, 255, 1)">=""</span><span style="color: rgba(255, 0, 0, 1)"> javaType</span><span style="color: rgba(0, 0, 255, 1)">=""</span><span style="color: rgba(0, 0, 255, 1)">></</span><span style="color: rgba(128, 0, 0, 1)">association</span><span style="color: rgba(0, 0, 255, 1)">></span> <span style="color: rgba(0, 0, 255, 1)"></</span><span style="color: rgba(128, 0, 0, 1)">resultMap</span><span style="color: rgba(0, 0, 255, 1)">></span></pre>
好啦，知道 resutMap 怎么配置后，咱们立即接着上面的 demo 来练习一下吧：
------------------------------------------------------------------ 下面是用 resultMap 处理一对多关系的映射的示例 -------------------------------------------------------------
一个 student 对应多个 course， 典型的一对多，咱们就来看看 mybatis 怎么配置这种映射吧：
studentDao.xml:
<mapper namespace="com.dy.dao.StudentDao"><span style="color: rgba(0, 128, 0, 1)"><!--</span><span style="color: rgba(0, 128, 0, 1)"> 这儿定义一个resultMap </span><span style="color: rgba(0, 128, 0, 1)">--></span> <span style="color: rgba(0, 0, 255, 1)"><</span><span style="color: rgba(128, 0, 0, 1)">resultMap </span><span style="color: rgba(255, 0, 0, 1)">type</span><span style="color: rgba(0, 0, 255, 1)">="student"</span><span style="color: rgba(255, 0, 0, 1)"> id</span><span style="color: rgba(0, 0, 255, 1)">="studentMap"</span><span style="color: rgba(0, 0, 255, 1)">></span> <span style="color: rgba(0, 128, 0, 1)"><!--</span><span style="color: rgba(0, 128, 0, 1)"> 数据库中主键是id, 但是我这儿却是指定idCard为主键，为什么？ 刚刚讲了，id用来表示唯一性， 我们可以认为只要idCard一样，那么他就是同一个学生。 如果此处用数据库中id， 那么mybatis将会认为数据库中每条记录都是一个student, 这显然不符合逻辑 </span><span style="color: rgba(0, 128, 0, 1)">--></span> <span style="color: rgba(0, 0, 255, 1)"><</span><span style="color: rgba(128, 0, 0, 1)">id </span><span style="color: rgba(255, 0, 0, 1)">property</span><span style="color: rgba(0, 0, 255, 1)">="idCard"</span><span style="color: rgba(255, 0, 0, 1)"> column</span><span style="color: rgba(0, 0, 255, 1)">="stu_id_card"</span><span style="color: rgba(0, 0, 255, 1)">/></span> <span style="color: rgba(0, 0, 255, 1)"><</span><span style="color: rgba(128, 0, 0, 1)">result </span><span style="color: rgba(255, 0, 0, 1)">property</span><span style="color: rgba(0, 0, 255, 1)">="id"</span><span style="color: rgba(255, 0, 0, 1)"> column</span><span style="color: rgba(0, 0, 255, 1)">="stu_id"</span><span style="color: rgba(0, 0, 255, 1)">/></span> <span style="color: rgba(0, 0, 255, 1)"><</span><span style="color: rgba(128, 0, 0, 1)">result </span><span style="color: rgba(255, 0, 0, 1)">property</span><span style="color: rgba(0, 0, 255, 1)">="name"</span><span style="color: rgba(255, 0, 0, 1)"> column</span><span style="color: rgba(0, 0, 255, 1)">="stu_name"</span><span style="color: rgba(0, 0, 255, 1)">/></span> <span style="color: rgba(0, 0, 255, 1)"><</span><span style="color: rgba(128, 0, 0, 1)">result </span><span style="color: rgba(255, 0, 0, 1)">property</span><span style="color: rgba(0, 0, 255, 1)">="deleteFlag"</span><span style="color: rgba(255, 0, 0, 1)"> column</span><span style="color: rgba(0, 0, 255, 1)">="stu_delete_flg"</span><span style="color: rgba(0, 0, 255, 1)">/></span> <span style="color: rgba(0, 128, 0, 1)"><!--</span><span style="color: rgba(0, 128, 0, 1)"> 这儿就是实现一对多的关键。 在Student中，courseList为List<Course>, 因此，ofType也应该与之对应（当然，我用了别名，不然要蛋疼的写全名了）。 collection的子标签是在指定Course的映射关系（由于Course的javabean的属性名与数据库的列名不一致） </span><span style="color: rgba(0, 128, 0, 1)">--></span> <span style="color: rgba(0, 0, 255, 1)"><</span><span style="color: rgba(128, 0, 0, 1)">collection </span><span style="color: rgba(255, 0, 0, 1)">property</span><span style="color: rgba(0, 0, 255, 1)">="courseList"</span><span style="color: rgba(255, 0, 0, 1)"> column</span><span style="color: rgba(0, 0, 255, 1)">="stu_course_id"</span><span style="color: rgba(255, 0, 0, 1)"> ofType</span><span style="color: rgba(0, 0, 255, 1)">="Course"</span><span style="color: rgba(0, 0, 255, 1)">></span> <span style="color: rgba(0, 0, 255, 1)"><</span><span style="color: rgba(128, 0, 0, 1)">id </span><span style="color: rgba(255, 0, 0, 1)">property</span><span style="color: rgba(0, 0, 255, 1)">="id"</span><span style="color: rgba(255, 0, 0, 1)"> column</span><span style="color: rgba(0, 0, 255, 1)">="course_id"</span><span style="color: rgba(0, 0, 255, 1)">/></span> <span style="color: rgba(0, 0, 255, 1)"><</span><span style="color: rgba(128, 0, 0, 1)">result </span><span style="color: rgba(255, 0, 0, 1)">property</span><span style="color: rgba(0, 0, 255, 1)">="name"</span><span style="color: rgba(255, 0, 0, 1)"> column</span><span style="color: rgba(0, 0, 255, 1)">="course_name"</span><span style="color: rgba(0, 0, 255, 1)">/></span> <span style="color: rgba(0, 0, 255, 1)"><</span><span style="color: rgba(128, 0, 0, 1)">result </span><span style="color: rgba(255, 0, 0, 1)">property</span><span style="color: rgba(0, 0, 255, 1)">="deleteFlag"</span><span style="color: rgba(255, 0, 0, 1)"> column</span><span style="color: rgba(0, 0, 255, 1)">="course_delete_flg"</span><span style="color: rgba(0, 0, 255, 1)">/></span> <span style="color: rgba(0, 0, 255, 1)"></</span><span style="color: rgba(128, 0, 0, 1)">collection</span><span style="color: rgba(0, 0, 255, 1)">></span> <span style="color: rgba(0, 0, 255, 1)"></</span><span style="color: rgba(128, 0, 0, 1)">resultMap</span><span style="color: rgba(0, 0, 255, 1)">></span> <span style="color: rgba(0, 128, 0, 1)"><!--</span><span style="color: rgba(0, 128, 0, 1)"> 这儿将返回类型设置成了上面指定的studentMap </span><span style="color: rgba(0, 128, 0, 1)">--></span> <span style="color: rgba(0, 0, 255, 1)"><</span><span style="color: rgba(128, 0, 0, 1)">select </span><span style="color: rgba(255, 0, 0, 1)">id</span><span style="color: rgba(0, 0, 255, 1)">="findStudentById"</span><span style="color: rgba(255, 0, 0, 1)"> resultMap</span><span style="color: rgba(0, 0, 255, 1)">="studentMap"</span><span style="color: rgba(0, 0, 255, 1)">></span><span style="color: rgba(0, 0, 0, 1)"> SELECT s.*, c.* FROM t_student s LEFT JOIN t_course c ON s.stu_course_id=c.course_id WHERE s.stu_id_card=#{idCard} </span><span style="color: rgba(0, 0, 255, 1)"></</span><span style="color: rgba(128, 0, 0, 1)">select</span><span style="color: rgba(0, 0, 255, 1)">></span></mapper>
StudentDaoTest.java:
package com.dy.dao;import java.io.IOException;
import java.util.List;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 org.junit.Test;import com.dy.entity.Course;
import com.dy.entity.Student;public class StudentDaoTest {
@Test </span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">void</span><span style="color: rgba(0, 0, 0, 1)"> findCourseById() { SqlSessionFactory sqlSessionFactory </span>=<span style="color: rgba(0, 0, 0, 1)"> getSessionFactory(); SqlSession sqlSession </span>=<span style="color: rgba(0, 0, 0, 1)"> sqlSessionFactory.openSession(); StudentDao studentDao </span>= sqlSession.getMapper(StudentDao.<span style="color: rgba(0, 0, 255, 1)">class</span><span style="color: rgba(0, 0, 0, 1)">); Student student </span>= studentDao.findStudentById("20140101"<span style="color: rgba(0, 0, 0, 1)">); List</span><Course> courseList =<span style="color: rgba(0, 0, 0, 1)"> student.getCourseList(); </span><span style="color: rgba(0, 0, 255, 1)">for</span><span style="color: rgba(0, 0, 0, 1)"> (Course course: courseList) { System.out.println(course.getId() </span>+ " " +<span style="color: rgba(0, 0, 0, 1)"> course.getName()); } } </span><span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)">Mybatis 通过SqlSessionFactory获取SqlSession, 然后才能通过SqlSession与数据库进行交互</span> <span style="color: rgba(0, 0, 255, 1)">private</span> <span style="color: rgba(0, 0, 255, 1)">static</span><span style="color: rgba(0, 0, 0, 1)"> SqlSessionFactory getSessionFactory() { SqlSessionFactory sessionFactory </span>= <span style="color: rgba(0, 0, 255, 1)">null</span><span style="color: rgba(0, 0, 0, 1)">; String resource </span>= "mybatis-conf.xml"<span style="color: rgba(0, 0, 0, 1)">; </span><span style="color: rgba(0, 0, 255, 1)">try</span><span style="color: rgba(0, 0, 0, 1)"> { sessionFactory </span>= <span style="color: rgba(0, 0, 255, 1)">new</span><span style="color: rgba(0, 0, 0, 1)"> SqlSessionFactoryBuilder().build(Resources .getResourceAsReader(resource)); } </span><span style="color: rgba(0, 0, 255, 1)">catch</span><span style="color: rgba(0, 0, 0, 1)"> (IOException e) { e.printStackTrace(); } </span><span style="color: rgba(0, 0, 255, 1)">return</span><span style="color: rgba(0, 0, 0, 1)"> sessionFactory; }}
相信通过以上 demo， 大家也能够使用 mybatis 的 select 和 resultMap 的用法了。上面 demo 只演示了一对多的映射，其实多对一、多对多也与它类似，所以我就没演示了，有兴趣的可以自己动手再做做。
好啦，本次就写到这儿了。（PS，生病一周了，所以到现在才更新博客）。
另附上 demo, 需要的童鞋可以前往下载：
demo 下载地址：http://pan.baidu.com/s/1qWjsDzA