深入浅出Mybatis系列（八）---mapper映射文件配置之select、resultMap

川长思鸟来 2022-09-21 13:13 185阅读 0赞

上篇《[深入浅出Mybatis系列（七）---mapper映射文件配置之insert、update、delete][Mybatis_---mapper_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
            
         
         id="selectPerson"
         
         
         parameterType="int"
         
         
         resultType="hashmap"
         
         
         resultMap="personResultMap"
         
         
         flushCache="false"
         
         
         useCache="true"
         
         
         timeout="10000"
         
         
         fetchSize="256"
         
         
         statementType="PREPARED"
         
         
         resultSetType="FORWARD_ONLY">

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:

![051626136276640.png][]

t\_student:

![051626476425015.png][]

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：

![051631424869037.png][]

Course.java:

package com.dy.entity;
    
    public class Course {
    
        private int id;
        private String name; 
        private int deleteFlag;
        
        public int getId() {
            return id;
        }
        public void setId(int id) {
            this.id = id;
        }
        public String getName() {
            return name;
        }
        public void setName(String name) {
            this.name = name;
        }
        public int getDeleteFlag() {
            return deleteFlag;
        }
        public void setDeleteFlag(int deleteFlag) {
            this.deleteFlag = deleteFlag;
        }
        
    }

Student.java:

package com.dy.entity;
    
    import java.util.List;
    
    public class Student {
    
        private int id;
        private String idCard;
        private String name;
        private List<Course> courseList;
        private int deleteFlag;
        
        public Student(int id, String idCard, String name, List<Course> courseList, int deleteFlag) {
            this.id = id;
            this.idCard = idCard;
            this.name = name;
            this.courseList = courseList;
            this.deleteFlag = deleteFlag;
        }
        
        public int getId() {
            return id;
        }
        public void setId(int id) {
            this.id = id;
        }
        public String getIdCard() {
            return idCard;
        }
        public void setIdCard(String idCard) {
            this.idCard = idCard;
        }
        public String getName() {
            return name;
        }
        public void setName(String name) {
            this.name = name;
        }
        public List<Course> getCourseList() {
            return courseList;
        }
        public void setCourseList(List<Course> courseList) {
            this.courseList = courseList;
        }
        public int getDeleteFlag() {
            return deleteFlag;
        }
        public void setDeleteFlag(int deleteFlag) {
            this.deleteFlag = deleteFlag;
        }
    }

CourseDao.java:

package com.dy.dao;
    
    import com.dy.entity.Course;
    
    public interface CourseDao {
    
        public Course findCourseById(int courseId);
        
    }

StudentDao.java:

package com.dy.dao;
    
    
    import com.dy.entity.Student;
    
    public interface StudentDao {
    
        public Student findStudentById(String idCard);
    }

courseDao.xml:

<mapper namespace="com.dy.dao.CourseDao">
        
        
        <select id="findCourseById"  resultType="course" >
            select course_id as id, course_name as name, course_delete_flg as deleteFlag from t_course where course_id=#{courseId}
        </select>
        
    </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
        public void findCourseById() {
            SqlSessionFactory sqlSessionFactory = getSessionFactory();
            SqlSession sqlSession = sqlSessionFactory.openSession(); 
            CourseDao courseDao = sqlSession.getMapper(CourseDao.class);
            Course course = courseDao.findCourseById(1);
        }
        
        
        //Mybatis 通过SqlSessionFactory获取SqlSession, 然后才能通过SqlSession与数据库进行交互
        private static SqlSessionFactory getSessionFactory() {  
            SqlSessionFactory sessionFactory = null;  
            String resource = "mybatis-conf.xml";  
            try {  
                sessionFactory = new SqlSessionFactoryBuilder().build(Resources  
                        .getResourceAsReader(resource));
            } catch (IOException e) {  
                e.printStackTrace();  
            }  
            return 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：

<resultMap type="" id="">
        
            
            <id property="" column=""/>
            
                
            <result property="" column=""/>
            
            
            <constructor>
                
                <idArg column=""/>
                
                <arg column=""/>
            </constructor>
            
            
            <collection property="" column="" ofType=""></collection>
            
            
            <association property="" column="" javaType=""></association>
        </resultMap>

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">
    
        
        <resultMap type="student" id="studentMap">
        
            
            <id property="idCard" column="stu_id_card"/>
            <result property="id" column="stu_id"/>
            <result property="name" column="stu_name"/>
            <result property="deleteFlag" column="stu_delete_flg"/>
            
            
            <collection property="courseList" column="stu_course_id" ofType="Course">
                <id property="id" column="course_id"/>
                <result property="name" column="course_name"/>
                <result property="deleteFlag" column="course_delete_flg"/>
            </collection>
        </resultMap>
        
        
        <select id="findStudentById" resultMap="studentMap">
            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}
        </select>
        
    </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
        public void findCourseById() {
            SqlSessionFactory sqlSessionFactory = getSessionFactory();
            SqlSession sqlSession = sqlSessionFactory.openSession(); 
            StudentDao studentDao = sqlSession.getMapper(StudentDao.class);
            Student student = studentDao.findStudentById("20140101");
            List<Course> courseList = student.getCourseList();
            for (Course course: courseList) {
                System.out.println(course.getId() + "   " + course.getName());
            }
        }
        
        
        //Mybatis 通过SqlSessionFactory获取SqlSession, 然后才能通过SqlSession与数据库进行交互
        private static SqlSessionFactory getSessionFactory() {  
            SqlSessionFactory sessionFactory = null;  
            String resource = "mybatis-conf.xml";  
            try {  
                sessionFactory = new SqlSessionFactoryBuilder().build(Resources  
                        .getResourceAsReader(resource));
            } catch (IOException e) {  
                e.printStackTrace();  
            }  
            return 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。

原文链接：[http://www.cnblogs.com/dongying/p/4073259.html][http_www.cnblogs.com_dongying_p_4073259.html]

[Mybatis_---mapper_insert_update_delete]: http://blog.csdn.net/qq_32786873/article/details/51722295
[051626136276640.png]: /images/20220721/a7efa9b159214bbfb8d0289394414af2.png
[051626476425015.png]: /images/20220721/6870c20ff87a4d47bc6e5cf9f29f37a9.png
[051631424869037.png]: /images/20220721/93e67056823f448ab86f77018e1146c6.png
[http_www.cnblogs.com_dongying_p_4073259.html]: http://www.cnblogs.com/dongying/p/4073259.html