mybatis学习 (四) 了解映射器的配置

野性酷女 2022-05-29 00:15 126阅读 0赞

映射器是javaBean和数据库表沟通的桥梁， 必须掌握它的使用  
它的内容如下表

![这里写图片描述][70]  
![这里写图片描述][70 1]

# select #

--------------------

这是最简单的用法了， 根据id来返回一个实体。

<select id="getStudent" parameterType="integer" resultType="student">
                select * from student where stuId = #{stuId}
            </select>

参数解释  
id 和接口中的方法定义一致  
parameterType 要传给sql的参数类型, 可以是全路径或别名  
resultType 结果类型，可以是全路径或别名

###### 统计姓王的学生人数， 步骤如下 ######

1 . 在接口中定义方法

public Student countFirstName(String name);

2 . 配置select标签  
这里需要注意的是like的 %写法

<select id="countFirstName" parameterType="string" resultType="integer">
                select count(*) FROM student where stuName like concat(#{firstName} ,'%')
            </select>

3 . 测试

@Test
        public void test() throws IOException {
            SqlSession sqlSession = null;
            try {
               sqlSession = SqlSessionFactoryUtil.openSqlSession();
               StudentMapper studentMapper = sqlSession.getMapper(StudentMapper.class);
                System.out.println(studentMapper.countFirstName("王"));
            } catch (Exception e){
                e.printStackTrace();
            }finally {
                if (sqlSession != null){
                    sqlSession.close();
                }
            }
        }

###### select的自动映射 ######

假如有一个这样的字段，在数据库中是stu\_id, 用下划线分割了两个词， 但是javabean中的命名为stuId， 两种不能互相对应。  
解决办法是 ，用as将放回列表重命名成javabean的属性

<select id="getStudent" parameterType="integer" resultType="student">
                SELECT stu_id AS stuId , stu_name AS stuName,stu_age AS stuAge,stu_major AS stuMajor,birthday  FROM student WHERE stu_Id = #{stuId}
            </select>

###### 多个值传递 ######

上面传递一个值还是挺简单的， 当有多个值的时候 用map将列名对应的参数传入.  
比如查找姓氏为王， 年龄18的学生  
接口方法

public List<Student> findByNameAndAge(Map<String, String> params);

xml

<select id="findByNameAndAge" parameterType="map" resultType="student">
           SELECT stu_id AS stuId , stu_name AS stuName,stu_age AS stuAge,stu_major AS stuMajor,birthday
            FROM student WHERE stu_name LIKE concat(#{name},'%') AND stu_age = #{age};
        </select>

测试

@Test
        public void test() throws IOException {
            SqlSession sqlSession = null;
            try {
               sqlSession = SqlSessionFactoryUtil.openSqlSession();
               StudentMapper studentMapper = sqlSession.getMapper(StudentMapper.class);
               Map<String, String> paramsMap = new HashMap<String, String>();
               paramsMap.put("name","王");
               paramsMap.put("age","18");
               System.out.println(studentMapper.findByNameAndAge(paramsMap));
            } catch (Exception e){
                e.printStackTrace();
            }finally {
                if (sqlSession != null){
                    sqlSession.close();
                }
            }
        }

###### 多值传递 –注解方式 ######

在接口方法参数上加上@Param(“name”) 也可以将值传入

public List<Student> findByNameAndAge(@Param("name") String name, @Param("age") Integer age);

xml, 不需要指定parameterType

<select id="findByNameAndAge"  resultType="student">
           SELECT stu_id AS stuId , stu_name AS stuName,stu_age AS stuAge,stu_major AS stuMajor,birthday
            FROM student WHERE stu_name LIKE concat(#{name},'%') AND stu_age = #{age};
        </select>

测试代码

studentMapper.findByNameAndAge("王",18)

###### 使用javabean 方式传入 ######

需要针对查询的参数来重新创建一个类，类的属性就是查询的参数，如下

public class StuParam {
        private String name;
        private Integer age;
    
        public String getName() {
            return name;
        }
    
        public void setName(String name) {
            this.name = name;
        }
    
        public Integer getAge() {
            return age;
        }
    
        public void setAge(Integer age) {
            this.age = age;
        }
    }

接口创建以这个参数类为参数的方法

public List<Student> findByNameAndAge(StuParam stuParam);

xml, 指定输入类型为StuParam类

<select id="findByNameAndAge"  parameterType="com.cyh.pojo.StuParam" resultType="student">
           SELECT stu_id AS stuId , stu_name AS stuName,stu_age AS stuAge,stu_major AS stuMajor,birthday
            FROM student WHERE stu_name LIKE concat(#{name},'%') AND stu_age = #{age};
        </select>

测试：

studentMapper.findByNameAndAge(new StuParam("王",18));

3种多值传递方法感觉还是注解的方便一些。

## insert ##

--------------------

插入的规则要比查询简单很多， 只要注意普通的查询是不返回主键的，如下代码

@Test
        public void test() throws IOException {
            SqlSession sqlSession = null;
            try {
               sqlSession = SqlSessionFactoryUtil.openSqlSession();
               StudentMapper studentMapper = sqlSession.getMapper(StudentMapper.class);
    
               Student stu = new Student();
               stu.setStuName("刘si");
               stu.setStuAge(12);
               stu.setStuMajor("软件");
               stu.setBirthday(new Date());
    
                studentMapper.insertStudent(stu);
                sqlSession.commit();
    
                System.out.println(stu.getStuId());
            } catch (Exception e){
                e.printStackTrace();
            }finally {
                if (sqlSession != null){
                    sqlSession.close();
                }
            }
        }

输出的id是null， 主键不回填， 后面若是需要对stu这个对象操作的话就麻烦了， 在xml中有解决的办法，如下 . 设置useGeneratedKeys为true ， keyProperty为pojo的主键属性

<insert id="insertStudent" parameterType="student"
                useGeneratedKeys="true" keyProperty="stuId">
                insert into student(stu_name, stu_age, stu_major, birthday) values(#{
       stuName},#{
       stuAge},#{
       stuMajor},#{
       birthday})
            </insert>

## update ##

参数传递的方式和select 一样  
接口如下

public Integer updateStudent(Student stu);

xml 如下

<update id="updateStudent" parameterType="student"  >
            UPDATE student SET stu_name=#{
       stuName} WHERE stu_id = #{
       stuId}
        </update>

测试

@Test
        public void test() throws IOException {
            SqlSession sqlSession = null;
            try {
               sqlSession = SqlSessionFactoryUtil.openSqlSession();
               StudentMapper studentMapper = sqlSession.getMapper(StudentMapper.class);
    
               Student stu = new Student();
               stu.setStuId(6);
               stu.setStuName("新名字哎");
               studentMapper.updateStudent(stu);
                sqlSession.commit();
            } catch (Exception e){
                e.printStackTrace();
            }finally {
                if (sqlSession != null){
                    sqlSession.close();
                }
            }
        }

注意一点是， 这里只按id修改了学生的姓名，没有传入stuMajor参数, 但是数据库不会将stu\_major字段设置为null， 而是保留上一次的内容

## delete ##

<delete id="deleteStudent" parameterType="integer">
             delete FROM student WHERE stu_id = #{stuid}
         </delete>

[70]: /images/20220529/460fae141a494f85a671cc6202b688ae.png
[70 1]: /images/20220529/2460a3ad5d2f4eddac490cefab50becf.png