Mybatis 注解

古城微笑少年丶 2022-12-05 04:56 147阅读 0赞

## 1.1 基本使用 ##

### 1.1.1 常用注解 ###

<table> 
 <thead> 
  <tr> 
   <th align="left">注解</th> 
   <th align="left">描述</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="left">@Insert</td> 
   <td align="left">实现新增</td> 
  </tr> 
  <tr> 
   <td align="left">@Update</td> 
   <td align="left">实现更新</td> 
  </tr> 
  <tr> 
   <td align="left">@Delete</td> 
   <td align="left">实现删除</td> 
  </tr> 
  <tr> 
   <td align="left">@Select</td> 
   <td align="left">实现查询</td> 
  </tr> 
  <tr> 
   <td align="left">@Result</td> 
   <td align="left">实现结果集封装</td> 
  </tr> 
  <tr> 
   <td align="left">@Results</td> 
   <td align="left">可以与 @Result 一起使用，封装多个结果集</td> 
  </tr> 
  <tr> 
   <td align="left">@One</td> 
   <td align="left">实现一对一结果集封装</td> 
  </tr> 
  <tr> 
   <td align="left">@Many</td> 
   <td align="left">实现一对多结果集封装</td> 
  </tr> 
 </tbody> 
</table>

### 1.1.2 MyBatis 增删改查 ###

**☞ 实体类**

public class Student { 
        private Long sId;
        private String sName;
        private Long sAge;
        private String sSex;
    	
    	// set and get
    
    }

**☞ DAO**

/** * Created with IntelliJ IDEA. * * @author Demo_Null * @date 2020/9/9 * @description Mybatis 注解 */
    public interface StudentDao { 
    
        @Select("select * from student")
        public List<Student> findAll();
    
        @Select("select * from where s_id = #{id} ")
        public Student findOne(Long id);
    
        @Insert("insert into student values (#{sId}, #{sName}), #{sAge}, #{sSex}")
        public int insert(Student student);
    
        @Update("update from student set s_id = #{sId}, s_name = #{sName}, s_age = #{sAge}, s_sex = #{sSex}")
        public int update(Student student);
        
        @Delete("delete from student where s_id = #{id} ")
        public int delete(Long id);
    }

**☞ 配置文件**

**☞ 测试**

/** * Created with IntelliJ IDEA. * * @author Demo_Null * @date 2020/9/9 * @description */
    public class MybatisDemo { 
    
        private StudentDao studentDao;
    
        @Before
        public void before() throws IOException { 
            // 加载核心配置文件
            InputStream resourceAsStream = Resources.getResourceAsStream("mybatis.xml");
            // 获得 sqlSession 工厂对象
            SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
            // 获得 sqlSession 对象
            SqlSession sqlSession = sqlSessionFactory.openSession();
            // 获取 mapper
            studentDao = sqlSession.getMapper(StudentDao.class);
        }
    
        @Test
        public void TestA() throws IOException { 
            List<Student> studentList = studentDao.findAll();
    
            System.out.println(studentList);
        }
    
    }

![在这里插入图片描述][20200909095331588.png_pic_center]  
  我们可以看到，明明结果已经查询出来了，为什么打印出来却是空的。这个是因为属性名和列名不一致造成的，类似于我们这种 `sId ☞ s_id` 可以打开驼峰之自动转换。如果二者之间没有任何联系就需要使用 @Results 一一映射。

@Select("select * from student")
    @Results(value = { 
                    @Result(property = "sId", column = "s_id"),
                    @Result(property = "sName", column = "s_name"),
                    @Result(property = "sAge", column = "s_age"),
                    @Result(property = "sSex", column = "s_sex")
    })
    public List<Student> findAll();

![在这里插入图片描述][20200909095828869.png_pic_center]

## 1.2 复杂映射 ##

### 1.2.1 注解详解 ###

<table> 
 <thead> 
  <tr> 
   <th align="left">注解</th> 
   <th align="left">说明</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="left">@Results</td> 
   <td align="left">代替的是标签该注解中可以使用单个@Result注解，也可以使用@Result集合<br>使用格式：@Results（{@Result（），@Result（）}）或@Results（@Result（））</td> 
  </tr> 
  <tr> 
   <td align="left">@Result</td> 
   <td align="left">代替了标签和标签<br>@Result中属性介绍：<br>column：数据库的列名<br>property：需要装配的属性名<br>one：需要使用的@One 注解（@Result（one=@One）（）））<br>many：需要使用的@Many 注解（@Result（many=@many）（）））</td> 
  </tr> 
  <tr> 
   <td align="left">@One(一对一)</td> 
   <td align="left">代替了标签，是多表查询的关键，在注解中用来指定子查询返回单一对象。<br>@One注解属性介绍：<br>select：指定用来多表查询的sqlmapper使用格式：@Result（column=""，property="".one=@One（select="））</td> 
  </tr> 
  <tr> 
   <td align="left">@Many(一对多)</td> 
   <td align="left">代替了标签，是是多表查询的关键，在注解中用来指定子查询返回对象集合。<br>使用格式：@Result（property="".column="，many=@Many（select="））</td> 
  </tr> 
 </tbody> 
</table>

### 1.2.2 一对一 ###

**☞ 实体类**

public class Student { 
        private Long sId;
        private String sName;
        private Long sAge;
        private String sSex;
        private Class class;
    
        // set and get
    }

public class Class { 
        private Long cId;
        private String cName;
        private String cAddr;
    
        // set and get
    }

**☞ DAO**

public interface ClassDao { 
        @Select("select * from class where c_id = #{id} ")
        public Class findById(Long id);
    }

public interface StudentDao { 
    
        @Select("select * from student")
        @Results({ 
                @Result(property = "sId", column = "s_id"),
                @Result(property = "sName", column = "s_name"),
                @Result(property = "sAge", column = "s_age"),
                @Result(property = "sSex", column = "s_sex"),
                // 类似于先查询出 student 然后拿 c_id 再去查 class
                @Result(property = "class", column = "c_id", javaType = Class.class,
                		one = @One(select = "com.software.mybatis.dao.ClassDao.findById"))
        })
        public List<Student> findAll();
    }

### 1.2.3 一对多 ###

**☞ 实体类**

public class Student { 
        private Long sId;
        private String sName;
        private Long sAge;
        private String sSex;
        private Long cId;
    
        // set and get
    }

public class Class { 
        private Long cId;
        private String cName;
        private String cAddr;
        private List<Student> students;
    
        // set and get
    }

**☞ DAO**

public interface StudentDao { 
        @Select("select * from where c_id = #{id} ")
        public List<Student> findByCid(Long id);
    }

public interface ClassDao { 
        @Select("select * from class")
        @Results({ 
                @Result(property = "cId", column = "c_id"),
                @Result(property = "cName", column = "c_name"),
                @Result(property = "cAddr", column = "c_addr"),
                // 类似于先查询 class 然后使用 c_id 查询 student
                @Result(property = "students", column = "c_id", javaType = List.class,
                        many = @Many(select = "com.software.mybatis.dao.StudentDao.findByCid")),
        })
        public List<Class> findAll();
    }

### 1.2.4 多对多 ###

**☞ 实体类**

public class Course { 
        private Integer cId;
        private String cName;
        private List<Student> students;
        
        // set and get
    }

public class Student { 
        private Integer sId;
        private String sName;
        private Long sAge;
        private String sSex;
        private List<Course> courses;
            
        // set and get
    }

**☞ DAO**

public interface CourseDao { 
        @Select("select * from course c, s_c sc where c.c_id = sc.c_id and sc.s_id = #{id} ")
        public List<Course> findBySid(Long id);
    }

public interface StudentDao { 
        @Select("select * from student")
        @Results({ 
                @Result(property = "sId", column = "s_id"),
                @Result(property = "sName", column = "s_name"),
                @Result(property = "sAge", column = "s_age"),
                @Result(property = "sSex", column = "s_sex"),
                // 类似于先查询 student 然后连接查询 course 和 s_c 表条件为 c.c_id = sc.c_id and s_id = sc.s_id
                @Result(property = "Courses", column = "s_id", javaType = List.class,
                		many = @Many(select = "com.software.mybatis.dao.CoursesDao.findBySid"))
        })
        public List<Student> findAll();
    }

[20200909095331588.png_pic_center]: /images/20221123/31c3f2c509f942a694fa4dd3affba586.png
[20200909095828869.png_pic_center]: /images/20221123/3b96cc9cc79f46f7bcc4e4ed4b296375.png