【MyBatis学习18】使用注解配置Mapper

r囧r小猫 2022-06-17 07:45 199阅读 0赞

本文博客地址：[http://blog.csdn.net/soonfly/article/details/67640653][http_blog.csdn.net_soonfly_article_details_67640653] （转载请注明出处）

MyBatis支持使用注解来配置映射的sql语句，这样可以省掉映射器xml文件。

## 一、映射语句 ##

1、insert   
比如入门实例中的：

public int insert(User user) throws Exception;
     
      
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<insert id="insert" parameterType="twm.mybatisdemo.pojo.User" useGeneratedKeys="true" keyProperty="id">
        
        insert into user (username,address,email) values (#{username},#{address},#{email})
    </insert>
     
      
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改成注解来配置映射：

@Insert("insert into user (username,address,email) values (#{username},#{address},#{email})") @Options(useGeneratedKeys = true, keyProperty = "id") public int insert(User user) throws Exception;
     
      
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这里使用`@Insert`注解来定义一个INSERT映射语句。   
并且使用`@Options`注解的`userGeneratedKeys` 和`keyProperty`属性，让[数据库][Link 1]auto\_increment生成的主键值，赋值到keyProperty标记的属性id中

还有一种获取主键的方法（[Oracle][]要用SELECT SEQ.NEXTVAL FROM DUAL ，且order设为before）

<insert id="insert" parameterType="twm.mybatisdemo.pojo.User" >
        
        <selectKey keyProperty="id" order="AFTER" resultType="java.lang.Integer">
               select last_insert_id()
        </selectKey>
        insert into user (username,address,email) values (#{username},#{address},#{email})
    </insert>
     
      
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相应的注解是：

@Insert("insert into user (username,address,email) values (#{username},#{address},#{email})") @SelectKey(statement="select last_insert_id()",keyProperty="id", resultType=int.class, before=true) public int insert(User user) throws Exception;
     
      
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2、select

@Select("select * from user where id=#{id}")
    public User selectById(int id) throws Exception;
     
      
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返回的是一个User对象，因此如果select语句返回多行记录，就会出现TooManyResultsException异常。

3、update

@Update("update user set username=#{username},address=#{address},email=#{email} where id=#{id}") public int update(User user) throws Exception;
     
      
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4、delete

@Delete("delete from user where id=#{id}")
    public int delete(int id) throws Exception;
     
      
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## 二、结果映射 ##

在xml配置文件中，将查询结果和JavaBean属性映射起来的标签是`<resultMap>`。对应的是`@Results`注解

@Select("select * from user")
    @Results({ @Result(id = true, column = "id", property = "id"),
            @Result(column = "username", property = "user_name"),
            @Result(column = "city", property = "city") })
    public List<User> selectAll() throws Exception;
     
      
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`@Results`注解没办法复用。譬如我们的`public User selectById(int id) throws Exception`也要用到同样的`@Results`注解，但还是要重新写一个一模一样的`@Results`

@Select("select * from user where id=#{id}")
    @Results({ @Result(id = true, column = "id", property = "id"),
            @Result(column = "username", property = "user_name"),
            @Result(column = "city", property = "city") })
    public User selectById(int id) throws Exception;
     
      
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如果要想使用可以复用的映射器，那么就使用`@ResultMap`注解。该注解依赖一个xml配置文件。   
在接口文件同目录下新建一个userMapper.xml文件，并定义一个名为userMap的resultMap。

<mapper namespace="twm.mybatisdemo.mapper.UserMapper">
        
        <resultMap id="userMap" type="twm.mybatisdemo.pojo.User">
            <id column="id" property="id" jdbcType="INTEGER" />
            <result property="user_name" column="username"></result>
            <result property="city" column="city"></result>
        </resultMap>
    </mapper>
     
      
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在userMapper.[Java][]中，使用`@ResultMap`引用名为userMap的resultMap，实现复用。

@Select("select * from user where id=#{id}")  
    @ResultMap("twm.mybatisdemo.mapper.UserMapper.userMap")  
    public User selectById(int id) throws Exception;
    
    @Select("select * from user")  
    @ResultMap("twm.mybatisdemo.mapper.UserMapper.userMap")  
    public List<User> selectAll() throws Exception;
     
      
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## 三、关联关系 ##

1、一对一关系   
MyBatis提供了@One注解来配合@Result注解，从而实现一对一关联查询数据的加载。   
比如所有user对象都有个配偶信息spouse(本例中配偶本身也是一个user对象)

@Select("select * from user")
    @Results({ @Result(id = true, column = "id", property = "id"),
            @Result(column = "username", property = "user_name"),
            @Result(column = "city", property = "city"),
            @Result(column = "account_id", property = "account",one = @One(select = "twm.mybatisdemo.mapper.AccountMapper.selectById")) })
    public List<User> selectAll() throws Exception;
     
      
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在这里`column = "account_id"`指定了向`twm.mybatisdemo.mapper.AccountMapper.selectById`方法传递的参数。如果`@OneSELECT`查询返回了多行结果，则会抛出TooManyResultsException异常。

调用:

UserMapper userMapper =sqlSession.getMapper(UserMapper.class); 
    List<User> userlist = userMapper.selectAll(); 
    System.out.println(userlist[0].getAccount().getBalance()); 
     
      
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如果结果映射使用的是xml文件,那么可以association 来配置映射：

<mapper namespace="twm.mybatisdemo.mapper.UserMapper">
        
        <resultMap id="userMapWithAccount" type="twm.mybatisdemo.pojo.User">
            <id column="id" property="id" jdbcType="INTEGER" />
            <result property="user_name" column="username"></result>
            <result property="city" column="city"></result>
            <association property="account" javaType="Account" column="account_id" select="twm.mybatisdemo.mapper.AccountMapper.selectById" />
        </resultMap>
    </mapper>
     
      
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接口如下：

@Select("select * from user")
    @ResultMap("twm.mybatisdemo.mapper.UserMapper.userMapWithAccount")
    public List<User> selectAll() throws Exception;
     
      
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2、一对多关系   
MyBatis提供了@Many注解来配合@Result注解，从而实现一对多关联查询数据的加载。   
现在让我们看一下如何使用@Many注解获取一个讲师及其教授课程列表信息：

public interface TutorMapper
    {
        @Select("select * from courses where tutor_id=#{tutorId}")
        @Results(
        {
            @Result(id = true, column = "course_id", property = "courseId"),
            @Result(column = "name", property = "name"),
            @Result(column = "description", property = "description"),
            @Result(column = "start_date" property = "startDate"),
            @Result(column = "end_date" property = "endDate")
        })
        List<Course> findCoursesByTutorId(int tutorId);
    
        @Select("SELECT tutor_id, name as tutor_name, email, addr_id FROM tutors where tutor_id=#{tutorId}")
        @Results(
        {
            @Result(id = true, column = "tutor_id", property = "tutorId"),
            @Result(column = "tutor_name", property = "name"),
            @Result(column = "email", property = "email"),
            @Result(property = "address", column = "addr_id",
            one = @One(select = " com.mybatis3. mappers.TutorMapper.findAddressById")),
            @Result(property = "courses", column = "tutor_id",
            many = @Many(select = "com.mybatis3.mappers.TutorMapper. findCoursesByTutorId"))
        })
        Tutor findTutorById(int tutorId);
    }
     
      
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同样也可以采用配置xml中配置resultMaps的方式

<mapper namespace="com.mybatis3.mappers.TutorMapper">
        <resultMap type="Course" id="CourseResult">
            <id column="course_id" property="courseId" />
            <result column="name" property="name" />
            <result column="description" property="description" />
            <result column="start_date" property="startDate" />
            <result column="end_date" property="endDate" />
        </resultMap>
        <resultMap type="Tutor" id="TutorResult">
            <id column="tutor_id" property="tutorId" />
            <result column="tutor_name" property="name" />
            <result column="email" property="email" />
            <collection property="courses" resultMap="CourseResult" />
        </resultMap>
    </mapper>
     
      
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public interface TutorMapper
    {
        @Select("SELECT T.TUTOR_ID, T.NAME AS TUTOR_NAME, EMAIL, A.ADDR_ID, STREET, CITY, STATE, ZIP, COUNTRY, COURSE_ID, C.NAME, DESCRIPTION, START_DATE, END_DATE FROM TUTORS T LEFT OUTER JOIN ADDRESSES A ON T.ADDR_ID=A.ADDR_ID LEFT OUTER JOIN COURSES C ON T.TUTOR_ID=C.TUTOR_ID WHERE T.TUTOR_ID=#{tutorId}")
        @ResultMap("com.mybatis3.mappers.TutorMapper.TutorResult")
        Tutor selectTutorById(int tutorId);
    }
     
      
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## 四、动态SQL ##

MyBatis提供了各种注解如@InsertProvider,@UpdateProvider,@DeleteProvider和@SelectProvider，来帮助构建动态SQL语句，然后让MyBatis执行这些SQL语句。

这里重点用@SelectProvider举例：

创建一个TutorDynaSqlProvider.java类，以及findTutorByIdSql()方法，如下所示：

package com.mybatis3.sqlproviders;
    import org.apache.ibatis.jdbc.SQL;
    public class TutorDynaSqlProvider
    {
        public String findTutorByIdSql(int tutorId)
        {
            return "SELECT TUTOR_ID AS tutorId, NAME, EMAIL FROM TUTORS WHERE TUTOR_ID=" + tutorId;
        }
    }
     
      
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在TutorMapper.java接口中创建一个映射语句，如下：

@SelectProvider(type=TutorDynaSqlProvider.class, method="findTutorByIdSql") Tutor findTutorById(int tutorId);  
     
      
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本文博客地址：[http://blog.csdn.net/soonfly/article/details/67640653][http_blog.csdn.net_soonfly_article_details_67640653] （转载请注明出处）

[http_blog.csdn.net_soonfly_article_details_67640653]: http://blog.csdn.net/soonfly/article/details/67640653
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