mybatis注解建立实体类属性与数据库表中列的对应关系

1.1 @Results

@Results注解来映射查询结果集到实体类属性

（1）@Results的基本用法。当数据库字段名与实体类对应的属性名不一致时，可以使用@Results映射来将其对应起来。

column为数据库字段名，porperty为实体类属性名，jdbcType为数据库字段数据类型，id为是否为主键。

    @Select("select * from user")
    @Results(value = {
            @Result(id = true, column = "id", property = "userId",jdbcType = JdbcType.INTEGER),
            @Result(column = "username", property = "userName",jdbcType = JdbcType.VARCHAR),
            @Result(column = "birthday", property = "userBirthday",jdbcType = JdbcType.DATE),
            @Result(column = "sex", property = "userSex",jdbcType = JdbcType.VARCHAR),
            @Result(column = "address", property = "userAddress",jdbcType = JdbcType.VARCHAR)
    })
    List<User> findAll();

如上所示的数据库字段名id与实体类属性名userId，就通过这种方式建立了映射关系。名字相同的可以省略。

1.2 @ResultMap

（2）@ResultMap的用法。当这段@Results代码需要在多个方法用到时，为了提高代码复用性，我们可以为这个@Results注解设置id，然后使用@ResultMap注解来复用这段代码。

　　@Select("select * from user")
    @Results(id = "userMap",value = {
            @Result(id = true, column = "id", property = "userId",jdbcType = JdbcType.INTEGER),
            @Result(column = "username", property = "userName",jdbcType = JdbcType.VARCHAR),
            @Result(column = "birthday", property = "userBirthday",jdbcType = JdbcType.DATE),
            @Result(column = "sex", property = "userSex",jdbcType = JdbcType.VARCHAR),
            @Result(column = "address", property = "userAddress",jdbcType = JdbcType.VARCHAR)
    })
    List<User> findAll();

用@ResultMap()注解来引用

    @Select("select * from user where id=#{id}")
    @ResultMap(value = {"userMap"})
    User findById(Integer id);

1.3 @One

（3）@One的用法。当我们需要通过查询到的一个字段值作为参数，去执行另外一个方法来查询关联的内容，而且两者是一对一关系时，可以使用@One注解来便捷的实现。

一对一：一般都用立即加载

一对多：一般都用懒加载/延迟加载

fetchType = FetchType.EAGER  //设置为立即加载

package com.jh.dao;

import com.jh.entity.Account;
import org.apache.ibatis.annotations.One;
import org.apache.ibatis.annotations.Result;
import org.apache.ibatis.annotations.Results;
import org.apache.ibatis.annotations.Select;
import org.apache.ibatis.mapping.FetchType;

import java.util.List;

public interface AccountDao {
    /**
     * 查询所有账户，并且获取每个账户所属的用户信息
     *
     * @return
     */
    @Select("select * from account")
    @Results(id = "accountMap", value = {
            @Result(id = true, property = "id", column = "id"),
            @Result(property = "uid", column = "uid"),
            @Result(property = "money", column = "money"),
            @Result(property = "user", column = "uid", one = @One(select = "com.jh.dao.UserDao.findById", fetchType = FetchType.EAGER))
    })
    List<Account> findAll();
}

package com.jh.dao;

import com.jh.entity.User;
import org.apache.ibatis.annotations.*;
import org.apache.ibatis.type.JdbcType;

import java.util.List;

public interface UserDao {
    @Select("select * from user")
    @Results(id = "userMap", value = {
            @Result(id = true, column = "id", property = "userId", jdbcType = JdbcType.INTEGER),
            @Result(column = "username", property = "userName", jdbcType = JdbcType.VARCHAR),
            @Result(column = "birthday", property = "userBirthday", jdbcType = JdbcType.DATE),
            @Result(column = "sex", property = "userSex", jdbcType = JdbcType.VARCHAR),
            @Result(column = "address", property = "userAddress", jdbcType = JdbcType.VARCHAR)
    })
    List<User> findAll();

    @Select("select * from user where id=#{id}")
    @ResultMap(value = {"userMap"})
    User findById(Integer id);
    }
}

1.4 @Many

（4）@Many的用法。与@One类似，只不过如果使用@One查询到的结果是多行，会抛出TooManyResultException异常，这种时候应该使用的是@Many注解，实现一对多的查询

package com.jh.dao;

import com.jh.entity.User;
import org.apache.ibatis.annotations.*;
import org.apache.ibatis.mapping.FetchType;
import org.apache.ibatis.type.JdbcType;

import java.util.List;

public interface UserDao {
    @Select("select * from user")
    @Results(id = "userMap", value = {
            @Result(id = true, column = "id", property = "userId", jdbcType = JdbcType.INTEGER),
            @Result(column = "username", property = "userName", jdbcType = JdbcType.VARCHAR),
            @Result(column = "birthday", property = "userBirthday", jdbcType = JdbcType.DATE),
            @Result(column = "sex", property = "userSex", jdbcType = JdbcType.VARCHAR),
            @Result(column = "address", property = "userAddress", jdbcType = JdbcType.VARCHAR),
            @Result(column = "id", property = "accounts",many = @Many(select = "com.jh.dao.AccountDao.findById",fetchType = FetchType.LAZY))
    })
    List<User> findAll();
    }
}

package com.jh.dao;

import com.jh.entity.Account;
import org.apache.ibatis.annotations.One;
import org.apache.ibatis.annotations.Result;
import org.apache.ibatis.annotations.Results;
import org.apache.ibatis.annotations.Select;
import org.apache.ibatis.mapping.FetchType;

import java.util.List;

public interface AccountDao {/**
     * 根据用户id查询账户信息
     * @param id
     * @return
     */
    @Select("select * from account where uid=#{uid}")
    List<Account> findById(Integer id);
}

1.5 @CacheNamespace

@CacheNamespace(blocking = true)：写在接口名上面，注解方式开启二级缓存

 
参考博客：https://blog.csdn.net/cherlshall/article/details/80950150