mybatis注解建立實體類屬性與數據庫表中列的對應關系

1.1 @Results

@Results注解來映射查詢結果集到實體類屬性

（1）@Results的基本用法。當數據庫字段名與實體類對應的屬性名不一致時，可以使用@Results映射來將其對應起來。

column為數據庫字段名，porperty為實體類屬性名，jdbcType為數據庫字段數據類型，id為是否為主鍵。

 

    @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();

 

如上所示的數據庫字段名id與實體類屬性名userId，就通過這種方式建立了映射關系。名字相同的可以省略。

1.2 @ResultMap

（2）@ResultMap的用法。當這段@Results代碼需要在多個方法用到時，為了提高代碼復用性，我們可以為這個@Results注解設置id，然后使用@ResultMap注解來復用這段代碼。

　　@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()注解來引用

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

1.3 @One

（3）@One的用法。當我們需要通過查詢到的一個字段值作為參數，去執行另外一個方法來查詢關聯的內容，而且兩者是一對一關系時，可以使用@One注解來便捷的實現。

一對一：一般都用立即加載

一對多：一般都用懶加載/延遲加載

fetchType = FetchType.EAGER //設置為立即加載

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 {
    /**
     * 查詢所有賬戶，並且獲取每個賬戶所屬的用戶信息
     *
     * @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。與@One類似，只不過如果使用@One查詢到的結果是多行，會拋出TooManyResultException異常，這種時候應該使用的是@Many注解，實現一對多的查詢

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 {/**
     * 根據用戶id查詢賬戶信息
     * @param id
     * @return
     */
    @Select("select * from account where uid=#{uid}")
    List<Account> findById(Integer id);
}

1.5 @CacheNamespace

@CacheNamespace(blocking = true)：寫在接口名上面，注解方式開啟二級緩存

 

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