引言

MyBatis是一款优秀的持久层框架，它提供了灵活且强大的关联关系配置功能。本文将介绍MyBatis中一对一、一对多和多对多关联关系的配置方法和实践经验，帮助读者更好地理解和应用这些关系。

一对一关联关系配置
一对一关联关系表示两个实体之间具有唯一对应关系。在MyBatis中配置一对一关联关系需要使用标签和标签。下面是一个示

一对一关联关系配置

例：

<resultMap id="OrderItemVoMap" type="com.niyin.vo.OrderltemVo">
  <result column="order_item_id" property="orderItemId"></result>
  <result column="product_id" property="productId"></result>
  <result column="quantity" property="quantity"></result>
  <result column="oid" property="oid"></result>
  <association property="order" javaType="com.niyin.model.Order">
    <result column="order_id" property="orderId"></result>
    <result column="order_no" property="orderNo"></result>
  </association>
</resultMap>
  
  
  
  <select id="selectByOrderItem" resultMap="OrderItemVoMap" parameterType="java.lang.Integer">
select * from t_hibernate_order o,
t_hibernate_order_item oi where o.order_id=oi.oid and oi.order_item_id=#{oiid}
  </select>

orderitemMap

package com.niyin.mapper;

import com.niyin.model.Orderitem;
import com.niyin.vo.OrderVo;
import com.niyin.vo.OrderltemVo;
import org.apache.ibatis.annotations.Param;

public interface OrderitemMapper {
    int deleteByPrimaryKey(Integer orderItemId);

    int insert(Orderitem record);

    int insertSelective(Orderitem record);

    Orderitem selectByPrimaryKey(Integer orderItemId);

    int updateByPrimaryKeySelective(Orderitem record);

    int updateByPrimaryKey(Orderitem record);

    OrderltemVo selectByOrderItem(@Param("oiid") Integer oiid);
}

biz

package com.niyin.biz;

import com.niyin.vo.OrderltemVo;
import org.apache.ibatis.annotations.Param;

public interface OrderItemBiz {
    OrderltemVo selectByOrderItem(Integer oiid);

}

impl

package com.niyin.biz.impl;

import com.niyin.biz.OrderItemBiz;
import com.niyin.mapper.OrderitemMapper;
import com.niyin.vo.OrderltemVo;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

@Service
public class OrderItemBizImpl implements OrderItemBiz {
    @Autowired
    private OrderitemMapper orderitemMapper;
    @Override
    public OrderltemVo selectByOrderItem(Integer oiid) {
        return orderitemMapper.selectByOrderItem(oiid);
    }
}

测试

package com.niyin.biz.impl;

import com.niyin.biz.Hboobiz;
import com.niyin.biz.HcategoryBiz;
import com.niyin.biz.OrderBiz;
import com.niyin.biz.OrderItemBiz;
import com.niyin.vo.CategoryVo;
import com.niyin.vo.HbookVo;
import com.niyin.vo.OrderVo;
import com.niyin.vo.OrderltemVo;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.test.context.ContextConfiguration;
import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;

import static org.junit.Assert.*;
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(locations={"classpath:spring-context.xml"})
public class OrderBizImplTest {

    @Autowired
    private OrderBiz orderBiz;
@Autowired
private OrderItemBiz orderItemBiz;

@Autowired
private Hboobiz hboobiz;


@Autowired
private HcategoryBiz hcategoryBiz;
    @Test
    public void selectByOid() {
       OrderVo orderVo= orderBiz.selectByOid(7);
        System.out.println(orderVo);
        orderVo.getOrderitems().forEach(System.out::println);
    }

@Test
    public void selectByOItem() {
   OrderltemVo orderltemVo= orderItemBiz.selectByOrderItem(27);
    System.out.println(orderltemVo);
    System.out.println(orderltemVo.getOrder());
}
@Test
    public void selectBybook(){
    HbookVo hbookVo = this.hboobiz.selectByBookID(8);
hbookVo.getCategories().forEach(System.out::println);
}

    @Test
    public void selectBcid(){
        CategoryVo categoryVo = this.hcategoryBiz.selectBycategoryId(8);
        System.out.println(categoryVo);
        categoryVo.getBooks().forEach(System.out::println);
    }


    }

下面案列均用这个测试类
结果

上述代码中，定义了orderitem实体的映射规则，表示与User实体关联的Profile实体，通过property属性指定了关联属性的名称，通过javaType属性指定了关联属性的类型。在SQL查询语句中，可以通过JOIN操作获取关联实体的数据。

一对多关联关系配置

一对多关联关系表示一个实体与多个实体之间存在关联关系。在MyBatis中配置一对多关联关系需要使用标签和标签。下面是一个示例：

 <resultMap id="OrderVoMap" type="com.niyin.vo.OrderVo" >
   <result column="order_id" property="orderId"></result>
    <result column="order_no" property="orderNo"></result>
    <collection property="orderitems" ofType="com.niyin.model.Orderitem">
      <result column="order_item_id" property="orderItemId"></result>
      <result column="product_id" property="productId"></result>
      <result column="quantity" property="quantity"></result>
      <result column="oid" property="oid"></result>
    </collection>
  </resultMap>

<select id="selectByOid" resultMap="OrderVoMap" parameterType="java.lang.Integer">
select * from t_hibernate_order o,
t_hibernate_order_item oi where o.order_id=oi.oid and o.order_id=#{oid}
</select>

vo

package com.niyin.vo;

import com.niyin.model.Order;
import com.niyin.model.Orderitem;

import java.util.ArrayList;
import java.util.List;

public class OrderVo extends Order {

private List<Orderitem> orderitems =new ArrayList<Orderitem>();

    public List<Orderitem> getOrderitems() {
        return orderitems;
    }
    public void setOrderitems(List<Orderitem> orderitems) {
        this.orderitems = orderitems;
    }

}

map

package com.niyin.mapper;

import com.niyin.model.Order;
import com.niyin.vo.OrderVo;
import org.apache.ibatis.annotations.Param;

public interface OrderMapper {
    int deleteByPrimaryKey(Integer orderId);

    int insert(Order record);

    int insertSelective(Order record);

    Order selectByPrimaryKey(Integer orderId);

    int updateByPrimaryKeySelective(Order record);

    int updateByPrimaryKey(Order record);

    OrderVo selectByOid(@Param("oid") Integer oid);

}

biz

package com.niyin.biz;


import com.niyin.vo.OrderVo;
import org.apache.ibatis.annotations.Param;

public interface OrderBiz  {
    OrderVo selectByOid(Integer oid);

}

impl

package com.niyin.biz.impl;

import com.niyin.biz.OrderBiz;
import com.niyin.mapper.OrderMapper;
import com.niyin.vo.OrderVo;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

@Service
public class OrderBizImpl implements OrderBiz {
@Autowired
private OrderMapper orderMapper;

    @Override
    public OrderVo selectByOid(Integer oid) {
        return orderMapper.selectByOid(oid);
    }
}

结果

结论

上述代码中，定义了Post实体的映射规则，表示与Post实体关联的Comment实体集合，通过property属性指定了关联属性的名称，通过ofType属性指定了集合中元素的类型。在SQL查询语句中，可以利用外键约束和嵌套查询等方式获取关联实体集合的数据。

多对多关联关系配置

多对多关联关系表示两个实体之间存在互相包含的关系。在MyBatis中配置多对多关联关系需要使用中间表和联合主键。下面是一个示例：

 <resultMap id="HbookMap" type="com.niyin.vo.HbookVo" >
<result column="book_id" property="bookId"></result>
    <result column="book_name" property="bookName"></result>
    <result column="price" property="price"></result>
    <collection property="categories" ofType="com.niyin.model.HCategory">

      <result column="category_id" property="categoryId"></result>
      <result column="category_name" property="categoryName"></result>
    </collection>

  </resultMap>

  <select id="selectByBookID" resultMap="HbookMap" parameterType="java.lang.Integer">
    select * from t_hibernate_book b,t_hibernate_book_category bc,t_hibernate_category c
where b.book_id =bc.bid and bc.cid=c.category_id and b.book_id=#{bid}
  </select>

vo

package com.niyin.vo;

import com.niyin.model.HBook;
import com.niyin.model.HCategory;

import java.util.List;

public class HbookVo extends HBook {


    private List<HCategory> categories;

    public List<HCategory> getCategories() {
        return categories;
    }

    public void setCategories(List<HCategory> categories) {
        this.categories = categories;
    }
}

map

package com.niyin.mapper;

import com.niyin.model.HBook;
import com.niyin.vo.HbookVo;
import org.apache.ibatis.annotations.Param;

public interface HBookMapper {
    int deleteByPrimaryKey(Integer bookId);

    int insert(HBook record);

    int insertSelective(HBook record);

    HBook selectByPrimaryKey(Integer bookId);

    int updateByPrimaryKeySelective(HBook record);

    int updateByPrimaryKey(HBook record);
    HbookVo selectByBookID(@Param("bid") Integer bid) ;
}

biz

package com.niyin.biz;

import com.niyin.vo.HbookVo;
import org.apache.ibatis.annotations.Param;

public interface Hboobiz {
    HbookVo selectByBookID( Integer bid) ;

}

impl

package com.niyin.biz;

import com.niyin.vo.HbookVo;
import org.apache.ibatis.annotations.Param;

public interface Hboobiz {
    HbookVo selectByBookID( Integer bid) ;

}

运行结果

示例2：

<resultMap id="categoryvoMap" type="com.niyin.vo.CategoryVo">
    <result column="category_id" property="categoryId"></result>
    <result column="category_name" property="categoryName"></result>
    <collection property="books" ofType="com.niyin.model.HBook">
      <result column="book_id" property="bookId"></result>
      <result column="book_name" property="bookName"></result>
      <result column="price" property="price"></result>
    </collection>
  </resultMap>
  
<select id="selectBycategoryId" resultMap="categoryvoMap" parameterType="java.lang.Integer">
  select * from t_hibernate_book b,t_hibernate_book_category bc,t_hibernate_category c
where b.book_id =bc.bid and bc.cid=c.category_id and c.category_id=#{cid}
</select>

vo

package com.niyin.vo;

import com.niyin.model.HBook;
import com.niyin.model.HCategory;

import java.util.ArrayList;
import java.util.List;

public class CategoryVo extends HCategory {
    private List<HBook>books=new ArrayList<>();

    public List<HBook> getBooks() {
        return books;
    }

    public void setBooks(List<HBook> books) {
        this.books = books;
    }
}

map

package com.niyin.mapper;

import com.niyin.model.HCategory;
import com.niyin.vo.CategoryVo;
import org.apache.ibatis.annotations.Param;

public interface HCategoryMapper {
    int deleteByPrimaryKey(Integer categoryId);

    int insert(HCategory record);

    int insertSelective(HCategory record);

    HCategory selectByPrimaryKey(Integer categoryId);

    int updateByPrimaryKeySelective(HCategory record);

    int updateByPrimaryKey(HCategory record);
    CategoryVo selectBycategoryId(@Param("cid") Integer cid);
}

biz

package com.niyin.biz;

import com.niyin.vo.CategoryVo;
import org.apache.ibatis.annotations.Param;

public interface HcategoryBiz {
    CategoryVo selectBycategoryId(Integer cid);

}

impl

package com.niyin.biz.impl;

import com.niyin.biz.HcategoryBiz;
import com.niyin.mapper.HCategoryMapper;
import com.niyin.vo.CategoryVo;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

@Service
public class HcategoryBizImpl implements HcategoryBiz {
    @Autowired
    private HCategoryMapper hCategoryMapper;
    @Override
    public CategoryVo selectBycategoryId(Integer cid) {
        return hCategoryMapper.selectBycategoryId(cid);
    }
}

结果

上述代码中，定义了User实体和Role实体的映射规则，并通过property属性指定了互相关联的属性名称。在SQL查询语句中，需要通过联合主键和中间表来获取两个实体之间的关联数据。

结论

本文介绍了MyBatis中一对一、一对多和多对多关联关系的配置方法和实践经验。在一对一关联关系中，使用标签配置关联属性映射；在一对多关联关系中，使用标签配置关联集合属性映射；在多对多关联关系中，需要通过中间表和联合主键来配置关联关系。通过灵活运用这些配置方法，可以轻松处理各种复杂的关联查询需求。

希望本文能够帮助读者更好地理解和应用MyBatis中的关联关系配置，并在实际项目中发挥作用。感谢阅读！