The teacher of the J2EE class asked us to complete the exercises after the class. The content is to use the nested result method and the nested query method to query the commodity table and the classification table respectively.
The contents of the table are as follows:
First of all, let's build a database and a table.
create DATABASE shop CHARACTER SET utf8 COLLATE utf8_general_ci;
use shop;
CREATE TABLE category
(
id int(32) PRIMARY KEY auto_increment,
typename VARCHAR(56)
)CHARACTER set utf8 COLLATE utf8_general_ci;
INSERT INTO category VALUES(1, '黑色家电');
INSERT INTO category VALUES(2, '白色家电');
CREATE TABLE product
(
id int(32) PRIMARY KEY auto_increment,
goodsname VARCHAR(56),
price DOUBLE,
category_id int(32) not null,
FOREIGN key(category_id) REFERENCES category(id)
)CHARACTER set utf8 COLLATE utf8_general_ci;
INSERT INTO product
VALUES(1,'电视机', 5000,1);
INSERT INTO product
VALUES(2,'冰箱', 4000,2);
INSERT INTO product
VALUES(3,'空调', 3000,2);
INSERT INTO product
VALUES(4,'洗衣机', 2000,2);Then create the entity class
package pojo;
import java.util.List;
public class Category {
private Integer id;
private String typename;
private List<Product> productList;
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getTypename() {
return typename;
}
public void setTypename(String typename) {
this.typename = typename;
}
public List<Product> getProductList() {
return productList;
}
public void setProductList(List<Product> productList) {
this.productList = productList;
}
@Override
public String toString() {
return "Category{" +
"id=" + id +
", typename='" + typename + '\'' +
", productList=" +'\'' + productList+ '\'' +
'}';
}
}
package pojo;
public class Product {
private Integer id;
private String goodsname;
private double price;
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getGoodsname() {
return goodsname;
}
public void setGoodsname(String goodsname) {
this.goodsname = goodsname;
}
public double getPrice() {
return price;
}
public void setPrice(double price) {
this.price = price;
}
@Override
public String toString() {
return "Product{" +
"id=" + id + ", goodsname='" + goodsname +
", price=" + price + '}';
}
}
Write xml file in mapper under Resources
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<!-- mapper为映射的根节点-->
<!-- mapper为映射的根节点，namespace指定Dao接口的完整类名
mybatis会依据这个接口动态创建一个实现类去实现这个接口，
而这个实现类是一个Mapper对象-->
<mapper namespace="pojo.CategoryMapper">
<!--id ="接口中的方法名"
parameterType="传入的参数类型"
resultType = "返回实体类对象，使用包.类名"-->
<!-- 一对多：查看某一商品的类别及其关联的商品信息-->
<select id="findCategoryWithProduct" parameterType="Integer"
resultMap="CategoryWithProductResult">
SELECT c.*, p.id as category_id, p.goodsname, p.price
from category c,
product p
WHERE c.id = p.category_id
and c.id = #{id}
</select>
<resultMap type="pojo.Category" id="CategoryWithProductResult">
<id property="id" column="id"/>
<result property="typename" column="typename"/>
<!-- 一对多关联映射：collection
ofType表示属性集合中元素的类型，List<Product>属性即Product类 -->
<collection property="productList"
ofType="pojo.Product">
<id property="id" column="category_id"/>
<result property="goodsname" column="goodsname"/>
<result property="price" column="price"/>
</collection>
</resultMap>
</mapper>
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<!-- mapper为映射的根节点-->
<!-- mapper为映射的根节点，namespace指定Dao接口的完整类名
mybatis会依据这个接口动态创建一个实现类去实现这个接口，
而这个实现类是一个Mapper对象-->
<mapper namespace="pojo.ProductMapper">
<!--id ="接口中的方法名"
parameterType="传入的参数类型"
resultType = "返回实体类对象，使用包.类名"-->
<!-- 一对多：查看某一商品的类别及其关联的商品信息-->
<select id="findProductById" parameterType="Integer" resultMap="Product">
select * from product where category_id IN(select id from category where id=#{id})
</select>
<resultMap type="pojo.Product" id="Product">
<id property="id" column="id"/>
<result property="typename" column="typename"/>
<!-- 一对多关联映射：collection
ofType表示属性集合中元素的类型，List<Product>属性即Product类 -->
</resultMap>
</mapper>
Then write the query statement in the test class
public class findCategoryTest {
@Test
public void findCategoryTest() {
String resources = "mybatis-config.xml";
Reader reader = null;
try {
reader = Resources.getResourceAsReader(resources);
} catch (IOException e) {
e.printStackTrace();
}
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);
SqlSession sqlSession = sqlSessionFactory.openSession();
/**
* author 诉衷情の麻雀
* date 2023/3/24
* 查询id为2的商品类信息
*/
String statement = "findCategoryWithProduct";
Category category = new Category();
category.setId(2);
category = sqlSession.selectOne(statement, category);
System.out.print(category);
sqlSession.close();
}
@Test
public void findProductTest() {
String resources = "mybatis-config.xml";
Reader reader = null;
try {
reader = Resources.getResourceAsReader(resources);
} catch (IOException e) {
e.printStackTrace();
}
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);
SqlSession sqlSession = sqlSessionFactory.openSession();
String statement = "findProductById";
Product product = new Product();
// product.setId(1);
product= sqlSession.selectOne(statement, 1);
System.out.print(product);
sqlSession.close();
}
}
It should be noted that in the xml file, the previously mapped statement is resultType but when mapped to an entity class, it is ResultMap
If you have the following error report in addition to checking the mybatis_config configuration, it is possible that there is a problem here
