In normal development, the field names in our table and the attribute names of the entity class corresponding to the table are not necessarily exactly the same. Let's demonstrate how to resolve the conflict between the field name and the entity class attribute name in this case.
1. Prepare the tables and data needed for the presentation
CREATE TABLE orders( order_id INT PRIMARY KEY AUTO_INCREMENT, order_no VARCHAR(20), order_price FLOAT ); INSERT INTO orders(order_no, order_price) VALUES('aaaa', 23); INSERT INTO orders(order_no, order_price) VALUES('bbbb', 33); INSERT INTO orders(order_no, order_price) VALUES('cccc', 22);
Second, define the entity class
1 package me.gacl.domain; 2 3 /** 4 * @author gacl 5 * Define the entity class corresponding to the orders table 6 */ 7 public class Order { 8 /** 9 * 10 CREATE TABLE orders( 11 order_id INT PRIMARY KEY AUTO_INCREMENT, 12 order_no VARCHAR(20), 13 order_price FLOAT 14 ); 15 */ 16 17 // The attribute name in the Order entity class and the field name in the orders table are different 18 private int id; //id===>order_id 19 private String orderNo; //orderNo===>order_no 20 private float price; //price===>order_price 21 22 public int getId() { 23 return id; 24 } 25 26 public void setId(int id) { 27 this.id = id; 28 } 29 30 public String getOrderNo() { 31 return orderNo; 32 } 33 34 public void setOrderNo(String orderNo) { 35 this.orderNo = orderNo; 36 } 37 38 public float getPrice() { 39 return price; 40 } 41 42 public void setPrice(float price) { 43 this.price = price; 44 } 45 46 @Override 47 public String toString() { 48 return "Order [id=" + id + ", orderNo=" + orderNo + ", price=" + price+ "]"; 49 } 50 }
3. Write test code
3.1, write SQL xml mapping file
1. Create an orderMapper.xml file. The content of orderMapper.xml is as follows:
1 <?xml version="1.0" encoding="UTF-8" ?> 2 <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd"> 3 <!-- 为这个mapper指定一个唯一的namespace，namespace的值习惯上设置成包名+sql映射文件名，这样就能够保证namespace的值是唯一的 4 例如namespace="me.gacl.mapping.orderMapper"就是me.gacl.mapping(包名)+orderMapper(orderMapper.xml文件去除后缀) 5 --> 6 <mapper namespace="me.gacl.mapping.orderMapper"> 7 8 <!-- 9 根据id查询得到一个order对象，使用这个查询是查询不到我们想要的结果的， 10 这主要是因为实体类的属性名和数据库的字段名对应不上的原因，因此无法查询出对应的记录 11 --> 12 <select id="getOrderById" parameterType="int" 13 resultType="me.gacl.domain.Order"> 14 select * from orders where order_id=#{id} 15 </select> 16 17 <!-- 18 根据id查询得到一个order对象，使用这个查询是可以正常查询到我们想要的结果的， 19 这是因为我们将查询的字段名都起一个和实体类属性名相同的别名，这样实体类的属性名和查询结果中的字段名就可以一一对应上 20 --> 21 <select id="selectOrder" parameterType="int" 22 resultType="me.gacl.domain.Order"> 23 select order_id id, order_no orderNo,order_price price from orders where order_id=#{id} 24 </select> 25 26 <!-- 27 根据id查询得到一个order对象，使用这个查询是可以正常查询到我们想要的结果的， 28 这是因为我们通过<resultMap>映射实体类属性名和表的字段名一一对应关系 --> 29 <select id="selectOrderResultMap" parameterType="int" resultMap="orderResultMap"> 30 select * from orders where order_id=#{id} 31 </select> 32 <!--通过<resultMap>映射实体类属性名和表的字段名对应关系 --> 33 <resultMap type="me.gacl.domain.Order" id="orderResultMap"> 34 <!-- 用id属性来映射主键字段 --> 35 <id property="id" column="order_id"/> 36 <!-- 用result属性来映射非主键字段 --> 37 <result property="orderNo" column="order_no"/> 38 <result property="price" column="order_price"/> 39 </resultMap> 40 41 </mapper>
2、在conf.xml文件中注册orderMapper.xml映射文件
<mappers> <!-- 注册orderMapper.xml文件， orderMapper.xml位于me.gacl.mapping这个包下，所以resource写成me/gacl/mapping/orderMapper.xml--> <mapper resource="me/gacl/mapping/orderMapper.xml"/> </mappers>
3.2、编写单元测试代码
1 package me.gacl.test; 2 3 import me.gacl.domain.Order; 4 import me.gacl.util.MyBatisUtil; 5 import org.apache.ibatis.session.SqlSession; 6 import org.junit.Test; 7 8 public class Test2 { 9 10 @Test 11 public void testGetOrderById(){ 12 SqlSession sqlSession = MyBatisUtil.getSqlSession(); 13 /** 14 * 映射sql的标识字符串， 15 * me.gacl.mapping.orderMapper是orderMapper.xml文件中mapper标签的namespace属性的值， 16 * getOrderById是select标签的id属性值，通过select标签的id属性值就可以找到要执行的SQL 17 */ 18 String statement = "me.gacl.mapping.orderMapper.getOrderById";//映射sql的标识字符串 19 //执行查询操作，将查询结果自动封装成Order对象返回 20 Order order = sqlSession.selectOne(statement,1);//查询orders表中id为1的记录 21 //使用SqlSession执行完SQL之后需要关闭SqlSession 22 sqlSession.close(); 23 System.out.println(order);//打印结果：null，也就是没有查询出相应的记录 24 } 25 26 @Test 27 public void testGetOrderById2(){ 28 SqlSession sqlSession = MyBatisUtil.getSqlSession(); 29 /** 30 * 映射sql的标识字符串， 31 * me.gacl.mapping.orderMapper是orderMapper.xml文件中mapper标签的namespace属性的值， 32 * selectOrder是select标签的id属性值，通过select标签的id属性值就可以找到要执行的SQL 33 */ 34 String statement = "me.gacl.mapping.orderMapper.selectOrder";//映射sql的标识字符串 35 //执行查询操作，将查询结果自动封装成Order对象返回 36 Order order = sqlSession.selectOne(statement,1);//查询orders表中id为1的记录 37 //使用SqlSession执行完SQL之后需要关闭SqlSession 38 sqlSession.close(); 39 System.out.println(order);//打印结果：Order [id=1, orderNo=aaaa, price=23.0] 40 } 41 42 @Test 43 public void testGetOrderById3(){ 44 SqlSession sqlSession = MyBatisUtil.getSqlSession(); 45 /** 46 * 映射sql的标识字符串， 47 * me.gacl.mapping.orderMapper是orderMapper.xml文件中mapper标签的namespace属性的值， 48 * selectOrderResultMap是select标签的id属性值，通过select标签的id属性值就可以找到要执行的SQL 49 */ 50 String statement = "me.gacl.mapping.orderMapper.selectOrderResultMap";//映射sql的标识字符串 51 //执行查询操作，将查询结果自动封装成Order对象返回 52 Order order = sqlSession.selectOne(statement,1);//查询orders表中id为1的记录 53 //使用SqlSession执行完SQL之后需要关闭SqlSession 54 sqlSession.close(); 55 System.out.println(order);//打印结果：Order [id=1, orderNo=aaaa, price=23.0] 56 } 57 }
执行单元测试的结果：
1、testGetOrderById方法执行查询后返回一个null。
2. After the testGetOrderById2 method and the testGetOrderById3 method execute the query, the desired result can be obtained normally.
4. Summary
The above test code demonstrates the problem that when the attribute name in the entity class is inconsistent with the field name in the table, the corresponding result cannot be queried when using MyBatis for query operation, and two methods are adopted for the problem:
Solution 1 : Define the alias of the field name in the sql statement of the query , so that the alias of the field name is consistent with the attribute name of the entity class, so that the field name of the table and the attribute name of the entity class can be in one-to-one correspondence. The way is to solve the mapping relationship between field names and attribute names by defining aliases in SQL statements.
Solution 2 : Use <resultMap> to map the one-to-one correspondence between field names and entity class attribute names. This method uses the solution provided by MyBatis to solve the mapping relationship between field names and attribute names.