First, prepare the table and need to use the data 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, the class definition entities
1 package me.gacl.domain;
2
3 /**
4 * @author gacl
5 * 定义orders表对应的实体类
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 //Order实体类中属性名和orders表中的字段名是不一样的
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 }Third, writing test code
3.1, write the SQL xml mapping files
1, create a orderMapper.xml file, the contents of orderMapper.xml 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, registration orderMapper.xml mapping file conf.xml file
<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, test code write unit
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 }Results unit testing:
1, testGetOrderById method After executing the query returns a null.
2, testGetOrderById2 methods and testGetOrderById3 method After executing a query can normally get the desired results.
IV Summary
The problem can not check out when the corresponding results of the above test code demonstrates the inconsistency when the entity class attributes and field names in the table, using MyBatis query operations and adopted two approaches for the problem:
Solution one: by defining field names in the sql statement query aliases, so that the property name and alias entity class field names the same, so that you can attribute name table field names and entities on the type of correspondence, such way is through over-defined alias in sql statement to resolve the mapping between field names and attribute names are.
Solution two: one relationship to the entity class mapping field names and attribute names by <resultMap>. This approach is the use of solutions provided by MyBatis to address mapping between field names and attribute names are.
Reproduced in: https: //my.oschina.net/zhanghaiyang/blog/606233