MyBatis select statement returns null values - jdbc

Question

I'm trying to run a simple MyBatis example, selecting all rows from the "trains" table.
The problem is that the query performs, but it returns a list with the correct number of elements, but populated with null values.
The same query runned directly with JDBC PreparedStatement works fine.
Perhaps it's a configuration problem, but I cannot figure out what I'm doing wrong.
Here is the code. Thanks in advance.
Train.java
package org.example.mybatis.domain;
public class Train implements Serializable
{
private int id;
private String type;
// getters and setters
}
TrainMapper.java
package org.example.mybatis.persistence;
public interface TrainMapper {
List<Train> getAllTrains();
}
TrainSelector.java
package org.example.mybatis.test;
public class TrainSelector implements TrainMapper {
private static String resource = "mybatis-config.xml";
private static SqlSessionFactory factory = null;
private SqlSessionFactory getSqlSessionFactory()
{
if (factory == null)
{
try {
InputStream inputStream = Resources.getResourceAsStream(resource);
factory = new SqlSessionFactoryBuilder().build(inputStream);
} catch (IOException e) {
e.printStackTrace();
}
}
return factory;
}
#Override
public List<Train> getAllTrains()
{
List<Train> trains = null;
SqlSession session = getSqlSessionFactory().openSession();
try {
TrainMapper mapper = session.getMapper(TrainMapper.class);
trains = mapper.getAllTrains();
} finally {
session.close();
}
return trains;
}
public static void main(String[] args) {
List<Train> trains = null;
TrainSelector trainSelector = new TrainSelector();
trains = trainSelector.getAllTrains();
System.out.println(trains);
}
}
mybatis-config.xml
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
<properties resource="database.properties" />
<typeAliases>
<typeAlias alias="Train" type="org.example.mybatis.domain.Train" />
<!--package name="org.example.mybatis.domain" />-->
</typeAliases>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC" />
<dataSource type="POOLED">
<property name="driver" value="${database.driver}" />
<property name="url" value="${database.url}" />
<property name="username" value="${database.username}" />
<property name="password" value="${database.password}" />
</dataSource>
</environment>
</environments>
<mappers>
<mapper resource="org/example/mybatis/persistence/TrainMapper.xml" />
</mappers>
</configuration>
TrainMapper.xml
<?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 namespace="org.example.mybatis.persistence.TrainMapper">
<cache />
<select id="getAllTrains" parameterType="list" resultType="Train">
SELECT *
FROM trains
</select>
</mapper>
JdbcStatementExample.java
package org.example.mybatis.test;
public class JdbcStatementExample {
private static void selectAllTrains() throws SQLException
{
String sql = "SELECT * FROM trains";
Connection conn = null;
PreparedStatement ps = null;
ResultSet rs = null;
String url = "jdbc:mysql://localhost/testing";
String user = "test";
String password = "test";
try {
conn = DriverManager.getConnection(url, user, password);
ps = conn.prepareStatement(sql);
rs = ps.executeQuery();
while (rs.next()) {
String id = rs.getString("train_id");
String type = rs.getString("train_type");
System.out.println("id: " + id);
System.out.println("type: " + type);
}
} catch (SQLException e) {
throw new RuntimeException(e);
} finally {
if (ps != null) {
ps.close();
}
if (conn != null) {
conn.close();
}
}
}
public static void main(String[] args)
{
try {
selectAllTrains();
} catch (SQLException e) {
e.printStackTrace();
}
}
}

Answer 1

The names of the columns in the result set are different from the names of the properties in the Train object. You need an explicit result map to let Mybatis know which column is to be mapped to which property.
<resultMap id="trainMap" type="Train>
<id property="id" column="train_id" javaType="java.lang.Integer" jdbcType="INTEGER"/>
<result property="type" column="train_type" javaType="java.lang.String" jdbcType="VARCHAR"/>
</resultMap>
Making your select element into
<select id="getAllTrains" parameterType="list" resultType="trainMap">
SELECT * FROM trains
</select>

Answer 2

Other option is to use column names an aliases.
The column names will be your database's and the aliases will be set to match with your Train object properties:
<select id="getAllTrains" parameterType="list" resultType="trainMap">
SELECT
train_id as id,
train_type as type
FROM trains
</select>

Answer 3

I had the same problem, but only for fields with multiple words. Of course my naming convention in SQL was user_id and in java was userId. This piece of config inside my mybatis-config.xml file saved the day:
<settings>
<setting name="mapUnderscoreToCamelCase" value="false"/>
</settings>
or for properties file:
mybatis.configuration.map-underscore-to-camel-case=true
credit: https://chois9105.github.io/spring/2017/12/31/configuring-mybatis-underscore-to-camel-case.html

Answer 4

Results can be mapped as described by Seeta or in the official docs here:
https://mybatis.org/mybatis-3/sqlmap-xml.html
In MyBatis 3.x the example doesn't work as you need to set resultMap rather than resultType. And you must not set both at the same time! Working example looks like:
<select id="getAllTrains" parameterType="list" resultMap="trainMap">
SELECT * FROM trains
</select>

Answer 5

if you are using spring boot, you can change the map-underscore-to-camel-case property as true like below. because most if the time we use _ (user_id) when create the table attributes. but in java we use camelCase (userId) for the variables. then mybatis don't know about that and when it tries to mapping, the error is thrown.
mybatis.configuration.map-underscore-to-camel-case=true