MyBatis select语句返回空值 [英] MyBatis select statement returns null values

问题描述

我试图运行一个简单的MyBatis示例，从火车"表中选择所有行.

I'm trying to run a simple MyBatis example, selecting all rows from the "trains" table.

问题是查询执行，但是它返回具有正确数目的元素但填充有空值的列表. 直接使用JDBC PreparedStatement运行的同一查询工作正常.

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.

也许这是一个配置问题，但我无法弄清楚自己在做错什么.

Perhaps it's a configuration problem, but I cannot figure out what I'm doing wrong.

这是代码.预先感谢.

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();
        }
    }
}

推荐答案

结果集中的列名称与Train对象中的属性名称不同.您需要一个明确的结果映射，以使Mybatis知道要将哪个列映射到哪个属性.

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>

将选择元素放入

<select id="getAllTrains" parameterType="list" resultType="trainMap">
    SELECT * FROM trains
</select>

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