关于mybatis多表联查返回null的问题

[magic_nian]

上代码

实体类：Classes

package com.lnn.mybatis.entity;

public class Classes {

	//定义实体类的属性，与class表中的字段对应
    private int id;            //id===>c_id
    private String name;    //name===>c_name
    
    /**
     * class表中有一个teacher_id字段，所以在Classes类中定义一个teacher属性，
     * 用于维护teacher和class之间的一对一关系，通过这个teacher属性就可以知道这个班级是由哪个老师负责的
     */
    private Teacher teacher;

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Teacher getTeacher() {
        return teacher;
    }

    public void setTeacher(Teacher teacher) {
        this.teacher = teacher;
    }

    @Override
    public String toString() {
        return "Classes [id=" + id + ", name=" + name + ", teacher=" + teacher+ "]";
    }
}

实体类：Teacher

package com.lnn.mybatis.entity;

public class Teacher {

	//定义实体类的属性，与teacher表中的字段对应
    private int id;            //id===>t_id
    private String name;    //name===>t_name

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "Teacher [id=" + id + ", name=" + name + "]";
    }
}

表结构：




映射文件：

<?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="com.lnn.mybatis.mapper.ClassMapper1">
  <resultMap type="com.lnn.mybatis.entity.Classes" id="ClassResultMap">
    <id property="id" column="c_id" />
    <result property="name" column="c_name" />
    <association property="teacher" javaType="com.lnn.mybatis.entity.Teacher">
      <id property="id" column="t_id" />
      <result property="name" column="t_name" />
    </association>
  </resultMap>


  <select id="getClass" parameterType="int" resultMap="ClassResultMap">
    select * from class c, teacher t where c.teacher_id=t.t_id and c.c_id=#{id}
  </select>

</mapper>

接口：

package com.lnn.mybatis.mapper;

import org.apache.ibatis.annotations.Param;
import org.apache.ibatis.annotations.Select;

import com.lnn.mybatis.entity.Classes;

public interface ClassMapper {
	
	@Select("select * from class c, teacher t where c.teacher_id=t.t_id and c.c_id=#{id}")
    public Classes getClass(@Param(value="id") int id);
}

测试类1：

package com.lnn.mybatis.test;

import java.io.InputStream;

import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.Test;

import com.lnn.mybatis.entity.Classes;
import com.lnn.mybatis.mapper.ClassMapper;

public class TestCRUDAnnotationMapper {
	
	@Test
	public void test(){
		String resource = "config.xml";
		InputStream is = TestCRUDAnnotationMapper.class.getClassLoader().getResourceAsStream(resource);
		SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(is);
		SqlSession session = factory.openSession();
//		String statement = "com.lnn.mybatis.mapper.ClassMapper1.getClass";
//		Classes clazz = session.selectOne(statement, 1);
		ClassMapper mapper = session.getMapper(ClassMapper.class);
		Classes clazz = mapper.getClass(1);
//		HashMap<String,Object> map  = mapper.getClass(1);
		session.close();
		System.out.println(clazz);
	}

}

测试类2：

package com.lnn.mybatis.test;

import java.io.InputStream;

import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.Test;

import com.lnn.mybatis.entity.Classes;
import com.lnn.mybatis.mapper.ClassMapper;

public class TestCRUDAnnotationMapper {
	
	@Test
	public void test(){
		String resource = "config.xml";
		InputStream is = TestCRUDAnnotationMapper.class.getClassLoader().getResourceAsStream(resource);
		SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(is);
		SqlSession session = factory.openSession();
		String statement = "com.lnn.mybatis.mapper.ClassMapper1.getClass";
		Classes clazz = session.selectOne(statement, 1);
//		ClassMapper mapper = session.getMapper(ClassMapper.class);
//		Classes clazz = mapper.getClass(1);
//		HashMap<String,Object> map  = mapper.getClass(1);
		session.close();
		System.out.println(clazz);
	}

}

问题：我用测试类1，打印出的结果是null,用测试类2打印出的结果是正确的，

用单表做测试的时候，测试类1和2返回相同的结果，我想问下mybatis多表联查时返回的值是不是有什么讲究，求解答。