springMVC mybatis 数据库sql查询有值 但java返回为空

package com.ssm.entity;

import java.sql.Timestamp;

/**
* Created by Administrator on 2016/6/27.
*/
//学生实体
public class Student {
private long sid ;
private String sname ; //姓名
private String gender ; //性别
private int age ; //年龄
private String address ; //地址
private Timestamp starttime;//开始时间
private Timestamp endtime;//结束时间

//学生中有老师实体

private Teacher teacher ; //老师对象

public long getSid() {
return sid;
}

public void setSid(long sid) {
this.sid = sid;
}

public String getSname() {
return sname;
}

public void setSname(String sname) {
this.sname = sname;
}

public String getGender() {
return gender;
}

public void setGender(String gender) {
this.gender = gender;
}

public int getAge() {
return age;
}

public void setAge(int age) {
this.age = age;
}

public String getAddress() {
return address;
}

public void setAddress(String address) {
this.address = address;
}

public Teacher getTeacher() {
return teacher;
}

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

public Timestamp getStarttime() {
return starttime;
}

public void setStart_time(Timestamp starttime) {
this.starttime = starttime;
}

public Timestamp getEndtime() {
return endtime;
}

public void setEnd_time(Timestamp endtime) {
this.endtime = endtime;
}

@Override
public String toString() {
return "Student{" +
"sid=" + sid +
", sname='" + sname + '\'' +
", gender='" + gender + '\'' +
", age=" + age +
", address='" + address + '\'' +
", teacher=" + teacher +
'}';
}
}

<?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.ssm.dao.StudentDao">

<!--增加学生操作-->
<insert id="insertStudent" parameterType="com.ssm.entity.Student">
INSERT into student(sname,gender,age,address,start_time,end_time)
VALUES(#{sname},#{gender},#{age},#{address},#{starttime},#{endtime});
</insert>

<!--删除学生操作-->
<delete id="deleteStudentById" parameterType="long">
delete from student where sid = #{sid};
</delete>

<!--更新操作-->
<update id="updateStudent" parameterType="com.ssm.entity.Student">
UPDATE student s set s.address = #{address},s.sname=#{sname},
s.gender=#{gender},s.age=#{age},s.start_time=#{starttime},s.end_time=#{endtime} WHERE s.sid = #{sid};
</update>

<!--根据ID查询-->
<select id="getStudentById" parameterType="long" resultType="com.ssm.entity.Student">
select s.address,s.sname,s.gender,s.age,s.start_time as starttime ,s.end_time as endtime from student s WHERE  sid = #{sid};
</select>

<!--查询多个学生-->
<select id="queryAllStudent" resultType="com.ssm.entity.Student">
select s.sid, s.address,s.sname,s.gender,s.age,s.start_time as starttime ,s.end_time as endtime
from student s order by sid desc limit #{offset},#{limit};
</select>
</mapper>