major打印为null

<select id="findStudent" resultMap="forStudent">
select * from student
<if test="id != null">
where id = #{id}
</if>
</select>
<resultMap id="forStudent" type="Student">
<collection property="courses" column="major" ofType="Course" select="selectCourse"></collection>
</resultMap>
<select id="selectCourse" resultType="Course">
select * from course WHERE major_name = #{major}
</select>

import java.util.List;

public class Student {

private int id;
private int reg_no;
private String name;
private String sex;
private int age;
private String grade;
private String major;
private List<Course> courses;

public List<Course> getCourses() {
return courses;
}

public void setCourses(List<Course> courses) {
this.courses = courses;
}

public Student() {
}

public Student(int id) {
this.id = id;
}

public Student(int id, int reg_no, String name, String sex, int age, String grade, String major) {
this.id = id;
this.reg_no = reg_no;
this.name = name;
this.sex = sex;
this.age = age;
this.grade = grade;
this.major = major;
}
public Student(int reg_no, String name, String sex, int age, String grade, String major) {
this.reg_no = reg_no;
this.name = name;
this.sex = sex;
this.age = age;
this.grade = grade;
this.major = major;
}

public int getId() {
return id;
}

@Override
public String toString() {
return "Student{" +
"id=" + id +
", reg_no=" + reg_no +
", name='" + name + '\'' +
", sex='" + sex + '\'' +
", age=" + age +
", grade='" + grade + '\'' +
", major='" + major + '\'' +
", courses=" + courses +
'}';
}

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

public int getReg_no() {
return reg_no;
}

public void setReg_no(int reg_no) {
this.reg_no = reg_no;
}

public String getName() {
return name;
}

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

public String getSex() {
return sex;
}

public void setSex(String sex) {
this.sex = sex;
}

public int getAge() {
return age;
}

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

public String getGrade() {
return grade;
}

public void setGrade(String grade) {
this.grade = grade;
}

public String getMajor() {
return major;
}

public void setMajor(String major) {
this.major = major;
}
}

import java.sql.Date;

public class Course {
private int id;
private String courseid;
private String major_name;
private String courname;
private Date courtime;
private String courteacher;

public Course(int id, String courseid, String major_name, String courname, Date courtime, String courteacher) {
this.id = id;
this.courseid = courseid;
this.major_name = major_name;
this.courname = courname;
this.courtime = courtime;
this.courteacher = courteacher;
}

public String getCourname() {
return courname;
}

public void setCourname(String courname) {
this.courname = courname;
}

public Course() {
}

public Course(int id, String courseid, String major_name, Date courtime, String courteacher) {
this.id = id;
this.courseid = courseid;
this.major_name = major_name;
this.courtime = courtime;
this.courteacher = courteacher;
}

public int getId() {
return id;
}

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

public String getCourseid() {
return courseid;
}

public void setCourseid(String courseid) {
this.courseid = courseid;
}

public String getMajor_name() {
return major_name;
}

public void setMajor_name(String major_name) {
this.major_name = major_name;
}

public Date getCourtime() {
return courtime;
}

public void setCourtime(Date courtime) {
this.courtime = courtime;
}

public String getCourteacher() {
return courteacher;
}

@Override
public String toString() {
return "Course{" +
"id=" + id +
", courseid='" + courseid + '\'' +
", major_name='" + major_name + '\'' +
", courname='" + courname + '\'' +
", courtime=" + courtime +
", courteacher='" + courteacher + '\'' +
'}';
}

public void setCourteacher(String courteacher) {
this.courteacher = courteacher;
}
}