我正在尝试用Java创建双向ManyToMany模型,在数据库中插入数据没有问题,但是当我尝试从这些表中检索数据时,它将开始无限递归...我尝试了此baeldung tutorial中的解决方案
但是它们都不对我有用,也许我把注解放错了地方。
我需要找到一名学生,以及分配给该学生的课程是什么。
数据库
楷模
学生
@Component
@Entity
@Table(name="Student")
public class Student {
@Id
@Column(name="student_id")
@GeneratedValue(strategy=GenerationType.IDENTITY)
private Long id;
@Column(name="student_name")
private String name;
@Column(name="student_address")
private String address;
@Column(name="student_email")
private String email;
@Column(name="student_username")
private String username;
@Column(name="student_password")
private String password;
@JsonManagedReference
@OneToMany(fetch = FetchType.LAZY, mappedBy="pk.student")
private Set<StudentCourse> studentCourses;
public Student() {
studentCourses = new HashSet<StudentCourse>(0);
}
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getAddress() {
return address;
}
public void setAddress(String address) {
this.address = address;
}
public String getEmail() {
return email;
}
public void setEmail(String email) {
this.email = email;
}
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
public Set<StudentCourse> getCourses() {
return studentCourses;
}
public void setCourses(Set<StudentCourse> studentCourses) {
this.studentCourses = studentCourses;
}
}
课程
@Component
@Entity
@Table(name="Course")
public class Course {
@Id
@Column(name="course_id")
@GeneratedValue(strategy=GenerationType.IDENTITY)
private Long id;
@Column(name="course_name")
private String name;
@Column(name="course_schedule")
private String schedule;
@ManyToOne
@JoinColumn(name = "teacher_id")
private Teacher teacher;
@JsonBackReference
@OneToMany(fetch = FetchType.LAZY, mappedBy="pk.course")
private Set<StudentCourse> studentCourses = new HashSet<StudentCourse>(0);
public Course() {
}
public Teacher getTeacher() {
return teacher;
}
public void setTeacher(Teacher teacher) {
this.teacher = teacher;
}
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getSchedule() {
return schedule;
}
public void setSchedule(String schedule) {
this.schedule = schedule;
}
public Set<StudentCourse> getStudentCourses() {
return studentCourses;
}
public void setStudentCourses(Set<StudentCourse> studentCourses) {
this.studentCourses = studentCourses;
}
}
学生课程
@Component
@Entity
@Table(name="Student_Course")
@AssociationOverrides({
@AssociationOverride(name = "pk.student",
joinColumns = @JoinColumn(name = "student_id")),
@AssociationOverride(name = "pk.course",
joinColumns = @JoinColumn(name = "course_id")) })
public class StudentCourse implements java.io.Serializable {
@EmbeddedId
private StudentCourseId pk = new StudentCourseId();
public StudentCourseId getPk() {
return pk;
}
public void setPk(StudentCourseId pk) {
this.pk = pk;
}
@Transient
public Student getStudent() {
return getPk().getStudent();
}
public void setStudent(Student student) {
getPk().setStudent(student);
}
@Transient
public Course getCourse() {
return getPk().getCourse();
}
public void setCourse(Course course) {
getPk().setCourse(course);
}
}
学生课程编号
@Embeddable
public class StudentCourseId implements java.io.Serializable {
@ManyToOne
private Student student;
@ManyToOne
private Course course;
public Student getStudent() {
return student;
}
public void setStudent(Student student) {
this.student = student;
}
public Course getCourse() {
return course;
}
public void setCourse(Course course) {
this.course = course;
}
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
StudentCourseId that = (StudentCourseId) o;
if (student != null ? !student.equals(that.student) : that.student != null) return false;
if (course != null ? !course.equals(that.course) : that.course != null)
return false;
return true;
}
public int hashCode() {
int result;
result = (student != null ? student.hashCode() : 0);
result = 31 * result + (course != null ? course.hashCode() : 0);
return result;
}
}
服务
//find a student by its ID
public Optional<Student> getStudentById(Long studentID) throws SQLException{
return studentRepo.findById(studentID);
}
控制者
//find a student by its ID
@GetMapping("/findStudent/{studentID}")
public ResponseEntity<?> getStudentById(@PathVariable Long studentID){
student = sts.getStudentById(studentID).orElse(new Student());
return new ResponseEntity<>(student, HttpStatus.OK);
}
我的Json回应
我希望在“ studentCourses”下,有该学生分配的课程列表,但我得到的只是同一个学生的无限递归...
提前致谢。
最佳答案
使用@JsonIdentityInfo
可以正常工作,在类之间的关系中没有任何改变。
课程
@Component
@JsonIdentityInfo(generator=ObjectIdGenerators.PropertyGenerator.class, property="id")
@Entity
@Table(name="Course")
public class Course {
@Id
@Column(name="course_id")
@GeneratedValue(strategy=GenerationType.IDENTITY)
private Long id;
@Column(name="course_name")
private String name;
@Column(name="course_schedule")
private String schedule;
@ManyToOne
@JoinColumn(name = "teacher_id")
private Teacher teacher;
@OneToMany(fetch = FetchType.LAZY, mappedBy="pk.course")
private Set<StudentCourse> studentCourses = new HashSet<StudentCourse>(0);
public Course() {
}
/*
@ManyToOne(cascade = CascadeType.PERSIST)
@JoinColumn(name="teacher_id")
public Teacher getTeacher() {
return teacher;
}
*/
public Teacher getTeacher() {
return teacher;
}
public void setTeacher(Teacher teacher) {
this.teacher = teacher;
}
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getSchedule() {
return schedule;
}
public void setSchedule(String schedule) {
this.schedule = schedule;
}
public Set<StudentCourse> getStudentCourses() {
return studentCourses;
}
public void setStudentCourses(Set<StudentCourse> studentCourses) {
this.studentCourses = studentCourses;
}
}
学生
@Component
@JsonIdentityInfo(generator=ObjectIdGenerators.PropertyGenerator.class, property="id")
@Entity
@Table(name="Student")
public class Student {
@Id
@Column(name="student_id")
@GeneratedValue(strategy=GenerationType.IDENTITY)
private Long id;
@Column(name="student_name")
private String name;
@Column(name="student_address")
private String address;
@Column(name="student_email")
private String email;
@Column(name="student_username")
private String username;
@Column(name="student_password")
private String password;
@OneToMany(fetch = FetchType.LAZY, mappedBy="pk.student")
private Set<StudentCourse> studentCourses;
public Student() {
studentCourses = new HashSet<StudentCourse>(0);
}
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getAddress() {
return address;
}
public void setAddress(String address) {
this.address = address;
}
public String getEmail() {
return email;
}
public void setEmail(String email) {
this.email = email;
}
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
public Set<StudentCourse> getCourses() {
return studentCourses;
}
public void setCourses(Set<StudentCourse> studentCourses) {
this.studentCourses = studentCourses;
}
}