Many-to-Many Read
💪 Tantangan Utama: Lazy Loading
Secara default, relasi @ManyToMany di Hibernate bersifat LAZY. Artinya, saat kita mengambil data Student, Hibernate tidak akan mengambil data Course sampai kita memanggilnya (student.getCourses()).
Permasalahannya: Jika Session sudah ditutup sebelum kita memanggil daftar kursus (Course), maka akan muncul error LazyInitializationException.
✅ Solusi: Menggunakan HQL dengan JOIN FETCH (Sangat Direkomendasikan)
Teknik ini memaksa Hibernate untuk mengambil data Siswa dan Kursus dalam satu query SQL saja. Ini adalah cara paling efisien untuk menghindari masalah N+1.
tip
Baca: Eager vs Lazy
🚀 Implementasi Entity
1️⃣ Entity Student (Sisi Owner)
Sisi "Owner" adalah entitas yang menentukan konfigurasi tabel penghubung menggunakan @JoinTable.
@Entity
@Table(name = "student")
public class Student {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Integer id;
@Column(name = "first_name")
private String firstName;
@Column(name = "last_name")
private String lastName;
@ManyToOne(cascade = {CascadeType.DETACH, CascadeType.MERGE, CascadeType.PERSIST, CascadeType.REFRESH})
@JoinColumn(name = "class_id")
private Classes classes;
@OneToMany(
fetch = FetchType.LAZY,
cascade = CascadeType.ALL)
@JoinColumn(name = "student_id") // FK yang akan dibuat di tabel 'grade'
private List<Grade> grades = new ArrayList<>();
@ManyToMany(
fetch = FetchType.LAZY,
cascade = {CascadeType.DETACH, CascadeType.MERGE, CascadeType.PERSIST, CascadeType.REFRESH})
@JoinTable(
name = "course_student",
joinColumns = @JoinColumn(name = "student_id"),
inverseJoinColumns = @JoinColumn(name = "course_id"))
private Set<Course> courses = new HashSet<>();
public Student() {
}
public Student(String firstName, String lastName) {
this.firstName = firstName;
this.lastName = lastName;
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getFirstName() {
return firstName;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
public String getLastName() {
return lastName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
public Classes getClasses() {
return classes;
}
public void setClasses(Classes classes) {
this.classes = classes;
}
public List<Grade> getGrades() {
return grades;
}
public void setGrades(List<Grade> grades) {
this.grades = grades;
}
public void addGrade(Grade grade) {
if (grades == null) {
grades = new ArrayList<>();
}
grades.add(grade);
}
public Set<Course> getCourses() {
return courses;
}
public void setCourses(Set<Course> courses) {
this.courses = courses;
}
public void addCourse(Course course) {
if (course == null) return;
if (courses == null) {
courses = new HashSet<>();
}
if (!courses.contains(course)) {
courses.add(course);
course.addStudent(this);
}
}
@Override
public String toString() {
return "Student [id=" + id + ", firstName=" + firstName + ", lastName=" + lastName + "]";
}
}
2️⃣ Entity Course (Sisi Inverse)
Sisi ini menggunakan atribut mappedBy yang merujuk pada nama variabel di class Student.
@Entity
@Table(name = "course")
public class Course {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Integer id;
@Column(name = "course_name")
private String courseName;
@ManyToMany(
mappedBy = "courses",
fetch = FetchType.LAZY,
cascade = {CascadeType.DETACH, CascadeType.MERGE, CascadeType.PERSIST, CascadeType.REFRESH})
private Set<Student> students = new HashSet<>();
public Course() { }
public Course(String courseName) {
this.courseName = courseName;
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getCourseName() {
return courseName;
}
public void setCourseName(String courseName) {
this.courseName = courseName;
}
public Set<Student> getStudents() {
return students;
}
public void setStudents(Set<Student> students) {
this.students = students;
}
public void addStudent(Student student) {
if (student == null) return;
if (students == null) {
students = new HashSet<>();
}
if (!students.contains(student)) {
students.add(student);
student.addCourse(this);
}
}
@Override
public String toString() {
return "Course [id=" + id + ", courseName=" + courseName + "]";
}
}
🛢 Implementasi DAO
- Membuat Interface DAO:
public interface SchoolDAO {
Student findStudentAndCourseByStudentId(Integer id);
Course findCourseAndStudentByCourseId(Integer id);
}
- Implementasi DAO simpan
Studentsekaligus denganCourse-nya:
@Repository
public class SchoolDAOImpl implements SchoolDAO {
private EntityManager entityManager;
public SchoolDAOImpl(EntityManager entityManager) {
this.entityManager = entityManager;
}
@Override
@Transactional(readOnly = true)
public Student findStudentAndCourseByStudentId(Integer id) {
TypedQuery<Student> query = entityManager.createQuery(
"SELECT s FROM Student s "
+ "JOIN FETCH s.courses "
+ "WHERE s.id = :id", Student.class);
query.setParameter("id", id);
Student student = query.getSingleResult();
return student;
}
@Override
@Transactional(readOnly = true)
public Course findCourseAndStudentByCourseId(Integer id) {
TypedQuery<Course> query = entityManager.createQuery(
"SELECT c FROM Course c "
+ "JOIN FETCH c.students "
+ "WHERE c.id = :id", Course.class);
query.setParameter("id", id);
Course course = query.getSingleResult();
return course;
}
}
▶️ Main Class
- Kita akan membuat read data
StudentbesertaCourse-nya. Contoh kasus menampilkan dataCoursebeserta dataStudentterkait berdasarkanCoursedengan id 4 (findCourseAndStudent) dan menampilkan dataStudentbesertaCourseterkait berdasarkanStudentdengan id 6 (findStudentAndCourse).
@SpringBootApplication
public class Application {
public static void main(String[] args) {
SpringApplication.run(Application.class, args);
}
// Bean ini akan dieksekusi di command line
@Bean
public CommandLineRunner commandLineRunner(SchoolDAO dao) {
return runner -> {
findStudentAndCourse(dao);
findCourseAndStudent(dao);
};
}
private void findCourseAndStudent(SchoolDAO dao) {
int id = 4;
Course course = dao.findCourseAndStudentByCourseId(id);
System.out.println("Load course: " + course);
System.out.println("student of course: : " + course.getStudents());
}
private void findStudentAndCourse(SchoolDAO dao) {
int id = 6;
Student student = dao.findStudentAndCourseByStudentId(id);
System.out.println("Load student: " + student);
System.out.println("student course: " + student.getCourses());
}
}
- Contoh output Hibernate:
Hibernate: select s1_0.id,s1_0.class_id,c1_0.student_id,c1_1.id,c1_1.course_name,s1_0.first_name,s1_0.last_name from student s1_0 join course_student c1_0 on s1_0.id=c1_0.student_id join course c1_1 on c1_1.id=c1_0.course_id where s1_0.id=?
Load student: Student [id=6, firstName=Abu, lastName=Nawas]
student course: [Course [id=1, courseName=Bahasa Arab], Course [id=2, courseName=Sejarah]]
Hibernate: select c1_0.id,c1_0.course_name,s1_0.course_id,s1_1.id,s1_1.class_id,s1_1.first_name,s1_1.last_name from course c1_0 join course_student s1_0 on c1_0.id=s1_0.course_id join student s1_1 on s1_1.id=s1_0.student_id where c1_0.id=?
Load course: Course [id=4, courseName=Bahasa Arab]
student of course: : [Student [id=7, firstName=Abu, lastName=Jahal]]