Studentlist application from the previous blogs will be used to add many-to-many relation by using JPA. The starting point application can be found from previous blog.

The goal is to add new Course entity which is linked to student with many-to-many link. User should also be able to add new courses to students.

The listpage cointains two actions for deleting students and adding courses to students (static/templates/students.html).

screenshot

List page shows also student’s courses in one column. This column is implemented by using thymeleaf iterator and iteration status. It shows all student’s courses separated by comma. See more info about iteration status from thymelead pages.

<tr th:each = "student : ${students}">
    <td th:text="${student.firstName} + ' ' + ${student.lastName}"></td>
    <td th:text="${student.email}"></td>
    <td th:text="${student.department}"></td>
    <td>
        <span th:each="course,iterStat : ${student.courses}">
        <span th:text="${course.name}"/><th:block th:if="${!iterStat.last}">,</th:block>
        </span>    		
    </td>
    <td>
        <a th:href="@{/addStudentCourse/{id}(id=${student.id})}" class="btn btn-success btn-xs">Add Course</a>
        <a th:href="@{/delete/{id}(id=${student.id})}" class="btn btn-danger btn-xs">Delete</a>
    </td>
</tr>

Course can be added to students by using the following form (static/templates/addStudentCourse.html).

screenshot

Many-to-many relationship is defined in Student and Course entity classes. @JointTable annotation creates join table. @JoinColumn annotation specifies a column for joining an entity association.

// Student entity

@ManyToMany(cascade = CascadeType.MERGE)
@JoinTable(
    name = "student_course", 
    joinColumns = { @JoinColumn(name = "id") }, 
    inverseJoinColumns = { @JoinColumn(name = "courseid") })
public Set<Course> getCourses() {
    return this.courses;
}
// Course entity

@ManyToMany(mappedBy = "courses")    
private Set<Student> students;  
    
public Set<Student> getStudents() {
    return students;
}

It is good to set Spring Boot property for logging sql statements. Then you can see sql statements for table creations in the console.

spring.jpa.show-sql=true

screenshot

Controller contains following changes for adding courses to students.

//For opening Add Course form

@RequestMapping(value = "addStudentCourse/{id}", method = RequestMethod.GET)
public String addCourse(@PathVariable("id") Long studentId, Model model){
    model.addAttribute("courses", crepository.findAll());
	model.addAttribute("student", repository.findOne(studentId));
    return "addStudentCourse";
}
    
//For saving added course

@RequestMapping(value="/student/{id}/courses", method=RequestMethod.GET)
public String studentsAddCourse(@PathVariable Long id,
 @RequestParam Long courseId, Model model) {
	Course course = crepository.findOne(courseId);
	Student student = repository.findOne(id);

	if (student != null) {
		if (!student.hasCourse(course)) {
			student.getCourses().add(course);
		}
		repository.save(student);
		model.addAttribute("student", crepository.findOne(id));
		model.addAttribute("courses", crepository.findAll());
		return "redirect:/students";
	}
	return "redirect:/students";
}

List of technologies used

  • Spring Boot
  • Spring security
  • Java Persistence API (JPA)
  • H2 database (Java based in-memory db)
  • Thymeleaf
  • Bootstrap

Project also contains testdata which are inserted at runtime by using Spring Boot CommandLineRunner.

The complete project code can be found from GitHub repository