多对多RelationShip Spring Boot的问题

@Entity
public class Student {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;
    
    @ManyToMany
    private List<Course> courses;
    
    // 省略其他属性和方法
}

@Entity
public class Course {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;
    
    @ManyToMany(mappedBy = "courses")
    private List<Student> students;
    
    // 省略其他属性和方法
}

@Entity
public class Student {
    // 省略其他属性和方法
    
    @ManyToMany
    @JoinTable(name = "student_course",
               joinColumns = @JoinColumn(name = "student_id"),
               inverseJoinColumns = @JoinColumn(name = "course_id"))
    private List<Course> courses;
    
    // 省略其他属性和方法
}

@Entity
public class Course {
    // 省略其他属性和方法
    
    @ManyToMany(mappedBy = "courses")
    private List<Student> students;
    
    // 省略其他属性和方法
}

@Repository
public interface StudentRepository extends JpaRepository<Student, Long> {
}

@Repository
public interface CourseRepository extends JpaRepository<Course, Long> {
}

@Service
public class StudentService {
    @Autowired
    private StudentRepository studentRepository;
    
    @Transactional
    public void addCourse(Long studentId, Long courseId) {
        Student student = studentRepository.findById(studentId).orElse(null);
        Course course = courseRepository.findById(courseId).orElse(null);
        
        if (student != null && course != null) {
            student.getCourses().add(course);
            course.getStudents().add(student);
        }
    }
}