服務層用於封裝業務邏輯，讓應用的其他部分可以不直接與數據層交互，而是通過服務層進行調用。

StudentService 是應用的業務層核心。它通過 StudentRepository 與數據層進行交互，以提供業務功能。

package com.example.demo.student;

import jakarta.transaction.Transactional;
import jdk.jfr.TransitionTo;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;
import org.springframework.stereotype.Service;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestBody;

import java.time.LocalDate;
import java.time.Month;
import java.util.List;
import java.util.Objects;
import java.util.Optional;

@Service
public class StudentService {
    private final StudentRepository studentRepository;

    @Autowired
    public StudentService(StudentRepository studentRepository){
        this.studentRepository = studentRepository;
    }

    public List<Student> getStudents() {
      return studentRepository.findAll();
    }
    @PostMapping
    public void addNewStudent( Student student) {
        Optional<Student> studentOptional = studentRepository
                .findStudentByEmail(student.getEmail());
        if (studentOptional.isPresent()) {
            throw new IllegalStateException("email token");
        }
        studentRepository.save(student);
    }

    public void deletStudent(Long studentId) {
        boolean exists = studentRepository.existsById(studentId);
        if (!exists) {
            throw new IllegalStateException("student with id" + studentId + "dose not exists");
        }
        studentRepository.deleteById(studentId);
    }
    @Transactional
    public void updateStudent(Long studentId,
                              String name,
                              String email) {
        Student student = studentRepository.findById(studentId)
                .orElseThrow(() -> new IllegalStateException(
                        "student with ID" + studentId + "does not exist"
                ));
        if(name != null && name.length() > 0 && !Objects.equals(student.getName(), name)) {
            student.setName(name);
        }
        if (email != null &&
                email. length() > 0 &&
                Objects .equals (student. getEmail(), email)) {
            Optional<Student> student0ptional = studentRepository.findStudentByEmail(email);
            if (student0ptional.isPresent()) {
                throw new IllegalStateException("email taken");
            }
            student.setEmail(email);
        }
    }
}

此服務層提供 CRUD 操作，並包含其他業務邏輯，例如檢查郵件唯一性。