如何在spring boot中保存有关系的实体

@Entity
@Table(name = "users")
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String username;

    // 定义与Role的多对多关系
    @ManyToMany
    @JoinTable(name = "user_role",
            joinColumns = @JoinColumn(name = "user_id"),
            inverseJoinColumns = @JoinColumn(name = "role_id"))
    private List<Role> roles;

    // 省略getter和setter方法
}

@Entity
@Table(name = "roles")
public class Role {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    // 定义与User的多对多关系
    @ManyToMany(mappedBy = "roles")
    private List<User> users;

    // 省略getter和setter方法
}

spring.datasource.url=jdbc:mysql://localhost:3306/mydatabase
spring.datasource.username=root
spring.datasource.password=123456
spring.jpa.hibernate.ddl-auto=update

@Repository
public interface UserRepository extends JpaRepository<User, Long> {
    // 自定义查询方法
    List<User> findByUsername(String username);
}

@Service
public class UserService {
    @Autowired
    private UserRepository userRepository;

    public void saveUserWithRoles(User user, List<Role> roles) {
        user.setRoles(roles);
        userRepository.save(user);
    }
}