Есть ли центральный репозиторий для получения информации о часовом поясе

why you are making code complex and How come primary key returns List<Posts>. Try with the below code.

Just make change below in your Entity and Repository.

   @Id
   @GeneratedValue(strategy = GenerationType.AUTO)
   @Column(name="user_id", unique=true)
   private Long id;
   //private Long userId;

   @Column(name="user_name", length=35, nullable=false)
   @Size(min=2,message="Name should have atleast 2 character")
   private String userName;

   public interface PostRepository extends JpaRepository<Posts,Long> 
   {

   }


   (or)

   public interface PostRepository extends CrudRepository<Posts,Long> 
   {
     //Optional<Posts> posts findByUserId(Long userId);
   }

  Service Layer : 
  Approach 1 : Using JPA Repository
  Posts posts = repo.getOne(userId);

  Approach 2 : Using Crud Repository
  Posts posts = repo.findById(userId).get();
  //Posts posts = repo.findByUserId(userId).get();