Spring Java API returning byte[] variable with ""

Question

Good day! My Api returns byte variable like this "..." but retrofit in my android application can read only like this [...] how can i change response.

Entity:

@Entity
@Table(name = "book")
public class bookEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private long books_id;
    private String title;
    private String description;
    @Column(columnDefinition = "LONGBLOB")
    private byte[] cover;

Service:

public Iterable<bookEntity> findAll()
    {
        return bookRepository.findAll();
    }

Controller:

@GetMapping
    public ResponseEntity getAllBook()
    {
        return ResponseEntity.ok(bookService.findAll());
    }