JPA: How to get entity based on field value other than ID?

Question

In JPA (Hibernate), when we automatically generate the ID field, it is assumed that the user has no knowledge about this key. So, when obtaining the entity, user would query

Answer 1

I solved a similar problem, where I wanted to find a book by its isbnCode not by your id(primary key).

@Entity
public class Book implements Serializable {
    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private Integer id;
    private String isbnCode;

...

In the repository the method was created like @kamalveer singh mentioned. Note that the method name is findBy+fieldName (in my case: findByisbnCode):

@Repository
public interface BookRepository extends JpaRepository {

    Book findByisbnCode(String isbnCode);
}

Then, implemented the method in the service:

@Service
public class BookService {

    @Autowired
    private BookRepository repo;
    
    
    public Book findByIsbnCode(String isbnCode) {
        Book obj = repo.findByisbnCode(isbnCode);
        return obj; 
    }
}