调用实体管理器时的 NullPointer

Question

我尝试使用 Java、Hibernate 和 Spring 编写一些程序。我在调用 SQL 查询时看到 NullPointerException。我可能尝试了互联网上的所有代码示例，但我不知道该怎么做。我该如何解决？这是我的示例代码。有我的完整程序代码：https://github.com/lukasz-chojn/films_database/tree/master/src/main/java/Films

    @Controller
    public class DataFromUserController {
        private FilmDaoImpl filmDaoImpl = new FilmDaoImpl();
        private DirectorDaoImpl directorDaoImpl = new DirectorDaoImpl();
        private Locale locale = Locale.getDefault();
        private Scanner sc = new Scanner(System.in).useLocale(locale);

        public FilmDs collectData() {
            //....

            filmDaoImpl.insert(film);
            directorDaoImpl.insert(director);

            return film;
        }

        private String collectTitle() {
            System.out.print("Podaj tytuł filmu: ");
            String tytul = sc.nextLine();
            List<String> existingEntry = filmDaoImpl.existingTitle();
            if (existingEntry.contains(tytul)) {
                System.out.println("Tytuł jest już w bazie. Podaj inny");
                return collectTitle();
            }
                if (tytul.isEmpty()) {
                    System.out.println("Tytuł nie może być pusty. Podaj go");
                    return collectTitle();
                }
            return tytul;
        }//......
    }

    @Service
    @Transactional
    public class FilmDaoImpl implements FilmDAO {

        @PersistenceContext
        private EntityManager entityManager;

        //......
        @Override
        @SuppressWarnings("unchecked")
        public List<String> existingTitle() {
            List<String> titleInTheDatabase;
    // here is NPE
    return titleInTheDatabase = entityManager.createQuery("SELECT film.tytul FROM FilmDs film").getResultList();
        }

        //....
    }

Answer 1

您自己创建实例，而不是让 spring 为您注入它，这就是为什么您会丢失所有依赖项，如 entitymanager 等。更改您的 DataFromUserController

@Controller
public class DataFromUserController {

@Autowired
private FilmDAO filmDao;

@Autowired
private DirectorDAO directorDao;

.....
}