Spring JPA配置：EntityManager抛出NullPointerException

Question

我试图在Spring中创建REST API，并且需要仅使用JavaConfig通过JPA建立数据库连接。这是我的配置类：

@Configuration
@ComponentScan
@EnableTransactionManagement
public class ApplicationConfig {
@Bean
   public LocalContainerEntityManagerFactoryBean entityManagerFactory() {
      final LocalContainerEntityManagerFactoryBean em = new  LocalContainerEntityManagerFactoryBean();
      em.setDataSource(dataSource());
      em.setPackagesToScan(new String[] { "com.backend.domain", "com.backend.infrastructure.persistence" } );
      JpaVendorAdapter vendorAdapter = new HibernateJpaVendorAdapter();
      em.setJpaVendorAdapter(vendorAdapter);
      em.setJpaProperties(additionalProperties());
      em.afterPropertiesSet();

      return em;
   }

   @Bean
   public DataSource dataSource(){
      DriverManagerDataSource dataSource = new DriverManagerDataSource();
      dataSource.setDriverClassName("com.mysql.jdbc.Driver");
      dataSource.setUrl("jdbc:mysql://localhost:3306/mydb");
      dataSource.setUsername( "root" );
      dataSource.setPassword( "root" ); 
      return dataSource;
   }

   @Bean
   public PlatformTransactionManager transactionManager(EntityManagerFactory emf){
      JpaTransactionManager transactionManager = new JpaTransactionManager();
      transactionManager.setEntityManagerFactory(emf);

      return transactionManager;
   }



   @Bean
   public PersistenceExceptionTranslationPostProcessor exceptionTranslation(){
      return new PersistenceExceptionTranslationPostProcessor();
   }

   Properties additionalProperties() {
      Properties properties = new Properties();
      properties.setProperty("hibernate.hbm2ddl.auto", "create-drop");
      properties.setProperty("hibernate.dialect", "org.hibernate.dialect.MySQL5Dialect");
      properties.setProperty("hibernate.show_sql", "true");
      properties.setProperty("hibernate.format_sql", "true");
      return properties;
   }
 }

我有一个自定义存储库实现：

@Repository
@Transactional
public class GenericRepositoryImpl<K, E> implements GenericRepository<K, E> {

 protected static final Logger LOG = LoggerFactory.getLogger(GenericRepositoryImpl.class);
 private Class<E> entityClass;

 @PersistenceContext()
 protected EntityManager entityManager;



 @SuppressWarnings("unchecked")
    public GenericRepositoryImpl() {
        ParameterizedType genericSuperclass = (ParameterizedType) getClass().getGenericSuperclass();
        this.entityClass = (Class<E>) genericSuperclass.getActualTypeArguments()[0];
    }

    public void start() throws DomainException {
        try {
            entityManager.getTransaction().begin();
        } catch (Exception e) {
            LOG.error(e.getMessage());
            throw new DomainException(e);
        }
    }

    public void commit() throws DomainException {
        try {
            entityManager.getTransaction().commit();
        } catch (Exception e) {
            LOG.error(e.getMessage());
            throw new DomainException(e);
        }
    }

    @Override
    public void detach(E entity) throws DomainException {
        try {
            entityManager.detach(entity);
        } catch (Exception e) {
            LOG.error(e.getMessage());
            throw new DomainException(e);
        }
    }

    @Override
    public E findById(K id) throws DomainException {
        try {
            return entityManager.find(entityClass, id);
        } catch (Exception e) {
            LOG.error(e.getMessage());
            throw new DomainException(e);
        }
    }

    @Override
    public E findLockedEntityById(K id) throws DomainException {
        try {
            return entityManager.find(entityClass, id, LockModeType.OPTIMISTIC);
        } catch (Exception e) {
            LOG.error(e.getMessage());
            throw new DomainException(e);
        }
    }

    @Override
    public void persist(E entity) throws DomainException {
        try {
            entityManager.persist(entity);
        } catch (Exception e) {
            LOG.error(e.getMessage());
            throw new DomainException(e);
        }
    }

    @Override
    public E merge(E entity) throws DomainException {
        try {
            return entityManager.merge(entity);
        } catch (Exception e) {
            LOG.error(e.getMessage());
            throw new DomainException(e);
        }
    }

    @Override
    public void flush() throws DomainException {
        try {
            entityManager.flush();
        } catch (Exception e) {
            LOG.error(e.getMessage());
            throw new DomainException(e);
        }
    }

    @Override
    public void remove(E entity) throws DomainException {
        try {
            entityManager.remove(entity);
        } catch (Exception e) {
            LOG.error(e.getMessage());
            throw new DomainException(e);
        }
    }

    @Override
    public void refresh(E entity) throws DomainException {
        try {
            entityManager.refresh(entity);
        } catch (Exception e) {
            LOG.error(e.getMessage());
            throw new DomainException(e);
        }
    }

    @Override
    public void refreshLockedEntity(E entity) throws DomainException {
        try {
            entityManager.refresh(entity, LockModeType.OPTIMISTIC);
        } catch (Exception e) {
            LOG.error(e.getMessage());
            throw new DomainException(e);
        }
    }

    @Override
    public void lock(E entity) throws DomainException {
        try {
            entityManager.lock(entity, LockModeType.OPTIMISTIC);
        } catch (Exception e) {
            LOG.error(e.getMessage());
            throw new DomainException(e);
        }
    }

}

它正确编译，我可以看到数据库连接已建立，但在尝试保存实体时，会调用merge（）方法并抛出NullPointerException。我在这里做错了什么？

提前致谢