JPA的EntityManager find方法不起作用

Question

我想写这让我回通过cardId的customerId的方法，但是这是不工作时，Eclipse说customerId不能被解析为一个变量

public CustomerMapping getCustomerMapping(String cardId) { 

      this.getEntityManager(); 
      return em.find(CustomerMapping.class, customerId); 
    } 




@Stateless 
public class CustomerMappingEJB { 

    private EntityManager em; 


    private void getEntityManager() { 
     em = MultiTenantEntityManagerFactory.getEntityManager(); 
    } 

    private void beginTransaction() { 
     em.getTransaction().begin(); 
    } 

    private void commitTransaction() { 
     em.getTransaction().commit(); 
    } 


    private void insert(CustomerMapping customerMapping) { 
     em.persist(customerMapping); 
    } 

    public CustomerMapping getCustomerMapping(long id) { 

     this.getEntityManager(); 
     return em.find(CustomerMapping.class, id); 
    } 

    public List<CustomerMapping> getCustomerMappings() { 

     TypedQuery<CustomerMapping> query = null; 

      this.getEntityManager(); 
      query = em.createNamedQuery("AllCustomerIds", CustomerMapping.class);  

     return query.getResultList(); 
    } 

    public CustomerMappingHelper getCustomerMappingHelper(String cardId) { 

     this.getEntityManager(); 
     return em.find(CustomerMappingHelper.class, cardId); 
    } 

    public CustomerMapping addNew(CustomerMapping customerMapping) { 
     this.getEntityManager(); 
     this.beginTransaction(); 
     this.insert(customerMapping); 
     this.commitTransaction(); 
     return customerMapping; 
    } 
} 

我没有把MultiTenantEntityManagerFactory这个类放在这里，因为我猜你不会需要这个类，那么当你调用它时，我有CustomerMapping类与“一堆列表”，并且只有一些设置和获取id的方法， cardId和customerId我没有复制，因为这将是非常大，如果你也想我也会做

public class CustomerMapping { 

    @Id 
    @GeneratedValue 
    public Long id; 

    @Basic 
    private String customerId; 

    @Basic 
    private String cardId; 

    @Basic 
    private String hashKey; 

} 

Answer 1

您可以通过cardId使用JPQL获取CustomerMapping。更优化的解决方案将是使用投影。

public String getCustomerMappingCustomerId(String cardId) { 
    getEntityManager();   
    CustomerMapping result = first(em.createQuery(
     "select c from CustomerMapping c where c.cardId = :cardId") 
     .setParameter("cardId", cardId).getResultList()); 
    return result == null ? null : result.getCustomerId(); 
} 

private static <T> T first(List<T> items) { 
    return items == null || items.isEmpty() ? null : items.get(0); 
}