java如何正确处理EntityExistsException？

Question

我有两个实体：问题和偏好中心。首次将问题添加到收藏夹时，应创建收藏夹中心

考虑一个用例，当两个用户试图同时向收藏夹添加一个问题时，当第二个用户调用了^ {CD2>}时，这会导致^ {CD1>}。

但是下面的代码不起作用，因为当抛出EntityExistsException时，容器将事务标记为仅回滚，并尝试返回getFavoritesCounter(question)失败，返回结果为javax.resource.ResourceException: Transaction is not active

@Stateless
public class FavoritesServiceBean implements FavoritesService {

  ...

  public void addToFavorites(Question question) {
    FavoritesCounter counter = getCounter(question);
    if (counter == null) {
      counter = createCounter(question);
    }
    //increase counter
  }

  private FavoritesCounter createCounter(Question question) {
    try {
      FavoritesCounter counter = new FavoritesCounter();
      counter.setQuestion(question);
      entityManager.persist(counter);
      entityManager.flush();
      return counter;
    } catch (EntityExistsException e) {
      return getFavoritesCounter(question);
    }
  }

  private FavoritesCounter getFavoritesCounter(Question question) {
    Query counterQuery = entityManager.createQery("SELECT counter FROM FavoritesCounter counter WHERE counter.question = :question");
    counterQuery.setParameter("question", question);
    List<FavoritesCounter> result = counterQuery.getResultList();
    if (result.isEmpty()) return null;
    return result.get(0);
  }

}

问题

@Entity
public class Question implements Serializable {

  @Id
  @GeneratedValue(strategy = GenerationType.AUTO)
  private Long id;

  //getter and setter for id

}

收藏夹中心

@Entity
public class FavoritesCounter implements Serializable {

  @Id
  @GeneratedValue(strategy = GenerationType.AUTO)
  private Long id;

  @OneToOne
  @Column(unique = true)
  private Question question;

  //getter and setter

}

在EntityExistsException之后返回已创建实体的最佳方式是什么

Answer 1

您可以将createCounter（）移动到另一个会话Bean，并用@TransactionAttribute(REQUIRES_NEW)标记该方法

或者，您可以将createCounter（）移动到FavoriteServiceBean的本地接口（同时确保添加@TransactionAttribute(REQUIRES_NEW)注释），并按如下方式调用它：

@Stateless
public class FavoritesServiceBean implements FavoritesService 
{

    ...
    @Resource
    protected SessionContext sessionContext;

    public void addToFavorites(Question question) 
    {
        FavoritesCounter counter = getCounter(question);
        if (counter == null) {
          counter = sessionContext.getBusinessObject(FavoritesService.class)
                                  .createCounter(question);
        }
       //increase counter
    }

    @TransactionAttribute(REQUIRES_NEW)
    public FavoritesCounter createCounter(Question question) 
    {
        ...
    }
    ...
}

您需要通过业务/本地接口，而不是直接调用createCounter（），以便容器知道如何启动新事务