我以为em.find找到的一个entity是em自动管理的，连出一个事务，但是下面的这个class好像相反。是我错了还是那堂课有什么错误？

@Stateful
@TransactionAttribute(NOT_SUPPORTED)
public class CustomerGateway {

  @PersistenceContext(unitName = "customersPU", type = EXTENDED)
  private EntityManager em;
  private Customer customer;

  public Customer find(Long id) {
    // customer is not managed!
    this.customer = em.find(Customer.class, id);
    // Print false!
    System.out.println("Method find: " + em.contains(customer));
    // Print false too (2 is the id of an entity)!
    System.out.println("Method find: " + em.contains(em.find(Customer.class, 2L));
    // A workaround
    customer = em.merge(customer);
    // Print true.
    System.out.println("Method find after merge: " + em.contains(customer));
    return this.customer;
  }

编辑 1:实体代码

@Entity
@NamedQuery(name = "Customer.all", query = "select c from Customer c")
public class Customer implements Serializable {
  private static final long serialVersionUID = 1L;
  @Id
  @GeneratedValue(strategy = GenerationType.AUTO)
  private Long id;
  private String name;

  public Customer() {
  }

  public Customer(String name) {
    this.name = name;
  }

  public Long getId() {
    return id;
  }

  public void setId(Long id) {
    this.id = id;
  }

  public String getName() {
    return name;
  }

  public void setName(String name) {
    this.name = name;
  }

  @Override
  public int hashCode() {
    int hash = 0;
    hash += (id != null ? id.hashCode() : 0);
    return hash;
  }

  @Override
  public boolean equals(Object object) {
    // TODO: Warning - this method won't work in the case the id fields are not set
    if (!(object instanceof Customer)) {
      return false;
    }
    Customer other = (Customer) object;
    if ((this.id == null && other.id != null) || (this.id != null && !this.id.equals(other.id))) {
      return false;
    }
    return true;
  }

  @Override
  public String toString() {
    return "entity.Customer[ id=" + id + " ]";
  }

}

有状态 EJB 的代码:

@Stateful
@TransactionAttribute(NOT_SUPPORTED)
public class CustomerGateway {

  @PersistenceContext(type = PersistenceContextType.EXTENDED)
  private EntityManager em;

  private Customer customer;

  public Customer getCustomer() {
    return customer;
  }

  public void create(Customer customer) {
    em.persist(customer);
    this.customer = customer;
  }

  public Customer find(Long id) {
    this.customer = em.find(Customer.class, id);
    System.out.println("customer managed ? " + em.contains(this.customer));
    // Workaround :
//    this.customer = em.merge(customer);
    return customer;
  }

  public void remove(Long id) {
    Customer cust = em.getReference(Customer.class, id);
    em.remove(cust);
  }

  @TransactionAttribute(REQUIRES_NEW)
  public void save() {
  }

  public List<Customer> findAllCustomers() {
    Query query = em.createNamedQuery("Customer.all");
    return query.getResultList();
  }

  @Remove
  public void close() {
  }

}

我使用 NetBeans 7.4、GlassFish 4.0、EJB 3.2、Java DB。

最佳答案

您所经历的一切都符合规范。当事务存在时，持久性上下文仍然存在(并且实体保持附加)。因此，在扩展的持久性上下文和 NOT_SUPPORTED 事务中，通过调用 find 方法检索到的对象是分离的。 -此外，如果您的 Customer 对象具有惰性关系并且您尝试访问它们，那么您很可能会遇到运行时异常。-

现在，为什么 merge 方法就可以了？好吧，首先要记住 merge 返回一个托管实体并将客户附加到持久性上下文。

其次，您有一个EXTENDED 持久化上下文，因此，它不会去更新数据库，直到您调用@Remove 注释方法。当此调用到达时，您可能会收到 TransactionRequiredException。

编辑 1 ------------------------------------------ ------------------------------------

根据您的意见:

• find 不需要在事务中，但是，如果您想要托管对象，则必须有一个。

• 该段是关于 EM 生命周期的(3.3 节)，在这种情况下，试图解释在事务范围的 bean 的方法结束时，实体将被分离，但是，在在扩展 EM 的情况下，实体将保持附加状态。

• 有 2 个有见地的段落:

1. When an EM with an extended persistence context is used, the persist, remove, merge and refresh operations can be called regardless of whether a transaction is active. The effects of these operations will be committed to the database when the extended persistence context is enlisted in a transaction and the transaction commits.

2. The persistence context is closed by the container when the @Remove method of the stateful session bean completes (or the stateful session bean instance is otherwise destroyed).

• 看起来您最初在带有 @TransactionAttribute(REQUIRES_NEW) 的问题中省略的方法是合并成功发生的地方。这就是为什么你也不异常(exception)。

编辑 2 ------------------------------------------ ------------------------------------

经过一些测试，GF4 有一个错误并已报告> https://java.net/jira/browse/GLASSFISH-20968

编辑 3 ------------------------------------------ ------------------------------------

20/May/2014:该错误已标记为:Must Fix for Glassfish 4.0.1。

关于java - 在有状态的 ejb 扩展中通过查找找到的 JPA 实体不受管理，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/21356448/