JPA懒惰加载

Question

我在JPA实体中有延迟加载属性的问题.我读了许多类似的问题,但它们与spring或hibernate有关,它们的问题不适用或有用.

该应用程序是JEE,在Wildfly应用程序服务器上运行JPA2.1.有两个实体,DAO会话bean和servlet将它们组合在一起:

@Entity
@Table(name = "base_user")
public class User implements Serializable {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id")
    int id;

    @OneToMany(fetch=FetchType.LAZY, mappedBy="user")
    List<OAuthLogin> oauthLogins;

}


@Entity
@Table(name = "oauth_login")
public class OAuthLogin implements Serializable {
    @ManyToOne
    @JoinColumn(name="user_id", nullable=false)
    User user;
}


@Stateless(name = "UserDAOEJB")
public class UserDAO {
    @PersistenceContext(unitName="OAUTHDEMO")
    EntityManager em;

    public User findById(int id) {
        User entity;
    entity = em.find(User.class, id);
        return entity;
    }
}


public class SaveUserServlet extends HttpServlet {
    @EJB
    UserDAO userDAO;

    @Transactional
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        User user = new User(name);
        user.setEmail(email);
        System.out.println("Persisting user " + user);
        userDAO.persist(user);

        OAuthLogin fbLogin1 = new OAuthLogin(user, OAuthProvider.FACEBOOK, "1501791394");
        loginDAO.persist(fbLogin1);

        User user2 = userDAO.findById(user.getId());
        List<OAuthLogin> oauthLogins = user2.getOauthLogins();

当我运行此代码时,它失败了:

org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: cz.literak.demo.oauth.model.entity.User.oauthLogins, could not initialize proxy - no Session
org.hibernate.collection.internal.AbstractPersistentCollection.throwLazyInitializationException(AbstractPersistentCollection.java:572)
org.hibernate.collection.internal.AbstractPersistentCollection.withTemporarySessionIfNeeded(AbstractPersistentCollection.java:212)
org.hibernate.collection.internal.AbstractPersistentCollection.initialize(AbstractPersistentCollection.java:551)
org.hibernate.collection.internal.AbstractPersistentCollection.read(AbstractPersistentCollection.java:140)
org.hibernate.collection.internal.PersistentBag.iterator(PersistentBag.java:294)
cz.literak.demo.oauth.servlets.SaveUserServlet.doPost(SaveUserServlet.java:66)

我使用了与WebLogic/JPA1非常相似的模式,并且运行顺畅.任何的想法？谢谢

PS.这是一个JPA应用程序,我没有hibernate会话等.

Answer 1

您可以使用的替代品很少:

使用级联持久性:

@OneToMany(fetch=FetchType.LAZY, mappedBy="user", cascade = {CascadeType.PERSIST})
List<OAuthLogin> oauthLogins;

在你的Servlet中:

User user = new User(name);
user.setEmail(email);
OAuthLogin fbLogin = new OAuthLogin(user, OAuthProvider.FACEBOOK, "1501791394");      
user.getOauthLogins().add(fbLogin) // this is enough assuming uni-directional association
userDAO.persist(user);
List<OAuthLogin> oauthLogins = user.getOauthLogins();

这应该做,而且你有一个事务和更少的JDBC调用.

这对于特定Servlet方法调用的特定用例很有帮助.

EJB中的预取集合

public User findById(int id, boolean prefetch) {
    User entity = em.find(User.class, id);
    if (prefetch) {
        // Will trigger 1 or size JDBC calls depending on your fetching strategy
        entity.getOauthLogins().size() 
    }
    return entity;
}

或者,使用条件覆盖提取模式

这对于您希望在保留a时获取OAuthLogin集合的每个案例都很有用,并且仅针对该特定集合进行避免. UserFetchType.LAZYLazyInitializationException

在View Filter中使用Open Entity Manager

只要谷歌它,你会发现很多例子

对于LazyInitializationException跨越每个实体应用程序而言,这将基本上阻止每个懒惰地获取的关联

PS:

1. 如果不使用Spring你为什么使用@Transactional(默认情况下甚至不适用HttpServlet)
2. 它曾适用于WebLogic,可能使用某种定制的解决方案