Hibernate懒惰初始化问题 [英] Hibernate Lazy initialization problem

问题描述

在学习一些关于hibernate的知识时，我遇到了下面提到的例子中的着名的懒惰初始化异常。我有一个类A，它引用了另一个类B并使用了hibernate我正在尝试使用它的id加载类A，并且在关闭会话后我试图打印一些B类属性，认为hibernate可能已经初始化了对象，但它所做的只是一个代理对象，下面是代码快照 p>

  Session session = HibernateUtil.getSessionFactory（）。openSession（）; 
 Transaction tx = session.getTransaction（）; 
尝试{
 tx.begin（）; 
 a =（A）session.load（A.class，40288184318d9a3c01318d9a3eea0001）; 
 tx.commit（）; 
 
} catch（Exception e）{
 log.error（无法完成transcation
 +Transcation将被回滚，e）; 
 e.printStackTrace（）; 
 tx.rollback（）; 
 
}最后{
 log.info（关闭会议）; 
 if（session！= null&& session.isOpen（）== true）{
 session.close（）; 
} 
} 
 System.out.println（a.getUuid（））; 
 System.out.println（a.getB（）。getShortDescription（））; 
 System.out.println（a.getB（）。getLongDescription（））; 
 System.out.println（a.getB（）。getName（））; 
  

这段代码给我 org.hibernate.LazyInitializationException：无法初始化代理 - 没有会话因为我知道它给了我只有代理B，当我试图获得B的属性Hibernate抛出异常

我做了在我的代码中的一个小改动，它在这里运行得很好，是改变后的版本。

  Session session = HibernateUtil.getSessionFactory（）。openSession ）; 
 Transaction tx = session.getTransaction（）; 
尝试{
 tx.begin（）; 
 a =（A）session.load（A.class，40288184318d9a3c01318d9a3eea0001）; 
 tx.commit（）; 
 
} catch（Exception e）{
 e.printStackTrace（）; 
 tx.rollback（）; 
 
} finally {
 //log.info(\"Closing the session）; 
 if（session！= null&& session.isOpen（）== true）{
 b = des.getB（）; 
 session.close（）; 
} 
} 
 a.setB（b）; 
 System.out.println（a.getUuid（））; 
 System.out.println（a.getB（）。getShortDescription（））; 
 System.out.println（a.getB（）。getLongDescription（））; 
 System.out.println（a.getB（）。getName（））; 
} 
  

虽然它正在运作，但我不确定这是否正确方法，请提出建议

解决方案

绝对不是正确的做法。

了解你想要什么：你想加载A，然后获得与A相关的所有B.
了解你做了什么：你加载了A，然后将B设置为'b'，这可能会或可能没有改变B。

您有许多选项可以正确执行此操作。

1）在您的应用中使用视图过滤器中的打开会话。有些人认为这是一种反模式，但很多开发人员都这样做，我个人认为它是可以的。

2）创建一个自定义查询，让您获得所有的东西需要一气呵成。 3）调用 a.getB（） / code>，而会话处于活动状态，并且hibernate会为你取回它。

4）使关联不是懒惰的，但我不会那么做它会很慢。如果你总是想要B时，你可以做到，但只有在这种情况下。

while learning a bit about hibernate i came across famous lazy initialization exception under below mentioned case

i have a class A which has reference to another class B and using hibernate i was trying to load class A using its id and than after closing the session i was trying to print some properties of class B thinking that hibernate might have initialized the object but all it did was a proxy object, here is the code snap

Session session=HibernateUtil.getSessionFactory().openSession();
        Transaction tx = session.getTransaction();
        try {
            tx.begin();
            a=(A)session.load(A.class, "40288184318d9a3c01318d9a3eea0001");
            tx.commit();

        } catch (Exception e) {
            log.error("Unable to complete the transcation"
                    + "Transcation will be roll backed", e);
            e.printStackTrace();
            tx.rollback();

        } finally {
            log.info("Closing the session");
            if (session != null && session.isOpen() == true){
                session.close();
            }
        }
        System.out.println(a.getUuid());
        System.out.println(a.getB().getShortDescription());
        System.out.println(a.getB().getLongDescription());
        System.out.println(a.getB().getName());

this code was giving me org.hibernate.LazyInitializationException: could not initialize proxy - no Session since i know it has given me only proxy for B and when i tried to get properties of B hibernate throws exception

i did a small change in my code and it worked well here is the changed version

    Session session=HibernateUtil.getSessionFactory().openSession();
    Transaction tx = session.getTransaction();
    try {
        tx.begin();
        a=(A)session.load(A.class, "40288184318d9a3c01318d9a3eea0001");
        tx.commit();

    } catch (Exception e) {
        e.printStackTrace();
        tx.rollback();

    } finally {
        //log.info("Closing the session");
        if (session != null && session.isOpen() == true){
            b=des.getB();
            session.close();
        }
    }
    a.setB(b);
    System.out.println(a.getUuid());
    System.out.println(a.getB().getShortDescription());
    System.out.println(a.getB().getLongDescription());
    System.out.println(a.getB().getName());
}

though it is working but i am not sure if this is correct approach or not, please suggest

解决方案

Definitely NOT the right approach.

Understand what you want: you want to load A, and then get all Bs associated with A. Understand what you did: you loaded A, then set B to 'b', which might or might not have changed the value of B.

You have a number of options to do it correctly.

1) Use an open session in view filter in your app. Some people consider it an anti-pattern, but a lot of developers do it, and I personally think its ok.

2) Create a custom query which gets you everything you need in one go. You can write some hql that will get all Bs associated to an A when you fetch an A.

3) Call a.getB() while the session is active, and hibernate will fetch it for you.

4) Make the association not lazy, but I would not do that as it will be slow. If you always want Bs when you get A, then you can do it, but only in that case.

这篇关于Hibernate懒惰初始化问题的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！