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BASIC: Lazy Loading Question

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mexihca

Post subject: BASIC: Lazy Loading Question

Posted: Wed Jan 12, 2005 1:33 pm

Newbie

Joined: Mon Jan 10, 2005 2:33 pm
Posts: 4

Hi! I hope this question deserve your attention and answer.

I got the following secuence:

- Open my session
- Get a list of objects using lazy loading
- Close the Session

The user pick one of those objects.
I try to load the pending objects (those ones related to this by many-to-one mapping) using the follwing code:

Code:

public void resolveLazyLoading(Object o){
   
   Ccliente c=(Ccliente)o;
   
   Session s = null;
   try {
      s = getSession();
      s.lock(c,LockMode.NONE);
      
      Integer cd=(c.getIddireccionbodega()!=null)?c.getIddireccionbodega().getIddireccion():null;
      cd=(c.getIddireccionfiscal()!=null)?c.getIddireccionfiscal().getIddireccion():null;
      c.getIddireccionoficina().getIddireccion();
      c.getIdempleado().getIdempleado();
      c.getIdsucursal().getIdsucursal();
      
   } catch (HibernateException e){
      e.printStackTrace();
   }finally {
      try {
         closeSession();
      } catch (HibernateException e1) {}
   }
}

Then When I try to use the object's properties, say c.getIddireccionoficina().getIddireccion();, I get the following error:

Code:

11:03:51,046 ERROR LazyInitializer:63 - Exception initializing proxy
net.sf.hibernate.HibernateException: Could not initialize proxy - the owning Session was closed
   at net.sf.hibernate.proxy.LazyInitializer.initialize(LazyInitializer.java:47)
   at net.sf.hibernate.proxy.LazyInitializer.initializeWrapExceptions(LazyInitializer.java:60)
   at net.sf.hibernate.proxy.LazyInitializer.getImplementation(LazyInitializer.java:164)
   at net.sf.hibernate.proxy.CGLIBLazyInitializer.intercept(CGLIBLazyInitializer.java:108)
   at net.sf.hibernate.proxy.HibernateProxy$$EnhancerByCGLIB$$61ab85cc.toString(<generated>)
   at com.ia.ventas.ui.UIPedidoDetalle.muestraCliente(UIPedidoDetalle.java:1043)
   at com.ia.ventas.ui.UIPedidoDetalle.inicializaVentana(UIPedidoDetalle.java:875)
   at com.ia.ventas.ui.UIPedidoDetalle.createDialogArea(UIPedidoDetalle.java:803)
   at org.eclipse.jface.dialogs.Dialog.createContents(Dialog.java:632)
   at org.eclipse.jface.window.Window.create(Window.java:348)
   at org.eclipse.jface.dialogs.Dialog.create(Dialog.java:925)
   at org.eclipse.jface.window.Window.open(Window.java:637)
   at com.ia.ventas.ui.UIPedido.agregaPedido(UIPedido.java:549)
   at com.ia.ventas.ui.UIPedido.access$2(UIPedido.java:536)
   at com.ia.ventas.ui.UIPedido$6.widgetSelected(UIPedido.java:224)
   at org.eclipse.swt.widgets.TypedListener.handleEvent(TypedListener.java:89)
   at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:82)
   at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:796)
   at org.eclipse.swt.widgets.Display.runDeferredEvents(Display.java:2772)
   at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:2431)
   at com.ia.Menu.main(Menu.java:112)

My question is I have to keep the session open all the time even when I already use those object's properties and supposedly they already loaded their state from database?

Thanks in advance!!
Rodolfo

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cardsharp

Post subject:

Posted: Wed Jan 12, 2005 5:57 pm

Regular

Joined: Tue Jun 22, 2004 8:01 pm
Posts: 106
Location: PowderTown, Utah, USA

If you are using Lazilly loaded collections or your object has a relationship to a proxied Hibernate object, your session must be open when you access the lazy property. Here's why:

For lazy collections Hibernate populates the collection with a set of "proxy" objects that only contain the ID and Version of the object. When you access the proxied object's non ID/Version properties the first time the data for the object is loaded from the DB and the proxy gets "filled in" with the real Hibernate object. This being the case, if your Hibernate session is closed, there's no way that the lazy data can be read.

This problem also happens with object relationships where the other side of the relationship is a Proxied object. In this case the related object is only loaded when a property of the object other than the ID or Version is accessed for the first time.

The solution is to keep your Hibernate Session open. If you're writing a GUI then the ThreadLocal pattern can work well. If you're writing a Web application then the "Open Session In View" pattern can work well also. I've used both of these with great success. The idea is that a Hibernate Session is associated with the currently running thread. When the Thread goes out of a certain scope, the session is closed automatically. In the Open Session In View case, the Session is managed by a Servlet Filter that is responsible for opening a session at the beginning of the request and closing it at the end.

There are MANY posts and resources available on this topic. Search the Hibernate site and you'll find them.

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mexihca

Post subject:

Posted: Thu Jan 13, 2005 5:51 pm

Newbie

Joined: Mon Jan 10, 2005 2:33 pm
Posts: 4

Thanks for your answer!!

I going to implement the patterns.

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