Servlet Session

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Hi, i have a problem with flex session servlet.
This code work, but have a great problem.
If i wish to store in session from Flex ,a collection of Pojo this 
code don't work.
For example i have a CustemerVO i wish to store it in session and 
used it by my servlet with simple session.getAttribute("CustomerVO") , 
it's possibile with flex to do it.
Can you help me please.

here my code,
Devis

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.macromedia.com/2003/mxml"; 
backgroundColor="#FFFFFF" height="80%" width="80%">
<mx:RemoteObject id="servlet" source="servlet"/>



<mx:Script>
<![CDATA[
import mx.controls.Alert;
import vo.as400.*;
var vopf:PfVO;

function genPDF() {
vopf=new PfVO();
vopf.de50pf="Balsemin";
vopf.stagione.cdc_stag="1995";
vopf.strepf="A";
servlet.session("set",vopf);

var pdf_url = "/test/view/JavaPdf.pdf";
getURL(pdf_url,'_blank');
}
]]>
</mx:Script>
<mx:Panel title="Printing to a pdf" height="100%" width="100%">
<mx:Label text="Enter some below and then press the print 
button." width="100%"/>

<mx:TextInput id="t1" text="Enter some text here!" width="80%"/>
<mx:ControlBar>
<mx:Button label="Print" click="genPDF()"/>
</mx:ControlBar>
</mx:Panel>
</mx:Application>