Tree-like data structure in J2EE

Hi,

In my project I'm trying to use the Value Object Patern (aka Bulck Data Obje
ct Patern)
Nut, my VOs can refer to other VOs like a tree in witch a node can refer to 
another node.
Is a structure like this one, is possible that in some point a VO references
 a VO in its owner upper
structure , like this

A -> B -> C -> A

The problem when loading this VO is that I get a infinite loop.
The client requests a VO from the server. In the server, a facade Stateless 
Session Bean is in
charge of compiling the request. For this , the bean consults various entity
 beans constructing the
various VO.
So to parcialy resolve the problem I use a POJO that acts as a memory (a cac
he). Each time a VO is
loaded is added to the memory. The next time is requested, the cached object
 is returned.
This works fine, the problem is that by caching the objects I lose control i
f they are updated or
not since another separated process is in charge of saving the VO send by th
e client. (this process
is implemented in the stateless bean). So, when saving I remove the object f
rom the loading memory,
and when loading I remove the object from the saving memory. (the same loop 
occurs when saving, so
use the memory to avoid saving the same object over and over).

For a single client acessing the server this works , but for multiple client
es I have a serious
cuncurrency problem. Because this is becaming very non-standard I ask you if
 anyone had a similar
problem and how it was workedarround. Or, if there is a special pattern for 
this kind of VO. Or, if
anyone as any idea , at all, that can help solving the cunccurency problem.

Thanks in advance.