Ejb cache with singelton implementation

[arik salv]

I would appreciate your advice about an issue I have.

I'm working with jboss, EJB 3.1 environment.

each time the user entering a specific screen, it should be a trigger for creating a tree data type which based on a data that saved on the database. this tree calculation can take some time, and it's can be heavy on performance.

the following requests are sent from UI :

1. refreshTree - a trigger for building the tree
2. isTreeReady - indicating whether the tree is ready to use, and called every x amount of time
3. getTree - return the tree.

when building this I should take into consideration that multiple users can try to perform each one of those actions simultaneously. it's also true that 2 users share the same tree if any refresh hasn't been done.

I've thought about implementing it as a cache as follow :

@singleton
public class TreeCache{

@EJB
MyTree tree;

boolean isTreeReady = false;

@Lock(LockType.WRITE)
public void refresh(){
      isTreeReady = false;
      tree = calulcateTree() \\ heavy calculation
      isTreeReady = true;
}

public boolean isTreeReady(){
      return isTreeReady;
}

public MyTree getTree(){
      return tree;
}

}

the issue I have with that is that there can be a scenario in which :

first user refreshing the tree - and the tree is built.
then second user start to build the tree, and initialize the isReady flag to false, before the first user has noticed about the tree calculation - in this case the first user will need to wait to the calculation to be completed (even though he could have use the tree).

I'm trying to think a bout using a read lock in some way (instead of the isTreeReady flag), but I can't think about any that will fit my needs.

do you have an idea what can I do?

thanks.