U can use it in following structure:

ParentException : which extends Exception, and is also abstract and have an          abstract method it can be called translateException().
DataTierException : which extends ParentException, so it supposed to implement translateException method, it's would be implemented like this:
          translateException( Exception ex ){
             if ( ex instanceof Nullpointer )
                   throw new DataTierException( SpecificErrorCode )

BusinessTierException : which extends ParentException, so it supposed to implement translateException method, it's would be implemented like this:
          translateException( Exception ex ){
             if ( ex instanceof Nullpointer )
                   throw new BusinessTierException ( SpecificErrorCode )
WebTierException : which extends ParentException, so it supposed to implement translateException method, it's would be implemented like this:
          translateException( Exception ex ){
             if ( ex instanceof Nullpointer )
                   throw new WebTierException ( SpecificErrorCode )

So in each tier can be like this:

try{
    // do sth here
    DataTier.methodCall();
}catch( DataTierException ex )
{
    throw businessException.translateException( ex );
}

Struts can be configured for exception  handling via struts-config. It
is called "Declarative Exception Handling" .
Take a look at below url for an overall view of this feature:
javaboutique.internet.com/tutorials/excep_struts/

As you might notice, the goal is to remove exception handling  from
code to configurations classes. You just define the type of exceptions
and the class  that will go to handle them. You can configure it for
each action  too.

I have not used this feature, but seem to be very useful. This applies
to struts  1.1 but maybe new features for this in struts1.2.