Currently, there doesn't seem to be a method of validating an indexed property (ie. a collection) directly or indirecetly within an action.
For instance, given an action OrderAction with a property order of type Order where order contains a collection trades of type Trade, the following is not possible:
The closest thing to support for something like the above is using the VisitorFieldValidator validator. Although it has the following disadvantages:
1. It must be placed on the target object (invasive and not context sensitive)
2. It does not work with an indirect collection (as in the example above)
Something like this should be possible (even struts1 validator supports indexedListProperty)
On a related topic, it should be possible to do the following to display the error in the resulting view: