This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
Struts has support for validating all the primitive types. If you one about a big decimal, you could write your own custom validator rule. See the book "Professional Struts" written by James Goodwill and Rick Hightower for step-by-step instructions for creating your own custom validator rule. Alternatively, you look at the source code for the existing validator rules.