2.4. Presentation layer validation

When working with JSF and JBoss Seam, one can triggers the validation process at the presentation layer using Seam's JSF tags <s:validate> and <s:validateAll/>, letting the constraints be expressed on the model, and the violations presented in the view 
<h:form>
    <div>
        <h:messages/>
    </div>
    <s:validateAll>
        <div>
            Country:
            <h:inputText value="#{location.country}" required="true"/>
        </div>
        <div>
            Zip code:
            <h:inputText value="#{location.zip}" required="true"/>
        </div>
        <div>
            <h:commandButton/>
        </div>
    </s:validateAll>
</h:form>
Going even further, and adding Ajax4JSF to the loop will bring client side validation with just a couple of additional JSF tags, again without validation definition duplication.
Check the JBoss Seam documentation for more information.