where is the best place of validation?

[acec acec]

Hi,
After I read "Best Practices Guide", I still have a
question: "where is the best place of validation?"

I found "Another good example is doing special
pre-processing and validation on parameters before
passing them to a service for processing" in that
guide, so the validation should not be a part of
service, then I try to find an example, which has
validation logic.
But I found all validations were done in service, such
as: 
DhlServices.java, 
public static String sendDhlRequest(String xmlString)
{
...
 if (xmlString == null) {
            throw new DhlConnectException("XML message
cannot be null");
        }
...
}

Thanks a lot.
tomcat acec


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