Resteasy (JBoss AS 6) validation - jboss

Question

I'm looking for a good pattern of providing custom validation of input for Resteasy services.
Let's say I've this service:
#Local
#Path("/example")
public interface IExample {
public Response doSomething ( #QueryParam("arg1") String arg1, #QueryParam("arg2") Integer arg2);
}
which I've implemented:
#Stateless
public class Example implements IExample {
#Override
public Response doSomething ( String arg1, Integer arg2 ) { ... }
}
What's the best practice to validate arg1 and arg2?
My ideas:
Validate inside doSomething(...) method. Drawback: when I add some parameter (ex. arg3) in the future, I could easily forget to validate it.
In custom javax.servlet.Filter. Drawback: I cannot access arg1 and arg2 there as they're not yet parsed by Resteasy framework.
I came up with this concept:
public class ExampleValidator implements IExample {
public static class ValidationError extends RuntimeException { ... }
#Override
public Response doSomething ( String arg1, Integer arg2 ) {
// here do validation. In case of failure, throw ValidationError
return null;
}
}
which can be used as follows:
#Stateless
public class Example implements IExample {
#Override
public Response doSomething ( String arg1, Integer arg2 ) {
try {
(new ExampleValidator()).doSomething(arg1, arg2);
} catch ( ValidationError e ) {
// return Response with 400
}
}
}
In that way, when I change IExample.doSomething method signature, I have to update Validator because of compile time error. In order for Resteasy NOT TO interpret ExampleValidator as a service, I used resteasy.jndi.resources instead of resteasy.scan, but it fails (Example bean is loaded after resteasy attempts to use it on deploy time).
Any ideas - are there any good patterns of validation?
Or is it possible to somehow get my concept to work?
EDIT: Or, which would be the best, there is some Filter counterpart in Resteasy? Some scheme by which my method (Filter) would be called before actual implementation, but with parameters (arg1, arg2) already parsed?
Thanks in advance, sorry for a lengthy post ;)
Kamil

Answer 1

(1) Probably the cleanest approach is to use Java EE 6 Bean Validation framework. This would require writing custom validation interceptor. In this case you would have to change your methods, so instead of
public Response doSomething ( String arg1, Integer arg2 )
you would use domain object as an argument
public Response doSomething ( JAXBElement<MyDomainObject> myOJaxb )
Then you need to trasform your request, so they provide XML or JSON formatted data, which can ba automatically converted to actual object.
(2) Another option is to use normal ServletFilter.
(3) Prepare custom annotations a'la Bean Validation, then you need to plug-in custom annotations processor (look at project Lombok, as an inspiration).
(4) The simplest solution is to use built-in REST validation
#Path("users/{username: [a-zA-Z][a-zA-Z_0-9]}")
but this applies to path parameters, not query parameters (I think, but didn't check with JAX-RS spec)
Your choice depends on how much flexibility you have with your interfaces and how many time you have.
If you would come up with a generic, pluggable to Resteasy solution similar to suggested in option (3) and make it open source on GitHub many people would love you :)