When a StringParameterUnmarshaller is registered for another class, the ResteasyProviderFactory will break with a null pointer exception if another type is requested.

if (stringParameterUnmarshallers.size() == 0) return null;
Class<? extends StringParameterUnmarshaller> un = stringParameterUnmarshallers.get(clazz);
StringParameterUnmarshaller<T> provider = getProviderInstance(un);

The map is not empty (size != 0), but there is no registration for clazz, thus un wil be null and on the last line a NullPointerException will be thrown.