What is the best way to handle domain exceptions in Tapestry web application?

Question

I'm having some dilemmas on how to process and handle checked exceptions that are thrown by my domain objects while using Tapestry to create the web GUI as a presentation layer.

Assume I have a domain object, Foo that throws FooException on one of its methods:

public class Foo {
    ...
    public void fooMethod throws FooException() {
        ...
    }
    ...
}

Now, assume I have a Tapestry page called Bar where Foo object is, for example, being edited with BeanEditor.

Now, to ensure illegal values are not passed to BeanEditor for creating Foo object, I can think of two basic ways to do that:

1. Tapestry field validation using @Validate annotation

   In this case, if we can filter and check inputs via regexes or by limiting values or doing similar actions made available by @Validate we'll get a nifty error message next to the field we're editing and the submit will fail, thus making the user think about what he wrote there and how to fix it.

2. Catching domain exception and doing actions based on it

   I assume this scenario offers more options regarding what can and cannot be done. For example, if the used needs to enter URL and makes a mistake while doing so, the URL constructor will throw its own MalformedURLException. We can catch that exception in our Java code, but my question is, what to do next and how?

Does Tapestry offer any special mechanisms of handing domain exceptions (checked and/or unchecked), other than that exception window that pops up when things break apart?

Are there any patterns on how to solve this particular problem?

Where do you draw the limit between common and ordinary exceptions like IndexOutOfBoundsException and some domain-specific ones like FooException?

:D

Answer 1

The best way is applying validations to fields. One way is using @Validate. Another one is to use the Bean Validation (JSR 303) annotations by adding tapestry-beanvalidator, which is explained here: http://tapestry.apache.org/bean-validation.html.

In addition, in Tapestry, all form field component trigger a "validate" event before the value is applied to the property.

http://tapestry.apache.org/forms-and-validation.html explains it all. For example, supposing you have a form field with a t:id of "count", you could validate it by declaring an event handler method:

void onValidateFromCount(Integer value) throws ValidationException {
    if (value == 13) throw new ValidationException("Thirteen is an unlucky number.");
}