Exception handling in the ActionServlet

[kl...@ubs.com]

Hi,

I would like to catch all of our project specific exceptions in one place (as a 
last resort, if nobody else catches them earlier on) and handle them in a 
generic way. The obvious way to do this, I think, is by overriding 
processActionPerform in our own <Project>ActionServlet and catch 
<Project>Exception's (superclass for all our own exceptions) there. But the 
Action.perform method only throws IOException & ServletException, so either all 
our own exceptions will have to inherit from either of those two classes (can't 
be the right thing to do), or I will have to change the Action.perform 
definition to also throw our <Project>Exception's (doesn't seem right either, 
I'd prefer not to have to alter any struts code)?

What should I do?

This actually brings me to another problem that doesn't have anything to do 
with struts, but is more a general Java question. Anyway, maybe somebody here 
has an elegant solution to it:

Assume the problem above is solved and furthermore that some of the project 
specific exceptions have to inherit from other existing exception classes (e.g. 
ServletException, IOException, etc.). Since we don't have multiple inheritance, 
I can't have a <Project>Exception that is a superclass to all my own 
exceptions. I still want to catch all our own exceptions (but no others) in one 
place. My first though was of course to define a <Project>Exception interface 
and let all our own exceptions implement this interface, thus being able only 
to catch <Project>Exception's. But Throwable is not an interface (why not? 
What's the idea behind this design?) that <Project>Exception could inherit from 
and thus this approach doesn't work, as the try{}catch(){} statement expects a 
Throwable object as parameter (try{}catch(<Project>Exception e){} doesn't 
compile).

Well, as a kind of a hack I then thought I'll catch all exceptions, check if it 
is a project specific one (<Project>Exception.class.isInstance(anException)), 
and if not throw it again, but that can't be right, because the method where 
this happens then will have to be declared to throw Exception (to generic).

I'll appreciate any help, thanks in advance and the best satisfactory answer 
wins a free beer in Zurich ;-)


Klaus Bucka-Lassen

Re: Exception handling in the ActionServlet

["Craig R. McClanahan" <cr...@apache.org>]

On Sat, 9 Jun 2001 klaus.bucka-lassen@ubs.com wrote:

> Hi,
> 
> I would like to catch all of our project specific exceptions in one place (as a 
> last resort, if nobody else catches them earlier on) and handle them in a 
> generic way. The obvious way to do this, I think, is by overriding 
> processActionPerform in our own <Project>ActionServlet and catch 
> <Project>Exception's (superclass for all our own exceptions) there. But the 
> Action.perform method only throws IOException & ServletException, so either all 
> our own exceptions will have to inherit from either of those two classes (can't 
> be the right thing to do), or I will have to change the Action.perform 
> definition to also throw our <Project>Exception's (doesn't seem right either, 
> I'd prefer not to have to alter any struts code)?
> 
> What should I do?
> 

How about having the base class for you project exceptions be a subclass
of ServletException?  That way, you can throw it if you really want
to.  Then you could declare an <error-page> element in your web.xml file
to define common handling (if you don't the default will be an ugly stack
trace on most containers).

> This actually brings me to another problem that doesn't have anything to do 
> with struts, but is more a general Java question. Anyway, maybe somebody here 
> has an elegant solution to it:
> 
> Assume the problem above is solved and furthermore that some of the project 
> specific exceptions have to inherit from other existing exception classes (e.g. 
> ServletException, IOException, etc.). Since we don't have multiple inheritance, 
> I can't have a <Project>Exception that is a superclass to all my own 
> exceptions. I still want to catch all our own exceptions (but no others) in one 
> place. My first though was of course to define a <Project>Exception interface 
> and let all our own exceptions implement this interface, thus being able only 
> to catch <Project>Exception's. But Throwable is not an interface (why not? 
> What's the idea behind this design?) that <Project>Exception could inherit from 
> and thus this approach doesn't work, as the try{}catch(){} statement expects a 
> Throwable object as parameter (try{}catch(<Project>Exception e){} doesn't 
> compile).
> 
> Well, as a kind of a hack I then thought I'll catch all exceptions, check if it 
> is a project specific one (<Project>Exception.class.isInstance(anException)), 
> and if not throw it again, but that can't be right, because the method where 
> this happens then will have to be declared to throw Exception (to generic).
> 
> I'll appreciate any help, thanks in advance and the best satisfactory answer 
> wins a free beer in Zurich ;-)
> 

I'd investigate extending ServletException for your application exception
classes.

Another off-the-wall idea would be to extend RuntimeException instead --
you can throw such an exception without it being listed in the
"throws" clause.  This is how things like IllegalArgumentException and
NullPointerException work.

> 
> Klaus Bucka-Lassen
> 
> 

Craig