Exceptions or not (Was: RE: ModSAX (SAX 1.1) Proposal)

[Miles Sabin]

Jarle Stabell wrote,
> True. I love exceptions and find that they greatly 
> improves the robustness of applications, but the 
> reason I'm not convinced about whether it is good 
> to be forced to specify what will be thrown is that 
> this in many cases seem to require psychic powers of 
> the designer (or that the "real" exceptions must be 
> catched and converted into an "acceptable" one, which 
> looses information).

What you need is (to coin a phrase) an 'exception 
tunnel'. For example,

  public class SAXExceptionTunnel
    extends SAXException
  {
    private Throwable itsThrowable;

    public SAXException(Throwable throwable)
    {
      itsThrowable = throwable;
    }

    public Throwable getThrowable()
    {
      return itsThrowable;
    }

    public void rethrow()
      throws Throwable
    {
      throw itsThrowable;
    }
  }

You could use it like this,

  public class Foo
  {
    public void someMethod()
      throws SAXException
    {
      try
      {
        // do some stuff
      }
      catch(SomeWierdException ex)
      {
        throw new SAXExceptionTunnel(ex);
      }
    }
  }

  Foo someFoo = new Foo();

  try
  {
    someFoo.someMethod();
  }
  catch(SAXExceptionTunnel ex)
  {
    // deal with tunnelled exceptions here
    System.err.println(ex.getThrowable().getMessage());
    ex.rethrow();
  }
  catch(SAXException ex)
  {
    // deal with ordinary exceptions here
  }

I've found this idiom extremely useful in many 
situations where I've wanted to register callback 
handlers without either imposing restrictions on them
(ie. no IOExceptions allowed to leak out of the handler)
or having to use ludicrously general throws clauses.

Cheers,


Miles

-- 
Miles Sabin                          Cromwell Media
Internet Systems Architect           5/6 Glenthorne Mews
+44 (0)181 410 2230                  London, W6 0LJ
msabin at cromwellmedia.co.uk           England


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