Re: SAX: Exception Handling

James Clark (jjc@jclark.com)
Mon, 04 May 1998 20:56:12 +0700

David Megginson wrote:
>
> James Clark writes:
>
> > With my application writer's hat on (I just converted XMLTest to the new
> > SAX), the current solution is a significant step backwards. The typical
> > simple SAX program that processes an XML document and produces some sort
> > of output is going to have to make calls in its DocumentHandler
> > implementation that throw IOExceptions, and each of these methods now
> > have to do:
> >
> > try {
> > ...
> > }
> > catch (IOException e) {
> > throw new SAXException(e);
> > }
> >
> > The original goal of SAX was to be simple and easy to use for
> > application writers. I don't think requiring this sort of mumbo jumbo
> > even for trivial programs is consistent with this goal.
>
> I stood by the same position for nearly five months, in the face of
> very loud opposition from people whose opinions I have great reason to
> respect (as I have yours).
>
> However, upon reflection (or perhaps simply mental exhaustion), I
> realised that throwing java.lang.Exception was a false economy. When
> I write
>
> public void endElement (String name)
> throws java.lang.Exception
> {
> [..]
> }
>
> The Java compiler will not help me discover which constructors or
> methods invoked in the body can throw exceptions that I have not yet
> considered.

Right, so don't do that. Declaring an *interface* as throws
java.lang.Exception does not constrain an *implementation* of that
interface to be declared as throws java.lang.Exception. It can be
declared to throw the appropriate subclass of java.lang.Exception.

If you declare the interface

public void endElement(String name) throws java.lang.Exception;

then I can declare my implementation as

public void endElement(String name) throws java.lang.IOException {

}

or

public void endElement(String name) throws java.awt.AWTException {
}

or whatever is in fact appropriate to my implementation. In other words,
it is only be declaring the interface as java.lang.Exception, that I can
correctly declare the exceptions that by implementation throws and
thereby take advantage of Java's exception checking.

I just don't follow your argument at all.

> > I don't see an ideal solution, but I can think of several possibilities
> > in addition to the old solution and the current solution, any of which I
> > think would be an improvement over the current solution:
> >
> > 1. The handler methods are declared to throw both SAXException and
> > IOException (as I proposed for parse). My guess is that throwing
> > IOException is going to be very common, and will avoid the need for the
> > user to wrap exceptions themselves in a large proportion of simple
> > programs. I can't see any disadvantages over the current proposal.
>
> Your guess supposes that one class of XML application dominates.
> Database applications (and they may come to dominate XML) will more
> typically need to throw java.sql.SQLException, while GUI applications
> will often need to throw java.awt.AWTException.

I am not saying it will dominate but I am saying it will be a
significant class, and the other classes wouldn't be hurt by making it
easier for this class. I don't see much merit in the argument that
because we can't make things simpler for *all* applications we therefore
shouldn't make things simpler for *some*.

James