[getting OT ...] Re: [antlr-interest] exceptions in the lexer

Wed Dec 8 01:25:34 PST 2004

Maybe I should cite here the slides from my Java course about Exception
handling ... a short summary of my advice:

RuntimeExceptions have three main uses:

a. Working around "checked exception" boundaries (usually by wrapping a
checked exception). However: Before doing this, change the interface so that
it allows the additional Exception. Only when you cannot do this (or don't
want to - see also c.), use the wrapping.

b. Signalling programmer errors (e.g. IllegalArgument, IllegalState,
NullPointer). Before catching them, spend as much time as possible on
*correcting* the programmer error. If this is not possible (external
sources) or if programmer errors are legitimate according to the spec
(high-availability software - programming Space Shuttle software in Java?),
then catching them is necessary - but that should raise a red flag in
project and quality management!

c. Sometimes, signalling "unexpected environment exceptions" (e.g.
EJBException, exceptions in JDO) that can happen "everywhere" in that
environment. There is still some debate whether one should use this design
at all, but the "no checked exceptions" party is growing; and if/when/where
they are right, catching those RTExceptions is right. However, this requires
a spec as to how to proceed: In many cases, the internal state of the
program is still ok, and then proceeding is no problem; however, in some
cases, a "subsystem" may have gone into an unusable state due to the
exception - and then you must either
(*) re-initialize the subsystem; or
(*) "shut it off", i.e., disable its use by other subsystems/services -
which may entail other disablings relying on those subsystems/services. If
you don't think about this, your system might "limp along" for some more
time and then still fail - but now, finding the problem is much harder than
with "early failure".

So, in general, I'm wary of catching RuntimeExceptions.
At least, log all of them reliably(!) somewhere where they can be and are(!)
read!

Errors have two main uses:

d. Signalling "very unexpected program state exceptions" (AssertError).
This is like b. above: Dont catch it, correct the problem.

e. Signalling "unexpected JVM exceptions" (OutOfMemory, ThreadDeath).
This is like c. above: Catch it if it pays; but be aware and specify what
should happen if subsystems go out of order.

Thus, I'm not so wary of catching Errors. But still, logging is required.

Regards
Harald M. Mueller

> 
> Yes it may not work in some cases but in those cases that it does
> you have allowed the user to save the work they have done and continue to
> work using other areas of the application .
> In the JEdit example, all this error means is that you cannot open that
> particular file
> as it is too large. Your existing edits are not dumped by a programmer who
> is too lazy (timid?) to try and recover from errors.
> 
> In those cases where you cannot recover, what have you lost by trying?
> What was the alternative to trying to recover?
> Just shutting down the application and loosing all the user's existing
> work!
> No merit points for doing that.  Your application will get a reputation as
> being
> unreliable.
> 
> I want to program robust applications.  I want to keep trying to recover
> from
> ALL Throwables untill the the JVM or the OS dies.
> The user deserves as much.  If the JVM or OS dies as a result of an error
> trying to recover does not make it worse.
> 
> Programming errors during the developement of my Parallel program
> (www.forward.com.au)
> regularly caused it to try and recovere from NullPointers,
> ArraysOutOfBounds
> and
> OutOfMemory.  These errors never shut the whole application down.
> 
> Java's advanced garbage collection means it is much more robust in the
> face
> of errors then
> previous languages.  But it seems programmers (and even the language
> designers) do not
> realize how robust it is.
> 
> In many cases you can sucessfully recover from Errors and
> RuntimeExceptions.
> 
> It is dissapointing that the text books have trained a generation of Java
> programmers who
> will not even try.
> 
> matthew
> ----- Original Message ----- 
> From: "Harald Mueller" <harald.m.mueller at bigfoot.com>
> To: <antlr-interest at yahoogroups.com>
> Sent: Tuesday, December 07, 2004 9:56 PM
> Subject: [getting OT ...] Re: [antlr-interest] exceptions in the lexer
> 
> 
> >
> > >
> > > Well JEdit catches this and recovers.
> > > Why can't I?
> >
> > You can :-) But there's no guarantee it works. Here is the text from the
> JLS
> > (section 8.4.4) - which I interpret as a more constructive comment than
> > mine:
> >
> > > Exceptions that are represented by the subclasses of class Error, for
> > example OutOfMemoryError, are thrown due to a failure in or of the
> virtual
> > machine. Many of these are the result of linkage failures and can occur
> at
> > unpredictable points in the execution of a program. Sophisticated
> programs
> > may yet wish to catch and attempt to recover from some of these
> conditions.
> >
> > Regards
> > Harald M. Mueller
> >
> > > matthew
> > >
> > > ----- Original Message ----- 
> > > From: "Harald Mueller" <harald.m.mueller at bigfoot.com>
> > > To: <antlr-interest at yahoogroups.com>
> > > Sent: Tuesday, December 07, 2004 8:48 PM
> > > Subject: Re: [antlr-interest] exceptions in the lexer
> > >
> > >
> > > >
> > > > >
> > > > > Hey, Just extend you user-defined exceptions from RuntimeException
> and
> > > > > throw
> > > > > them.
> > > >
> > > > He explicitly didn't want to do this - but I also do it (using a
> > > > WrappingRuntimeException which I need at other places anyway (e.g.
> for
> > > the
> > > > Runnable.run() method)).
> > > >
> > > > > You catch all your RuntimeExceptions don't you?
> > > > > Remember OutOfMemoryException is not necessarly fatal.
> > > >
> > > > This is not an Exception, but an Error - you would never catch this.
> > > >
> > > > Regards
> > > > Harald M. Müller
> > > >
> > > > > matthew
> > > > > ----- Original Message ----- 
> > > > > From: "Paul J. Lucas" <pauljlucas at mac.com>
> > > > > To: <antlr-interest at yahoogroups.com>
> > > > > Sent: Tuesday, December 07, 2004 11:10 AM
> > > > > Subject: Re: [antlr-interest] exceptions in the lexer
> > > > >
> > > > >
> > > > > >
> > > > > > On Mon, 6 Dec 2004, Terence Parr wrote:
> > > > > >
> > > > > > > Are you familiar with the filter option on the lexer grammar?
> > > > > >
> > > > > > Yes in the sense that I've read that bit of the documentation,
> > > > > > but I've never used it.
> > > > > >
> > > > > > I don't want my entire lexer to be a filter since I need it to
> > > > > > be a real lexer for a parser.
> > > > > >
> > > > > > My particular case at hand is dealing with pragmas that can
> > > > > > appear anywhere comments can.  Since comments are most easily
> > > > > > handled in the lexer, I handle pragmas there to for the same
> > > > > > reason.  The problem is what to do if there's an error in a
> > > > > > pragma.  The lexer can't throw user-defined exceptions.  (I
> > > > > > might be able to work around it by hiding the real exception
> > > > > > inside a Runtime exception, but that's ugly.
> > > > > >
> > > > > > - Paul
> > > > > >
> > > > > >
> > > > > >
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