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Failure and Exceptions
A Conversation with James Gosling, Part II
by Bill Venners
September 22, 2003

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Exception Handling versus the One True Path

Bill Venners: What percentage of time do you think people should be spending programming the one true path—the path taken if nothing goes wrong—versus handling all the potential errors?

James Gosling: It's all over the map. Ideally you should be spending almost all of your time on the one true path. But if you look at various domains, the percentages are different. The worst case would be people doing avionics software. If they spend a few percent of their time working on the one true path, they feel very good about themselves. That's the kind of area where the high nineties percentage of your time is spent dealing with things that could go wrong, and speculating about things that could go wrong. In that area, life is truly dynamic. There are real random number generators up there, with turbulence doing strange things to wing tips and all kinds of weird stuff that may never actually happen.

Almost as bad as avionics is networking software. When you're trying to write some piece of software that is dealing with something across the network, trying to make that truly reliable can be a lot of work. I've done code that tries to maintain reliable database access, for example, and that can be 90% error handling. You look at most protocol implementations and at some level they're almost all error handling. If you believed everything was perfect all you would do was spew packets. You wouldn't worry about timeouts, flow control negotiations, and any of the other things that go on. Networking is all about how you deal with errors at all various different levels. But when you're in a fairly well-controlled regime like, "Here's a web request, compute the result please," there shouldn't be a whole lot that you're actually worried about.

Bill Venners: Yeah, I think there's a spectrum. At one end you have pace makers and airplanes. If you don't deal with certain kinds of unexpected situations people can die. At the other end you have one off scripts. I remember one script I wrote that was pulling information out of web pages and putting it into XML files, and the script worked on maybe 90% of the pages I was processing. And that was good enough because it was faster for me to fix the other 10% by hand than to make the script better at dealing with badly formed input files. In that case, there was a system administrator who was quite happy to spend an hour fixing things by hand, because that was cheaper than spending 2 or 3 hours trying to get the script to be more solid.

James Gosling: Right. It all depends on what the consequences of failure are. If you're just extracting information from pages like that and you're a system administrator, the consequences of failure are pretty minor. But if you're writing transaction reconciliation software for some bank, you could find that oh, thirteen billion dollars leaked out of some system because of a bug. I actually had a friend who basically lost thirteen billion dollars in one night because of a bug.

Bill Venners: Thirteen billion dollars?

James Gosling: Yep. Thirteen billion. They were actually able to reconstruct it from transaction logs, but it was a real big mess. Thirteen billion dollars is real money, so you're actually probably willing to test, willing to spend some time in dealing with handling those exceptions and making things reliable.

Bill Venners: I have heard a lot of people complain that in practice Java's checked exceptions encourage a lot of empty catch clauses. This empty catch clause problem seems to be more common than exceptions being propagated in throws clauses going all the way up the call stack. The compiler forces a programmer to catch an exception, so they catch it but they don't do anything. Then if the exception is ever thrown, it ends up being swallowed. It is not handled and not propagated. It is lost. Do you think that in a lot of places there's a lack of a culture of programmers wanting to deal with failure, perhaps because in the past they didn't have to deal with failure as much?

James Gosling: Anytime somebody has an empty catch clause they should have a creepy feeling. There are definitely times when it is actually the correct thing to do, but at least you have to think about it. In Java you can't escape the creepy feeling.

It really is a culture thing. When you go through college and you're doing assignments, they just ask you to code up the one true path. I certainly never experienced a college course where error handling was at all discussed. You come out of college and the only stuff you've had to deal with is the one true path. You get a job working in some IT department's data center, and they're using the software in production runs. If it doesn't work, it's a real problem. All of a sudden there's all this painful stuff that you don't like to do, and you're feeling grumpy about it because this isn't as much fun as just writing clean code.

There's really nothing a hacker likes more than having an empty editor buffer. You can just start to write code. That's the way university assignments are: here's a problem, just write the code. The real world is much cruftier.

But there's also a spectrum. You talk to people in banks, where large quantities of money get lost if there's a problem. They take failure very seriously. The spookiest folks are the people who have been doing real time software for a long time. I've spent a certain amount of time with that crowd. By and large these folks are very conservative and very careful. A lot of them know what it's like to go visit the family of the deceased and explain the bug to them. There are a number of places that have policies that if a test pilot augers in, then once it's all figured out what happened, the people who engineered the thing that failed have to go explain it to the family. I've actually only met one person who has ever actually had to do that. That would change your attitude about dealing with failure really quickly.

Next Week

Come back Monday, September 29 for Part V of a conversation with C#'s creator Anders Hejlsberg. I am now staggering the publication of several interviews at once, to give the reader variety. The next installment of this interview with James Gosling will appear in the near future. If you'd like to receive a brief weekly email announcing new articles at Artima.com, please subscribe to the Artima Newsletter.

Talk Back!

Have an opinion about refactoring tools, program visualization, or JavaDoc? Discuss this article in the News & Ideas Forum topic, Failure and Exceptions.

Resources

James Gosling's Home Page:
http://java.sun.com/people/jag/

Anders Hejlsberg's comments on checked exceptions are in this interview, "The Trouble with Checked Exceptions":
http://www.artima.com/intv/handcuffs.html

Bruce Eckel wrote an essay questioning the value of checked exceptions, "Does Java Need Checked Exceptions?"
http://www.mindview.net/Etc/Discussions/CheckedExceptions

Bill Venners has interviewed James Gosling each year for the past five years. The four previous interviews are:

James Gosling discusses semantic models, mobile behavior, abstraction versus vagueness, testing, and repetitive stress injury. (February 2002):
http://www.artima.com/intv/goslingD.html

James Gosling speaks on inheritance and composition, JSPs and Servlets, community design processes, and more. (May 2001):
http://www.artima.com/intv/gosling3.html

James Gosling discusses developer tools, the realtime JVM, mobile objects, strict interfaces, and more. (May 2000):
http://www.artima.com/intv/gosling2.html

James Gosling speaks on interfaces and protocols, servers and services, Jini, Java in the enterprise, and more. (May 1999):
http://www.artima.com/intv/gosling1.html

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