Monday, July 02, 2007

Good code tells you where it hurts

I've been meaning to write an entry on exception handling and how important it is to not throw away information. So, here it is. I'll start with one my pet peeves below:
try {
...hard stuff here...
} catch(Exception problem) {
problem.printStackTrace();
}

OK, first off, I'm usually skeptical of catch all blocks at low levels. But, there can be good reasons for them, but the problem I have with code above is printing the stack to the standard out and continuing. At the very least, this is only good for when you are doing command line utilities and even then it's not good. Worse yet, this is the default from Eclipse! I've been bitten too many times where code went wrong and we couldn't find out why because the stack was written to the standard out and we couldn't see what it was. Or maddeningly the code continued and caused other problems further down. Fun to debug those problems it really is. My solution is if it's informational and you can continue log it, if not re-throw the exception. But, there's times when you catch a checked exception, but it's something that's serious enough to stop current processing. I've seen this:
try {
...something hard here...
} catch(Exception ex) {
throws new RuntimeException("Something bad happened");
}

Thankfully, I haven't seen the above code so much since 1.4 added support for wrapped exceptions. But, I still see it. The problem with the above is the original exception is dropped. When debugging or trying to find a problem, the above code gives me no information of the original context. It's useless, but it did stop bad things from happening further down. But, it would be almost possible to find out what really went wrong.

Always catch the most specific exception and leave the catch all exception handlers to the top level code. It will save you headaches, I promise you.

My rule of thumb to exception handling is: if something goes wrong, what would I want to know? I see too much code where this is forgotten or it is written as if nothing will go wrong. In a perfect world, all code works beautifully and there is no need for exception handling, but we don't live in that world. As always think about helping the poor developer that has to come behind you.

1 comment:

malcolmsparks said...

I complete agree with you Blaine about not throwing away information. Keep everything, and add to that information whenever you have something to add. For example, when a caller catches an exception, the caller always has some indisputable and unique information it can add: exactly what it was trying to attempt when the failure happened.

Dropping this information is like using a shredder. It's not responsible behaviour for developers to shred information that might be useful to the poor sod who had to fix your buggy code at 4am in a production environment.

Actually, such information is often the 'single golden clue' that leads to a rapid diagnosis of the error. For this reason, I believe it's justified to add such information even when unchecked exceptions occur. "Yes, you got a NullPointerException, but what exactly were you trying to do when you caught the NullPointerException?"

For that reason only, I disagree with you about catching only the most specific exception- I always catch, wrap and rethrow any subclass of java.lang.Exception. I started handling exceptions in Java this way back in 1997, and haven't seen anything since that has caused me to change my mind (but I'm open to debate).

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