a little madness

What do you do when your app explodes with a StackOverflowError after oh, say, a week of operation? Of course, you just follow the stack trace to get to the line that caused the explosion, and work from there. Nice theory, but what about when the end of your stack trace looks like:

at java.util.Collections$UnmodifiableCollection$1.(Collections.java:1007)
at java.util.Collections$UnmodifiableCollection.iterator(Collections.java:1006)
at java.util.Collections$UnmodifiableCollection$1.(Collections.java:1007)
at java.util.Collections$UnmodifiableCollection.iterator(Collections.java:1006)
at java.util.Collections$UnmodifiableCollection$1.(Collections.java:1007)
at java.util.Collections$UnmodifiableCollection.iterator(Collections.java:1006)

? Last time I checked Collections weren’t runnable … so we’ve only got part of the trace, and the more worthless part at that. Well,at least it gives us some idea. With a little bit of mental gymnastics, and a bit of luck, you might guess that the temporal nature of the problem has something to do with that enterprise scheduler you’re using. Nice guess! Bump up the frequency of the schedule, throw a breakpoint into Collections$UnmodifiableCollection.iterator and voila, there’s your culprit!

But hang on a second, inspecting the state of your scheduler in your new-fangled-IDE’s (IDEA) debugger shows nothing out of the ordinary. The collection looks perfectly normal, it seems impossible that creating an iterator would explode the stack! That’s when (if you’re lucky) you realise that the IDE is helpfully interpreting the data structure for you. It sees an unmodifiable collection and says to itself “Aha! I know what that is! It’s just a regular collection wrapped in a bit of useless noise. Let me get that noise out of the way for you, sir.” Good in theory, but in this case that noise was just the problem I was trying to debug! IDEA had the same “Aha!” moment 4000-odd times and completely unwrapped a mess of nested unmodifiable collections, the kind of mess that gets you into trouble with the stack police.

Moral of the story: IDE magic is nice, but seeing the truth is nicer.

By the way, if you happen to be using Quartz, beware:

http://jira.opensymphony.com/browse/QUARTZ-399

——-
Into continuous integration? Want to be? Try pulse.

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This entry was posted on Wednesday, April 12th, 2006 at 3:20 am and is filed under Java. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.