Just some thoughts between nappy changes :-)
I seem to remember seeing the actual exception creation time being an
issue when profiling the code a while ago. I think it may be due to the
generation of the stack trace.
It would be interesting to see how much of a performance improvement you
would get if you reused exception objects rather than creating a new
exception object each time a class is not found. The problem with
reusing exceptions is that the stack trace isn't going to be correct.
Maybe we could have an optimization that doesn't have the stack trace or
some partial stack information by instantiating a subclass of the
ClassNotFoundException that overrides the fillInStackTrace() method
(which is called by the Throwable constructor) and possibly calls the
setStackTrace method.
You would want the ability to turn this optimization on or off. I the
optimization should be off by default, as it should be up to the user to
decide whether they want to live with less information to debug the
exception.
Regards,
John
Matt Hogstrom wrote:
I did some profiling of DayTrader deploy (which takes almost 60
seconds) to understand what is taking so long. It appears that we
spend a significant amount of time
MultiParentClassLoader.loadClass(). We've been in here before and
added some code to direct requests for specific class types to the
SystemClassLoader (like java., double, int, etc.). This help
performance a lot. But that's old news.
I've been gathering some information on where we are spending the time
and it appears that the single item that hurts us the most is classes
that are not being found. I add some code to cache the classes that
are known to not exist in a hierarchy and the results were fairly
impressive.
Startup time on my Mac goes from about 19 seconds to 15 seconds. That
is just about a 20% improvement in startup time.
Deploy time for DayTrader goes from 62 seconds to 36 seconds (about
1/2 the time).
I talked to Dain about this and its not really clear how to handle
this. One option is to implement the cache of classes that have not
been found. However, for entries in the ClassPath that are a
directory its possible that a user might drop a class in and we would
never find it once we failed. We could put some kind of timer
mechanism in to throw out entries that have not been referenced in
some time interval.
I wanted to solicit some input on alternatives.
For those interested in how many times a loadClass fails with a CNFE I
captured each failure by classloader and appended a number of times
the class was looked for. This can be found at
http://people.apache.org/~hogstrom/classNotFoundList.txt (It's about 2MB)
or you can grab a zip file if you prefer.
http://people.apache.org/~hogstrom/classNotFoundList.zip
