Reducing Garbage Generated by URLClassLoader

Scott Palmer swpalmer at
Sun Dec 4 21:21:30 UTC 2016

Excuse me if this is the wrong list for this discussion.  Please direct me to the right place if this isn’t it.

When doing an analysis of garbage generation in our application we discovered a significant number of redundant strings generated by the class loader.  In my case there are hundreds of jars on the classpath - everything in the application is a plugin.  I figured on average 10kB of useless garbage char[]s were generated per findResource call for plugin resources.

This is caused mostly by the ZipFile implementation.  What is the purpose of’s byte[] getBytes(String s) method?  It seems to simply be a custom implementation of string.getBytes(CharSet cs) and as such needs to first make a copy of the char[] to work on.  This combined with the need to operate on byte[] path names internally in the ZipFile implementation means that URLClassLoader generates a lot of unnecessary garbage in a findResource call - proportional to the number of jars on the classpath.

Since JarFile forces the ZipFile to be open with UTF-8 always, if there was some API exposed that took a byte[] for the resource name, all of that extra string copying and encoding could be hoisted out of the loop in sun.misc.URLClassPath. Would this be worth it creating an internal class for something like a ‘ClasspathJarFile’ to and tweaking ZipFile so the byte[] based method is protected instead of private?

I also noticed that, boolean) usually had nothing useful to do but still made three copies of the string passed in anyway (two char arrays to work on, and the String returned).



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