17 December 2014 by Tobias Roeser

SBuild 0.7.7 fixes a potentially thread deadlock in the procject’s and plugin’s classloaders.

Read the full changelog here.

I’m happy to say that I finally found the culprit after haunting that bug for over a half year now. It turned out, it isn’t that easy to write a proper ClassLoader implementation in Scala.

ClassLoaders in Scala

To implement a non-hierarchical ClassLoader for Java 7 and above it must be parallel loading capable. And to enable this capability you must invoke a caller sensitive protected static method from the same ClassLoader class but before the class gets constructed. Turns out, this isn’t possible in Scala (whereas in Java it would be a simple static block). That’s also the reason why the previous solution I implemented in SBuild 0.7.6 and before worked almost always but not for the first created instance, which of course is not good enough.

But there is a possible workaround (read: hack) and because it’s Scala, we can encapsulated that hack in a nice and safe way. We now create a dummy instance of the classloader and register its parallel capability from inside of that instance. Only after that we drop that instance and create another instance which now initializes correctly. We hide all this behind the apply method in the ClassLoader’s companion object and make the constructor private. Elegant solution for a rather hacky technically motivated workaround. If you are interested in details, this commit might be interesting to you.

Merry Christmas!


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