The Generator class caches filehandles while generating files. There is no
upper limit on the number of filehandles it will cache, so when generating many
files with the same generator, it eventually hits the per-process limits on the
number of open files. This then causes failures in the resource loader because
it cannot open any more files. I've seen this problem on OS X and Linux, but it
should be a pretty universal problem.
You can work around this problem by increasing the number of filehandles
available to the ant task, but that isn't always easy to do when running it as a
subtask from an IDE, and it should be fairly simple to implement a basic LRU scheme.
An alternative fix would be to more accurately report the error - the current
implementation just throws a ResourceNotFound exception, making it very hard to
track down the root cause of the error.