Invoking the Kernel background downloader when the Kernel JRE is already complete causes the directory lib/bundles to be recreated. The presence of the lib/bundles directory in a completed JRE causes certain operations to fail; for instance running the background downloader again will generate the exception:

Exception in thread "main" java.lang.NullPointerException
        at sun.jkernel.DownloadManager.getBundleNames(DownloadManager.java:468)
        at sun.jkernel.DownloadManager.performCompletionIfNeeded(DownloadManager.java:504)
        at sun.jkernel.BackgroundDownloader.doBackgroundDownloads(BackgroundDownloader.java:35)
        at sun.jkernel.BackgroundDownloader.main(BackgroundDownloader.java:172)

To reproduce this bug, run the command "java -Dkernel.background.download=false sun.jkernel.BackgroundDownloader" on a completed Kernel JRE. The lib/bundles directory will be recreated, and the next attempt to perform a background download will display the exception above.

As part of fixing this bug, we should ensure that a spurious lib/bundles directory in a completed JRE will not cause any problems.