Ant/Maven: "javadoc: warning - Error fetching URL"
<link offline="true" href="http://docs.oracle.com/javase/6/docs/api/" packagelistLoc="javadoc/javase6/package-list" />should be:
<link offline="true" href="http://docs.oracle.com/javase/6/docs/api/" packagelistLoc="javadoc/javase6/" />packagelistLoc is a path to directory containing package-list file.
It works fine, but for clarity, you need to download the package-list file from the Oracle website first, e.g. http://docs.oracle.com/javase/6/docs/api/package-list, and save that file to the folder you specified.