Initially I found that Generator was throwing OutOfMemoryError exception no matter how much RAM I allocated to JVM. I fixed the problem by moving URLFilters, URLNormalizers and ScoringFilters to top-level class as singletons and re-using them in all Generator mapper instances.
Then I found the same problem in Injector and applied analogical fix.
Now it seems that this issue may be common in all Nutch Mapper implementations.
I was wondering if it would it be possible to integrate this kind of change
in the upstream code base and potentially update all vulnerable Mapper classes.