Selector that filters files based on whether they appear in another
directory tree. It can contain a mapper element, so isn't available
as an ExtendSelector (since those parameters can't hold other
This sets whether to select a file if its dest file is present.
It could be a negate boolean, but by doing things
this way, we get some documentation on how the system works.
A user looking at the documentation should clearly understand
that the ONLY files whose presence is being tested are those
that already exist in the source directory, hence the lack of
a destonly option.
fp - An attribute set to either srconlyboth.
public void verifySettings()
Checks to make sure all settings are kosher. In this case, it
means that the targetdir attribute has been set and we have a mapper.