For java.base gensrc we do the following:

# Copy two Java files that need no preprocessing.
$(SUPPORT_OUTPUTDIR)/gensrc/java.base/java/lang/%.java: $(CHARACTERDATA_TEMPLATES)/%.java.template
$(call LogInfo, Generating $(@F))
$(call install-file)

GENSRC_CHARACTERDATA += $(SUPPORT_OUTPUTDIR)/gensrc/java.base/java/lang/CharacterDataUndefined.java \
    $(SUPPORT_OUTPUTDIR)/gensrc/java.base/java/lang/CharacterDataPrivateUse.java

What this means is that we have two "template" files that are just compiled as java files, but only after being copied to gensrc. :-)

We should just rename these files to java and move them to the normal source directory.