Calling FileUtils.chmod on a symlink tries to call File.lchmod if available. It checks this by calling File.lchmod with an empty file list. This succeeds, because POSIX.lchmod is not actually called in this case:

This leads to the following error, because lchmod does not exist under linux:

NotImplementedError: No message available
  lchmod at org/jruby/RubyFile.java:1019
   chmod at /data/software/jruby/9.2.5.0/lib/ruby/stdlib/fileutils.rb:1320
   chmod at /data/software/jruby/9.2.5.0/lib/ruby/stdlib/fileutils.rb:1000
    each at org/jruby/RubyArray.java:1792
   chmod at /data/software/jruby/9.2.5.0/lib/ruby/stdlib/fileutils.rb:999
  <main> at ./test.rb:15

This is the test program I used, which includes a workaround that makes sure File.lchmod is not considered available:

#!/usr/bin/env jruby

require 'fileutils'

# uncomment to fix NotImplementedError
# class FileUtils::Entry_
#   def have_lchmod?
#     false
#   end
# end

FileUtils.rm_f "link"
FileUtils.ln_s "target", "link"

FileUtils.chmod 400, "link"

I am using jruby 9.2.5.0 under RHEL 7.4.