Question

my problem is that I want to add some .class files from a normal directory into an .jar file. Do I have to extract it before or can I add the files “on the fly”?

Answer 1

If it was me I would almost certainly just call the jar command within Ruby to do this:

system 'jar uf jar_file.jar input_file(s).class'
# or
%x[ 'jar uf jar_file.jar input_file(s).class' ]

Reference here.

If you still want to do this without calling jar you should be able to do it with rubyzip, since JAR files are just ZIP files with a particular structure. Something like this:

require 'zip/zip'

filename = 'class_file.class'

Zip::ZipOutputStream::open "jar_file.jar" do |zip|
  zip.put_next_entry 'dest/path/in/jar/' + filename  # don't forget the path

  File.open filename, 'rb' {|f| zip.write f.read }
end

There are also a few Ruby wrappers for libarchive that could do this. E.g.