Android Studioでjarファイルをrubyで簡単に作成する ref: http://qiita.com/suzukaze/items/7f54f1b76369b397cbf2

build.gradle

dependencies {
  compile files('libs/Hoge.jar')
}

file1.txt

$ ./gradlew clean
$ ./gradlew aR

file2.txt

$ gem install rubyzip

file4.txt

$ ruby make_jar.rb ./Hoge.aar

make_jar.rb

require 'zip'

if ARGV.size == 0
  puts 'usage : ruby unzip.rb [aar filname]'
  exit
end

aar_file = ARGV[0]

TEMP_DIR = 'temp'
Dir.mkdir(TEMP_DIR) unless FileTest.exist?(TEMP_DIR)
FileUtils.copy(aar_file, "#{TEMP_DIR}/#{aar_file}", {})
Dir.chdir(TEMP_DIR) do
  Zip::File.open(aar_file) do |zip|
    zip.each do |entry|
      zip.extract(entry, entry.to_s) { true }
    end
  end
end

filename = File.basename(aar_file, ".aar")
lib_filename = "#{filename}.jar"

Dir.chdir(TEMP_DIR) do
  FileUtils.mv('classes.jar', lib_filename)
end

FileUtils.copy("#{TEMP_DIR}/#{lib_filename}", "#{lib_filename}")

FileUtils.rm_rf(TEMP_DIR)