Get compiled version of jar files.

jar_version.rb

#!/usr/bin/env ruby -Ku

require 'rubygems'
require 'zipruby'

version_map = {
  45.3 => "J2SE 1.1",
  46.0 => "J2SE 1.2",
  47.0 => "J2SE 1.3",
  48.0 => "J2SE 1.4",
  49.0 => "Java SE 5",
  50.0 => "Java SE 6"
}

def version(file)
  a = file.read(10)
  "#{a[6]*0xff+a[7]}.#{a[4]*0xff+a[5]}".to_f
end

ARGV.each do |path|
  Zip::Archive.open(path) do |ar|
    ar.each do |f|
      next if f.directory? || !(f.name =‾ /\.class$/)
      ar.fopen(f.name) do |h|
        puts version_map[version(h)]
      end
      break
    end
  end
end