# File lib/FileCache.rb, line 40
  def initialize(url_cache_dir,native_file_cache_dir)
    @url_cache_dir=url_cache_dir
    @native_file_cache_dir=native_file_cache_dir
    @url_cache_list="#{url_cache_dir}/cache.urls"
    Dir.mkdir(url_cache_dir) unless File.exists?(url_cache_dir)
    raise "cache directory #{url_cache_dir} invalid" unless File.directory?(url_cache_dir)
    Dir.mkdir(native_file_cache_dir) unless File.exists?(native_file_cache_dir)
    raise "cache directory #{native_file_cache_dir} invalid" unless File.directory?(native_file_cache_dir)
  end