要点在于 Net::HTTP.new("example.com").get(url, headers) 的时候 headers 中调用 cookie 方法，在没有 @cookie 变量的时候就登录，然后后期如果得到的返回是 302, (如果 302 代表需要登录). 那就清除 @cookie 变量再 retry

require "net/http"

module PubPage
  class HTTP

    class << self
      def get(path)
        @@instance ||= self.new
        @@instance.get(path)
      end
    end

    def get(path)
      uri = URI(path)
      url = "#{uri.path}?#{uri.query}"
      resp = http.get(url, headers)
      raise NeedLoginException if resp.code == "302"
      resp
      rescue NeedLoginException
      @cookie = nil
      retry
    end

    private
    def http
      @http ||= Net::HTTP.new("example.com")
    end

    def headers
      { "Cookie" => cookie }
    end

    def login_data
      config = YAML.load_file(Pow(Rails.root, "config/pub_page.yml"))
      account = config["account"]
      password = config["password"]
      "action=login&loginName=#{account}&pwd=#{password}"
    end

    def login
      @cookie = http
      .post("/login.json", login_data)
      .get_fields('set-cookie')
      .select {|e|
        e.match(/SESSION_ID/)
      }.map {|e|
        e.split(/; /)[0]
      }.first
    end

    def cookie
      login unless @cookie
      @cookie
    end
  end
end