满城尽带黄金甲，源码尽在routing.rb:

module ActionController
  module Routing

    class Route
      attr_accessor :segments, :requirements, :conditions
    end

    class RouteBuilder
      def build(path, options)
        path = "/#{path}" unless path[0] == ?/
        path = "#{path}/" unless path[-1] == ?/
        segments = segments_for_route_path(path)
        defaults, requirements, conditions = divide_route_options(segments, options)
        requirements = assign_route_options(segments, defaults, requirements)
        route = Route.new
        route.segments = segments
        route.requirements = requirements
        route.conditions = conditions
        if !route.significant_keys.include?(:action) && !route.requirements[:action]
          route.requirements[:action] = "index"
          route.significant_keys << :action
        end
        if !route.significant_keys.include?(:controller)
          raise ArgumentError, "Illegal route: the :controller must be specified!"
        end
        route
      end
    end

    class RouteSet

      class Mapper
        def initialize(set)
          @set = set
        end
        def connect(path, options = {})
          @set.add_route(path, options)
        end
        def root(*args, &proc)
          super unless args.length >= 1 && proc.nil?
          @set.add_named_route("root", *args)
        end
        def method_missing(route_name, *args, &proc)
          super unless args.length >= 1 && proc.nil?
          @set.add_named_route(route_name, *args)
        end
      end
      
      class NamedRouteCollection
        include Enumerable
        attr_reader :routes, :helpers
        def initialize
          clear!
        end
        def clear!
          @routes = {}
          @helpers = []
          @module ||= Module.new
          @module.instance_methods.each do |selector|
            @module.send :remove_method, selector
          end
        end
        def add(name, route)
          routes[name.to_sym] = route
          define_named_route_methods(name, route)
        end
        def install(destinations = [ActionController::Base, ActionView::Base])
          Array(destinations).each { |dest| dest.send :include, @module }
        end
        private
          def define_named_route_methods(name, route)
            {:url => {:only_path => false}, :path => {:only_path => true}}.each do |kind, opts|
              hash = route.defaults.merge(:use_route => name).merge(opts)
              define_hash_access route, name, kind, hash
              define_url_helper route, name, kind, hash
            end
          end
          def define_url_helper(route, name, kind, options)
            selector = url_helper_name(name, kind)
            segment_keys = route.segments.collect do |segment|
              segment.key if segment.respond_to? :key
            end.compact
            hash_access_method = hash_access_name(name, kind)
            @module.send :module_eval, <<-end_eval
              def #{selector}(*args)
                opts = if args.empty? || Hash === args.first
                  args.first || {}
                else
                  args.zip(#{segment_keys.inspect}).inject({}) do |h, (v, k)|
                    h[k] = v
                    h
                  end
                end
                url_for(#{hash_access_method}(opts))
              end
            end_eval
            @module.send(:protected, selector)
            helpers << selector
          end
          def url_helper_name(name, kind = :url)
            :"#{name}_#{kind}"
          end
      end

      attr_accessor :routes, :named_routes

      def initialize
        self.routes = []
        self.named_routes = NamedRouteCollection.new
      end

      def builder
        @builder ||= RouteBuilder.new
      end

      def draw
        clear!
        yield Mapper.new(self)
        named_routes.install
      end

      def clear!
        routes.clear
        named_routes.clear
        @combined_regexp = nil
        @routes_by_controller = nil
      end

      def add_route(path, options = {})
        route = builder.build(path, options)
        routes << route
        route
      end
  
      def add_named_route(name, path, options = {})
        named_routes[name] = add_route(path, options)
      end

    end

    Routes = RouteSet.new
  end
end

该文件就是大名鼎鼎的RAILS_ROOT/config/routes.rb文件里的程序逻辑的源代码出处
让我们看看ActionController::Routing::Routes.draw do |map|到底是怎样工作的:
1) Routes = RouteSet.new，draw方法当然也是调用的RouteSet对象的draw方法
2) draw方法则yield Mapper.new(self)，即RouteSet将方法调用继续传递给它的内部类Mapper
3) Mapper类定义了两个方法: connect和method_missing
connect就是我们在传说中的routes.rb文件中用来定义URL routing的map.connect
method_missing则调用了@set.add_named_route，即RouteSet类的add_named_route方法
4)RouteSet定义了routes和named_routes这两个实例变量，分别用来存放connect route和named_route
5)add_named_route方法中调用了add_route方法中的RouteBuilder.build方法来构造一个route
而NamedRouteCollection定义了add方法，其中调用了define_named_route_methods -> define_url_helper
其中定义了#{route_name}_url和#{route_name}_path方法，而且使用了UrlWriter模块的url_for方法

我们还要注意几点:
1，routes有priority，routes.rb文件里从上到下routing规则的priority为从高到低
2，map.root方法在Rails 2.0中用来作为map.connect ''的shortcut，实际上是@set.add_named_route("root", *args)，我们可以使用root_url和root_path方法
3，将*path这种route rule放在routes.rb文件的最后，用来做Route globbing，即前面的routes规则都没匹配时，使用该规则来匹配，如:

map.wildcard "*wildcard", :controller => "website", :action => "not_found"