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      <h2>Rails 应用的初始化过程</h2><p>本章节介绍了 Rails 4 应用启动的内部流程,适合有一定经验的Rails应用开发者阅读。</p><p>通过学习本章节，您会学到如下知识：</p>
<ul>
<li>如何使用 <code>rails server</code>；</li>
<li>Rails应用初始化的时间序列；</li>
<li>Rails应用启动过程都用到哪些文件；</li>
<li>Rails::Server接口的定义和使用；</li>
</ul>


                <div id="subCol">
            <h3 class="chapter"><img src="images/chapters_icon.gif" alt="" />Chapters</h3>
            <ol class="chapters">
<li>
<a href="#%E5%90%AF-%E5%8A%A8--bang">启 动 !</a>

<ul>
<li><a href="#railties/bin/rails"><code>railties/bin/rails</code></a></li>
<li><a href="#railties/lib/rails/app_rails_loader.rb"><code>railties/lib/rails/app_rails_loader.rb</code></a></li>
<li><a href="#bin/rails"><code>bin/rails</code></a></li>
<li><a href="#config/boot.rb"><code>config/boot.rb</code></a></li>
<li><a href="#rails/commands.rb"><code>rails/commands.rb</code></a></li>
<li><a href="#rails/commands/command_tasks.rb"><code>rails/commands/command_tasks.rb</code></a></li>
<li><a href="#actionpack/lib/action_dispatch.rb"><code>actionpack/lib/action_dispatch.rb</code></a></li>
<li><a href="#rails/commands/server.rb"><code>rails/commands/server.rb</code></a></li>
<li><a href="#%E5%90%AF-%E5%8A%A8--bang-rack:-lib/rack/server.rb">Rack: <code>lib/rack/server.rb</code></a></li>
<li><a href="#config/application"><code>config/application</code></a></li>
<li><a href="#rails::server#start"><code>Rails::Server#start</code></a></li>
<li><a href="#config/environment.rb"><code>config/environment.rb</code></a></li>
<li><a href="#config/application.rb"><code>config/application.rb</code></a></li>
</ul>
</li>
<li>
<a href="#%E5%8A%A0%E8%BD%BD-rails">加载 Rails</a>

<ul>
<li><a href="#railties/lib/rails/all.rb"><code>railties/lib/rails/all.rb</code></a></li>
<li><a href="#%E5%9B%9E%E5%88%B0-config/environment.rb">回到 <code>config/environment.rb</code></a></li>
<li><a href="#railties/lib/rails/application.rb"><code>railties/lib/rails/application.rb</code></a></li>
<li><a href="#%E5%8A%A0%E8%BD%BD-rails-rack:-lib/rack/server.rb">Rack: lib/rack/server.rb</a></li>
</ul>
</li>
</ol>

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  <div id="container">
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      <div id="mainCol">
        <p>本章节通过介绍一个基于Ruby on Rails框架默认配置的 Rails 4 应用程序启动过程中的方法调用，详细介绍了每个调用的细节。通过本章节，我们将了解当你执行<code>rails server</code>命令启动你的Rails应用时，背后究竟都发生了什么。</p><p>提示：本章节中的路径如果没有特别说明都是指Rails应用程序下的路径。</p><p>提示：如果你想浏览Rails的源代码<a href="https://github.com/rails/rails">sourcecode</a>，强烈建议您使用快捷键 <code>t</code>快速查找Github中的文件。</p><h3 id="启-动--bang">1 启 动 !</h3><p>我们现在准备启动和初始化一个Rails 应用。 一个Rails 应用经常是以运行命令 <code>rails console</code> 或者 <code>rails server</code> 开始的。</p><h4 id="railties/bin/rails">1.1 <code>railties/bin/rails</code>
</h4><p>Rails应用中的 <code>rails server</code>命令是Rails应用程序所在文件中的一个Ruby的可执行程序，该程序包含如下操作：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
version = "&gt;= 0"
load Gem.bin_path('railties', 'rails', version)

</pre>
</div>
<p>如果你在Rails 控制台中使用上述命令，你将会看到载入<code>railties/bin/rails</code>这个路径。作为 <code>railties/bin/rails.rb</code>的一部分，包含如下代码：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
require "rails/cli"

</pre>
</div>
<p>模块<code>railties/lib/rails/cli</code> 会调用<code>Rails::AppRailsLoader.exec_app_rails</code>方法.</p><h4 id="railties/lib/rails/app_rails_loader.rb">1.2 <code>railties/lib/rails/app_rails_loader.rb</code>
</h4><p><code>exec_app_rails</code>模块的主要功能是去执行你的Rails应用中<code>bin/rails</code>文件夹下的指令。如果当前文件夹下没有<code>bin/rails</code>文件，它会到父级目录去搜索，直到找到为止（Windows下应该会去搜索环境变量中的路径），在Rails应用程序目录下的任意位置(命令行模式下)，都可以执行<code>rails</code>的命令。</p><p>因为<code>rails server</code>命令和下面的操作是等价的：</p><div class="code_container">
<pre class="brush: plain; gutter: false; toolbar: false">
$ exec ruby bin/rails server

</pre>
</div>
<h4 id="bin/rails">1.3 <code>bin/rails</code>
</h4><p>文件<code>railties/bin/rails</code>包含如下代码：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
#!/usr/bin/env ruby
APP_PATH = File.expand_path('../../config/application', __FILE__)
require_relative '../config/boot'
require 'rails/commands'

</pre>
</div>
<p><code>APP_PATH</code>稍后会在<code>rails/commands</code>中用到。<code>config/boot</code>在这被引用是因为我们的Rails应用中需要<code>config/boot.rb</code>文件来载入Bundler,并初始化Bundler的配置。</p><h4 id="config/boot.rb">1.4 <code>config/boot.rb</code>
</h4><p><code>config/boot.rb</code> 包含如下代码:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
# Set up gems listed in the Gemfile.
ENV['BUNDLE_GEMFILE'] ||= File.expand_path('../../Gemfile', __FILE__)

require 'bundler/setup' if File.exist?(ENV['BUNDLE_GEMFILE'])

</pre>
</div>
<p>在一个标准的Rails应用中的<code>Gemfile</code>文件会配置它的所有依赖项。<code>config/boot.rb</code>文件会根据<code>ENV['BUNDLE_GEMFILE']</code>中的值来查找<code>Gemfile</code>文件的路径。如果<code>Gemfile</code>文件存在，那么<code>bundler/setup</code>操作会被执行，Bundler执行该操作是为了配置Gemfile依赖项的加载路径。</p><p>一个标准的Rails应用会包含若干Gem包，特别是下面这些：</p>
<ul>
<li>actionmailer</li>
<li>actionpack</li>
<li>actionview</li>
<li>activemodel</li>
<li>activerecord</li>
<li>activesupport</li>
<li>arel</li>
<li>builder</li>
<li>bundler</li>
<li>erubis</li>
<li>i18n</li>
<li>mail</li>
<li>mime-types</li>
<li>polyglot</li>
<li>rack</li>
<li>rack-cache</li>
<li>rack-mount</li>
<li>rack-test</li>
<li>rails</li>
<li>railties</li>
<li>rake</li>
<li>sqlite3</li>
<li>thor</li>
<li>treetop</li>
<li>tzinfo</li>
</ul>
<h4 id="rails/commands.rb">1.5 <code>rails/commands.rb</code>
</h4><p>一旦<code>config/boot.rb</code>执行完毕，接下来要引用的是<code>rails/commands</code>文件，这个文件于帮助解析别名。在本应用中，<code>ARGV</code> 数组包含的 <code>server</code>项会被匹配：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
ARGV &lt;&lt; '--help' if ARGV.empty?

aliases = {
  "g"  =&gt; "generate",
  "d"  =&gt; "destroy",
  "c"  =&gt; "console",
  "s"  =&gt; "server",
  "db" =&gt; "dbconsole",
  "r"  =&gt; "runner"
}

command = ARGV.shift
command = aliases[command] || command

require 'rails/commands/commands_tasks'

Rails::CommandsTasks.new(ARGV).run_command!(command)

</pre>
</div>
<p>提示： 如你所见，一个空的ARGV数组将会让系统显示相关的帮助项。</p><p>如果我们使用<code>s</code>缩写代替 <code>server</code>，Rails系统会从<code>aliases</code>中查找匹配的命令。</p><h4 id="rails/commands/command_tasks.rb">1.6 <code>rails/commands/command_tasks.rb</code>
</h4><p>当你键入一个错误的rails命令，<code>run_command</code>函数会抛出一个错误信息。如果命令正确，一个与命令同名的方法会被调用。</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
COMMAND_WHITELIST = %(plugin generate destroy console server dbconsole application runner new version help)

def run_command!(command)
  command = parse_command(command)
  if COMMAND_WHITELIST.include?(command)
    send(command)
  else
    write_error_message(command)
  end
end

</pre>
</div>
<p>如果执行<code>server</code>命令，Rails将会继续执行下面的代码：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
def set_application_directory!
  Dir.chdir(File.expand_path('../../', APP_PATH)) unless File.exist?(File.expand_path("config.ru"))
end

def server
  set_application_directory!
  require_command!("server")

  Rails::Server.new.tap do |server|
    # We need to require application after the server sets environment,
    # otherwise the --environment option given to the server won't propagate.
    require APP_PATH
    Dir.chdir(Rails.application.root)
    server.start
  end
end

def require_command!(command)
  require "rails/commands/#{command}"
end

</pre>
</div>
<p>这个文件将会指向Rails的根目录（与<code>APP_PATH</code>中指向<code>config/application.rb</code>不同），但是如果没找到<code>config.ru</code>文件，接下来将需要<code>rails/commands/server</code>来创建<code>Rails::Server</code>类。</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
require 'fileutils'
require 'optparse'
require 'action_dispatch'
require 'rails'

module Rails
  class Server &lt; ::Rack::Server

</pre>
</div>
<p><code>fileutils</code> 和 <code>optparse</code> 是Ruby标准库中帮助操作文件和解析选项的函数。</p><h4 id="actionpack/lib/action_dispatch.rb">1.7 <code>actionpack/lib/action_dispatch.rb</code>
</h4><p>动作分发(Action Dispatch)是Rails框架中的路径组件。它增强了路径，会话和中间件的功能。</p><h4 id="rails/commands/server.rb">1.8 <code>rails/commands/server.rb</code>
</h4><p>这个文件中定义的<code>Rails::Server</code>类是继承自<code>Rack::Server</code>类的。当<code>Rails::Server.new</code>被调用时，会在 <code>rails/commands/server.rb</code>中调用一个<code>initialize</code>方法：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
def initialize(*)
  super
  set_environment
end

</pre>
</div>
<p>首先，<code>super</code>会调用父类<code>Rack::Server</code>中的<code>initialize</code>方法。</p><h4 id="启-动--bang-rack:-lib/rack/server.rb">1.9 Rack: <code>lib/rack/server.rb</code>
</h4><p><code>Rack::Server</code>会为所有基于Rack的应用提供服务接口，现在它已经是Rails框架的一部分了。</p><p><code>Rack::Server</code>中的<code>initialize</code> 方法会简单的设置一对变量：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
def initialize(options = nil)
  @options = options
  @app = options[:app] if options &amp;&amp; options[:app]
end

</pre>
</div>
<p>在这种情况下，<code>options</code> 的值是 <code>nil</code>，所以在这个方法中相当于什么都没做。</p><p>当<code>Rack::Server</code>中的<code>super</code>方法执行完毕后。我们回到<code>rails/commands/server.rb</code>，此时此刻，<code>Rails::Server</code>对象会调用 <code>set_environment</code> 方法，这个方法貌似看上去什么也没干： </p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
def set_environment
  ENV["RAILS_ENV"] ||= options[:environment]
end

</pre>
</div>
<p>事实上，<code>options</code>方法在这做了很多事情。<code>Rack::Server</code> 中的这个方法定义如下：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
def options
  @options ||= parse_options(ARGV)
end

</pre>
</div>
<p>接着<code>parse_options</code>方法部分代码如下：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
def parse_options(args)
  options = default_options

  # Don't evaluate CGI ISINDEX parameters.
  # http://www.meb.uni-bonn.de/docs/cgi/cl.html
  args.clear if ENV.include?("REQUEST_METHOD")

  options.merge! opt_parser.parse!(args)
  options[:config] = ::File.expand_path(options[:config])
  ENV["RACK_ENV"] = options[:environment]
  options
end

</pre>
</div>
<p><code>default_options</code>方法的代码如下：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
def default_options
  environment  = ENV['RACK_ENV'] || 'development'
  default_host = environment == 'development' ? 'localhost' : '0.0.0.0'

  {
    :environment =&gt; environment,
    :pid         =&gt; nil,
    :Port        =&gt; 9292,
    :Host        =&gt; default_host,
    :AccessLog   =&gt; [],
    :config      =&gt; "config.ru"
  }
end

</pre>
</div>
<p><code>ENV</code>中没有<code>REQUEST_METHOD</code>项，所以我们可以忽略这一行。接下来是已经在 <code>Rack::Server</code>被定义好的<code>opt_parser</code>方法：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
def opt_parser
  Options.new
end

</pre>
</div>
<p><strong>这个</strong>方法已经在<code>Rack::Server</code>被定义过了，但是在<code>Rails::Server</code> 使用不同的参数进行了重载。他的 <code>parse!</code>方法如下：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
def parse!(args)
  args, options = args.dup, {}

  opt_parser = OptionParser.new do |opts|
    opts.banner = "Usage: rails server [mongrel, thin, etc] [options]"
    opts.on("-p", "--port=port", Integer,
            "Runs Rails on the specified port.", "Default: 3000") { |v| options[:Port] = v }
  ...

</pre>
</div>
<p>这个方法为<code>options</code>建立一些配置选项，以便给Rails决定如何运行服务提供支持。<code>initialize</code>方法执行完毕后。我们将回到<code>rails/server</code>目录下，就是<code>APP_PATH</code>中的路径。</p><h4 id="config/application">1.10 <code>config/application</code>
</h4><p>当<code>require APP_PATH</code>操作执行完毕后。<code>config/application.rb</code> 被载入了 (重新调用<code>bin/rails</code>中的<code>APP_PATH</code>), 在你的应用中，你可以根据需求对该文件进行配置。</p><h4 id="rails::server#start">1.11 <code>Rails::Server#start</code>
</h4><p><code>config/application</code>载入后，<code>server.start</code>方法被调用了。这个方法定义如下：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
def start
  print_boot_information
  trap(:INT) { exit }
  create_tmp_directories
  log_to_stdout if options[:log_stdout]

  super
  ...
end

private

  def print_boot_information
    ...
    puts "=&gt; Run `rails server -h` for more startup options"
    ...
    puts "=&gt; Ctrl-C to shutdown server" unless options[:daemonize]
  end

  def create_tmp_directories
    %w(cache pids sessions sockets).each do |dir_to_make|
      FileUtils.mkdir_p(File.join(Rails.root, 'tmp', dir_to_make))
    end
  end

  def log_to_stdout
    wrapped_app # touch the app so the logger is set up

    console = ActiveSupport::Logger.new($stdout)
    console.formatter = Rails.logger.formatter
    console.level = Rails.logger.level

    Rails.logger.extend(ActiveSupport::Logger.broadcast(console))
  end

</pre>
</div>
<p>这是Rails初始化过程中的第一次控制台输出。这个方法创建了一个<code>INT</code>中断信号，所以当你在服务端控制台按下<code>CTRL-C</code>键后，这将终止Server的运行。我们可以看到，它创建了<code>tmp/cache</code>,<code>tmp/pids</code>, <code>tmp/sessions</code>和<code>tmp/sockets</code>等目录。在创建和声明<code>ActiveSupport::Logger</code>之前，会调用 <code>wrapped_app</code>方法来创建一个Rake 应用程序。</p><p><code>super</code>会调用<code>Rack::Server.start</code> 方法，该方法定义如下：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
def start &amp;blk
  if options[:warn]
    $-w = true
  end

  if includes = options[:include]
    $LOAD_PATH.unshift(*includes)
  end

  if library = options[:require]
    require library
  end

  if options[:debug]
    $DEBUG = true
    require 'pp'
    p options[:server]
    pp wrapped_app
    pp app
  end

  check_pid! if options[:pid]

  # Touch the wrapped app, so that the config.ru is loaded before
  # daemonization (i.e. before chdir, etc).
  wrapped_app

  daemonize_app if options[:daemonize]

  write_pid if options[:pid]

  trap(:INT) do
    if server.respond_to?(:shutdown)
      server.shutdown
    else
      exit
    end
  end

  server.run wrapped_app, options, &amp;blk
end

</pre>
</div>
<p>上述Rails 应用有趣的部分在最后一行，<code>server.run</code>方法。它再次调用了<code>wrapped_app</code>方法(温故而知新)。</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
@wrapped_app ||= build_app app

</pre>
</div>
<p>这里的<code>app</code>方法定义如下：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
def app
  @app ||= options[:builder] ? build_app_from_string : build_app_and_options_from_config
end
...
private
  def build_app_and_options_from_config
    if !::File.exist? options[:config]
      abort "configuration #{options[:config]} not found"
    end

    app, options = Rack::Builder.parse_file(self.options[:config], opt_parser)
    self.options.merge! options
    app
  end

  def build_app_from_string
    Rack::Builder.new_from_string(self.options[:builder])
  end

</pre>
</div>
<p><code>options[:config]</code>中的值默认会从 <code>config.ru</code> 中获取，包含如下代码：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
# This file is used by Rack-based servers to start the application.

require ::File.expand_path('../config/environment', __FILE__)
run &lt;%= app_const %&gt;

</pre>
</div>
<p><code>Rack::Builder.parse_file</code>方法会从<code>config.ru</code>中获取内容，包含如下代码：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
app = new_from_string cfgfile, config

...

def self.new_from_string(builder_script, file="(rackup)")
  eval "Rack::Builder.new {\n" + builder_script + "\n}.to_app",
    TOPLEVEL_BINDING, file, 0
end

</pre>
</div>
<p><code>Rack::Builder</code>中的<code>initialize</code>方法会创建一个新的<code>Rack::Builder</code>实例，这是Rails应用初始化过程中主要内容。接下来<code>config.ru</code>中的<code>require</code>项<code>config/environment.rb</code>会继续执行：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
require ::File.expand_path('../config/environment', __FILE__)

</pre>
</div>
<h4 id="config/environment.rb">1.12 <code>config/environment.rb</code>
</h4><p>这是<code>config.ru</code> (<code>rails server</code>)和信使(Passenger)都要用到的文件，是两者交流的媒介。之前的操作都是为了创建Rack和Rails。</p><p>这个文件是以引用 <code>config/application.rb</code>开始的：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
require File.expand_path('../application', __FILE__)

</pre>
</div>
<h4 id="config/application.rb">1.13 <code>config/application.rb</code>
</h4><p>这个文件需要引用<code>config/boot.rb</code>：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
require File.expand_path('../boot', __FILE__)

</pre>
</div>
<p>如果之前在<code>rails server</code>中没有引用上述的依赖项，那么它<strong>将不会</strong>和信使(Passenger)发生联系。</p><p>现在，有趣的部分要开始了！</p><h3 id="加载-rails">2 加载 Rails</h3><p><code>config/application.rb</code>中的下一行是这样的：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
require 'rails/all'

</pre>
</div>
<h4 id="railties/lib/rails/all.rb">2.1 <code>railties/lib/rails/all.rb</code>
</h4><p>本文件中将引用和Rails框架相关的所有内容：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
require "rails"

%w(
  active_record
  action_controller
  action_view
  action_mailer
  rails/test_unit
  sprockets
).each do |framework|
  begin
    require "#{framework}/railtie"
  rescue LoadError
  end
end

</pre>
</div>
<p>这样Rails框架中的所有组件已经准备就绪了。我们将不会深入介绍这些框架的内部细节，不过强烈建议您去探索和发现她们。</p><p>现在，我们关心的模块比如Rails engines,I18n 和 Rails configuration 都已经准备就绪了。</p><h4 id="回到-config/environment.rb">2.2 回到 <code>config/environment.rb</code>
</h4><p><code>config/application.rb</code>为<code>Rails::Application</code>定义了Rails应用初始化之后所有需要用到的资源。当<code>config/application.rb</code> 加载了Rails和命名空间后，我们回到<code>config/environment.rb</code>，就是初始化完成的地方。比如我们的应用叫‘blog’，我们将在<code>rails/application.rb</code>中调用<code>Rails.application.initialize!</code>方法。</p><h4 id="railties/lib/rails/application.rb">2.3 <code>railties/lib/rails/application.rb</code>
</h4><p><code>initialize!</code>方法部分代码如下： </p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
def initialize!(group=:default) #:nodoc:
  raise "Application has been already initialized." if @initialized
  run_initializers(group, self)
  @initialized = true
  self
end

</pre>
</div>
<p>如你所见，一个应用只能初始化一次。初始化器通过在<code>railties/lib/rails/initializable.rb</code>中的<code>run_initializers</code>方法运行：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
def run_initializers(group=:default, *args)
  return if instance_variable_defined?(:@ran)
  initializers.tsort_each do |initializer|
    initializer.run(*args) if initializer.belongs_to?(group)
  end
  @ran = true
end

</pre>
</div>
<p><code>run_initializers</code>代码本身是有点投机取巧的，Rails在这里要做的是遍历所有的祖先，查找一个<code>initializers</code>方法，之后根据名字进行排序，并依次执行它们。举个例子，<code>Engine</code>类将调用自己和祖先中名为<code>initializers</code>的方法。</p><p><code>Rails::Application</code> 类是在<code>railties/lib/rails/application.rb</code>定义的。定义了<code>bootstrap</code>, <code>railtie</code>和 <code>finisher</code>模块的初始化器。<code>bootstrap</code>的初始化器在应用被加载以前就预加载了。(类似初始化中的日志记录器)，<code>finisher</code>的初始化器则是最后加载的。<code>railtie</code>初始化器被定义在<code>Rails::Application</code>中，执行是在<code>bootstrap</code>和 <code>finishers</code>之间。</p><p>这些完成后，我们将回到<code>Rack::Server</code> 。</p><h4 id="加载-rails-rack:-lib/rack/server.rb">2.4 Rack: lib/rack/server.rb</h4><p>上次我们离开的时候，<code>app</code> 方法代码如下：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
def app
  @app ||= options[:builder] ? build_app_from_string : build_app_and_options_from_config
end
...
private
  def build_app_and_options_from_config
    if !::File.exist? options[:config]
      abort "configuration #{options[:config]} not found"
    end

    app, options = Rack::Builder.parse_file(self.options[:config], opt_parser)
    self.options.merge! options
    app
  end

  def build_app_from_string
    Rack::Builder.new_from_string(self.options[:builder])
  end

</pre>
</div>
<p>此时此刻，<code>app</code>是Rails 应用本身(中间件)。接下来就是Rack调用所有的依赖项了(提供支持的中间件)：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
def build_app(app)
  middleware[options[:environment]].reverse_each do |middleware|
    middleware = middleware.call(self) if middleware.respond_to?(:call)
    next unless middleware
    klass = middleware.shift
    app = klass.new(app, *middleware)
  end
  app
end

</pre>
</div>
<p>必须牢记，<code>Server#start</code>最后一行中调用了<code>build_app</code>方法(被<code>wrapped_app</code>调用)了。接下来我们看看还剩下什么：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
server.run wrapped_app, options, &amp;blk

</pre>
</div>
<p>此时此刻，调用<code>server.run</code> 方法将依赖于你所用的Server类型 。比如，如果你的Server是Puma， 那么就会是下面这个结果：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
...
DEFAULT_OPTIONS = {
  :Host =&gt; '0.0.0.0',
  :Port =&gt; 8080,
  :Threads =&gt; '0:16',
  :Verbose =&gt; false
}

def self.run(app, options = {})
  options  = DEFAULT_OPTIONS.merge(options)

  if options[:Verbose]
    app = Rack::CommonLogger.new(app, STDOUT)
  end

  if options[:environment]
    ENV['RACK_ENV'] = options[:environment].to_s
  end

  server   = ::Puma::Server.new(app)
  min, max = options[:Threads].split(':', 2)

  puts "Puma #{::Puma::Const::PUMA_VERSION} starting..."
  puts "* Min threads: #{min}, max threads: #{max}"
  puts "* Environment: #{ENV['RACK_ENV']}"
  puts "* Listening on tcp://#{options[:Host]}:#{options[:Port]}"

  server.add_tcp_listener options[:Host], options[:Port]
  server.min_threads = min
  server.max_threads = max
  yield server if block_given?

  begin
    server.run.join
  rescue Interrupt
    puts "* Gracefully stopping, waiting for requests to finish"
    server.stop(true)
    puts "* Goodbye!"
  end

end

</pre>
</div>
<p>我们没有深入到服务端配置的细节，因为这是我们探索Rails应用初始化过程之旅的终点了。</p><p>高层次的阅读将有助于您提高编写代码的水平，成为Rail开发高手。如果你想要知道更多，那么去读Rails的源代码将是你的不二选择。</p>

        <h3>反馈</h3>
        <p>
          欢迎帮忙改善指南质量。
        </p>
        <p>
          如发现任何错误，欢迎修正。开始贡献前，可先行阅读<a href="http://edgeguides.rubyonrails.org/contributing_to_ruby_on_rails.html#contributing-to-the-rails-documentation">贡献指南：文档</a>。
        </p>
        <p>翻译如有错误，深感抱歉，欢迎 <a href="https://github.com/ruby-china/guides/fork">Fork</a> 修正，或至此处<a href="https://github.com/ruby-china/guides/issues/new">回报</a>。</p>
        <p>
          文章可能有未完成或过时的内容。请先检查 <a href="http://edgeguides.rubyonrails.org">Edge Guides</a> 来确定问题在 master 是否已经修掉了。再上 master 补上缺少的文件。内容参考 <a href="ruby_on_rails_guides_guidelines.html">Ruby on Rails 指南准则</a>来了解行文风格。
        </p>
        <p>最后，任何关于 Ruby on Rails 文档的讨论，欢迎到 <a href="http://groups.google.com/group/rubyonrails-docs">rubyonrails-docs 邮件群组</a>。
        </p>
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