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<!DOCTYPE html>
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<h2>Rails 啟動過程</h2><p>本篇介紹 Rails 4 啟動過程的內部原理。非常深入的一篇指南，推薦進階的 Rails 開發者閱讀。</p><p>讀完本篇，您將了解：</p>
<ul>
<li>如何使用 <code>rails server</code>。</li>
<li>Rails 啟動過程的時間軸。</li>
<li>啟動時 <code>require</code> 了那些檔案。</li>
<li><code>Rails::Server</code> 介面是如何定義的，以及如何使用。</li>
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<h3 class="chapter"><img src="images/chapters_icon.gif" alt="" />Chapters</h3>
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<li>
<a href="#%E5%95%9F%E5%8B%95%EF%BC%81">啟動！</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%95%9F%E5%8B%95%EF%BC%81-rack-lib-rack-server-rb">Rack: <code>lib/rack/server.rb</code></a></li>
<li><a href="#%E5%95%9F%E5%8B%95%EF%BC%81-config-application-rb"><code>config/application.rb</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="#%E5%95%9F%E5%8B%95%EF%BC%81-config-application-rb"><code>config/application.rb</code></a></li>
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<a href="#%E8%BC%89%E5%85%A5-rails">載入 Rails</a>
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<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="#%E8%BC%89%E5%85%A5-rails-rack-lib-rack-server-rb">Rack: lib/rack/server.rb</a></li>
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<div class="wrapper">
<div id="mainCol">
<p>本篇針對 Rails 4，走一遍啟動 Rails 所需的每個方法呼叫。過程中詳細解釋每個步驟的用途，特別針對從 <code>rails server</code>， 到應用程式啟動起來之間的過程做解說。</p><div class="note"><p>除非特別聲明，本篇提及的路徑都是相對於 <a href="https://github.com/rails/rails">Rails 原始碼</a>的目錄，或相對於 Rails 應用程式的路徑。</p></div><div class="info"><p>若想跟著瀏覽 Rails 的<a href="https://github.com/rails/rails">原始碼</a>，推薦<a href="https://github.com/blog/793-introducing-the-file-finder">使用 GitHub 提供的檔案搜索</a>功能，來快速找到檔案，在 GitHub Repository 頁面按 <code>t</code> 即可使用。</p></div><h3 id="啟動！">1 啟動！</h3><p>從初始化（Initialize）與啟動（boot）應用程式開始。Rails 應用程式通常在執行 <code>rails server</code> 或 <code>rails console</code> 時會啟動。</p><h4 id="railties-bin-rails">1.1 <code>railties/bin/rails</code>
</h4><p><a href="https://github.com/rails/rails/blob/master/railties/bin/rails">View Source</a></p><p><code>rails server</code> 命令裡的 <code>rails</code>，是放在載入路徑（load path）下的 Ruby 執行檔。這個執行檔的內容如下：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
version = ">= 0"
load Gem.bin_path('railties', 'rails', version)
</pre>
</div>
<p>若在 Rails Console 裡試這個命令，會看到這個命令載入了 <a href="https://github.com/rails/rails/blob/master/railties/bin/rails"><code>railties/bin/rails</code></a>。</p><p><code>railties/bin/rails</code> 裡有這一行：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
require "rails/cli"
</pre>
</div>
<p><a href="https://github.com/rails/rails/blob/master/railties/lib/rails/cli.rb"><code>railties/lib/rails/cli</code></a> 接著呼叫 <code>Rails::AppRailsLoader.exec_app_rails</code>。</p><h4 id="railties-lib-rails-app-rails-loader-rb">1.2 <code>railties/lib/rails/app_rails_loader.rb</code>
</h4><p><a href="https://github.com/rails/rails/blob/master/railties/lib/rails/app_rails_loader.rb">View Source</a></p><p><code>exec_app_rails</code> 的主要目的是執行應用程式的 <code>bin/rails</code>，若當前目錄沒有 <code>bin/rails</code>，會往上搜索，看找不找的到 <code>bin/rails</code>。這也是為什麼可以在 rails 應用程式裡的任何目錄下使用 <code>rails</code> 命令。</p><p><code>rails server</code> 實際上等於下面這個命令：</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><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> 常數之後會被 <code>rails/commands</code> 使用。這裡引用的 <code>config/boot</code> 檔案是指 <code>config/boot.rb</code>，負責載入與設定 Bundler。</p><h4 id="config-boot-rb">1.4 <code>config/boot.rb</code>
</h4><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>標準的 Rails 應用程式裡，會有一個檔案，裡面記錄所有依賴的 RubyGems：<code>Gemfile</code>。<code>config/boot.rb</code> 將 <code>ENV['BUNDLE_GEMFILE']</code> 設為 <code>Gemfile</code> 的位置。若 <code>Gemfile</code> 存在，則需要 <code>require 'bundler/setup'</code>。這一行是 Bundler 用來設定 <code>Gemfile</code> 內所有相依 RubyGems 的載入路徑。</p><p>標準的 Rails 應用程式依賴以下 RubyGems：</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>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>tzinfo</li>
</ul>
<h4 id="rails-commands-rb">1.5 <code>rails/commands.rb</code>
</h4><p><a href="https://github.com/rails/rails/blob/master/railties/lib/rails/commands.rb">View Source</a></p><p><code>config/boot.rb</code> 執行完畢後，下個 <code>require</code> 的檔案是 <code>rails/commands</code>，用來展開命令的別名（alias）。在 <code>rails server</code> 這個情況裡，<code>ARGV</code> 的內容是 <code>server</code>，無需展開：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
ARGV << '--help' if ARGV.empty?
aliases = {
"g" => "generate",
"d" => "destroy",
"c" => "console",
"s" => "server",
"db" => "dbconsole",
"r" => "runner"
}
command = ARGV.shift
command = aliases[command] || command
require 'rails/commands/commands_tasks'
Rails::CommandsTasks.new(ARGV).run_command!(command)
</pre>
</div>
<div class="info"><p>如上所見，<code>ARGV</code> 為空時，Rails 會印出幫助訊息。</p></div><p>若用了別名，如 <code>rails s</code>，便會用 <code>aliases</code> 展開成對應的命令：</p><h4 id="rails-commands-command-tasks-rb">1.6 <code>rails/commands/command_tasks.rb</code>
</h4><p><a href="https://github.com/rails/rails/blob/master/railties/lib/rails/commands/commands_tasks.rb">View Source</a></p><p>當輸入錯的 Rails 命令時，<code>run_command!</code> 負責拋出錯誤訊息。若命令是有效的，則會呼叫與命令同名的方法。</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>假設傳入的命令是 <code>server</code>，Rails 會執行以下的程式碼：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
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
private
def set_application_directory!
Dir.chdir(File.expand_path('../../', APP_PATH)) unless File.exist?(File.expand_path("config.ru"))
end
def require_command!(command)
require "rails/commands/#{command}"
end
</pre>
</div>
<p>沒找到 <code>config.ru</code> 時，會切換到 Rails 的根目錄（從 <code>APP_PATH</code> 往上兩層，<code>APP_PATH</code> 指向 <code>config/application.rb</code> ）。接著 <code>require</code> <code>rails/commands/server</code>（[rails/commands/server.rb](rails/commands/server），會把 <code>Rails::Server</code> 類別設定好。</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 < ::Rack::Server
</pre>
</div>
<p><code>fileutils</code> 和 <code>optparse</code> 是 Ruby 的標準函式庫，用來處理檔案與解析命令行參數。</p><h4 id="actionpack-lib-action-dispatch-rb">1.7 <code>actionpack/lib/action_dispatch.rb</code>
</h4><p><a href="https://github.com/rails/rails/blob/master/actionpack/lib/action_dispatch.rb">View Source</a></p><p>Action Dispatch 是 Rails 框架負責處理路由的元件。為 Rails 加入像是路由、Session 以及常見的 Middlewares。</p><h4 id="rails-commands-server-rb">1.8 <code>rails/commands/server.rb</code>
</h4><p><a href="https://github.com/rails/rails/blob/master/railties/lib/rails/commands/server.rb">View Source</a></p><p><code>Rails::Server</code> 在這個檔案裡定義，繼承自 <code>Rack::Server</code>。呼叫 <code>Rails::Server.new</code> 時，會呼叫 <a href="https://github.com/rails/rails/blob/master/railties/lib/rails/commands/server.rb"><code>rails/commands/server.rb</code></a> 裡的 <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>首先呼叫 <code>super</code>，<code>super</code> 會呼叫 <code>Rack::Server</code> 的 <code>initialize</code>。</p><h4 id="啟動！-rack-lib-rack-server-rb">1.9 Rack: <code>lib/rack/server.rb</code>
</h4><p><a href="https://github.com/rack/rack/blob/master/lib/rack/server.rb">View Source</a></p><p><code>Rack::Server</code> 負責給所有基於 Rack 的應用程式，提供通用的伺服器接口（interface），Rails 也是基於 Rack 的應用程式。</p><p><code>Rack::Server</code> 的 <code>initialize</code> 方法只是設定幾個變數而已：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
def initialize(options = nil)
@options = options
@app = options[:app] if options && options[:app]
end
</pre>
</div>
<p>這個情況裡，<code>options</code> 會是 <code>nil</code>，所以 <code>initialize</code> 什麼也沒做。</p><p><code>super</code> 結束之後，回到 <code>rails/commands/server.rb</code>。接著在 <code>Rails::Server</code> 的上下文裡呼叫 <code>set_environment</code>，猛一看好像沒做什麼：</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>實際上 <code>options</code> 方法做了很多事情。這個方法在 <code>Rack::Server</code> 的定義是：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
def options
@options ||= parse_options(ARGV)
end
</pre>
</div>
<p><code>parse_options</code> 方法的內容：</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> 的內容：</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 => environment,
:pid => nil,
:Port => 9292,
:Host => default_host,
:AccessLog => [],
:config => "config.ru"
}
end
</pre>
</div>
<p>接著看到，因為 <code>ENV</code> 裡沒有 <code>REQUEST_METHOD</code>，可以忽略 <code>args.clear</code>。下一行 <code>options.merge! opt_parser.parse!(args)</code>，把從命令行來的參數與 <code>opt_parser</code> 的選項合併，<code>opt_parser</code> 在 <code>Rack::Server</code> 裡定義：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
def opt_parser
Options.new
end
</pre>
</div>
<p>雖然 <a href="https://github.com/rack/rack/blob/master/lib/rack/server.rb#L6-L87"><code>parse!</code></a> 是在 <code>Rack::Server</code> 裡定義，但在 <code>Rails::Server</code> <a href="https://github.com/rails/rails/blob/master/railties/lib/rails/commands/server.rb#L9-L47">被覆寫了</a>，因為要收不同的參數。<code>Rails::Server</code> 定義的 <code>parse!</code> 方法開頭是：</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>這個方法會設定好 <code>options</code> 所有的鍵，Rails 根據這些鍵，決定伺服器該怎麼執行。在 <code>initialize</code> 結束之後，回到 <a href="https://github.com/rails/rails/blob/master/railties/lib/rails/commands/commands_tasks.rb"><code>rails/commands/command_tasks.rb</code></a>，見 <code># Back to here</code>：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
def server
set_application_directory!
require_command!("server")
# Back to here
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
</pre>
</div>
<h4 id="啟動！-config-application-rb">1.10 <code>config/application.rb</code>
</h4><p>當 <code>require APP_PATH</code> 執行時，會載入 <code>config/application.rb</code>。回想一下，<code>APP_PATH</code> 在 Rails 應用程式下的 <code>bin/rails</code> 裡定義：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
APP_PATH = File.expand_path('../../config/application', __FILE__)
</pre>
</div>
<p><code>config/application.rb</code> 裡放的是任何要對應用程式修改的設定。</p><h4 id="rails-server-start">1.11 <code>Rails::Server#start</code>
</h4><p><a href="https://github.com/rails/rails/blob/master/railties/lib/rails/commands/server.rb#L70-L81">View Source</a></p><p><code>config/application.rb</code> 載入完畢後，呼叫了 <code>server.start</code>。<code>#start</code> 方法的定義是：</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 "=> Run `rails server -h` for more startup options"
...
puts "=> 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>Rails 啟動過程“初次輸出訊息”的地方。這個方法會捕捉 <code>INT</code> 信號，所以當你按下 <code>CTRL-C</code> 時，才能從進程（process）裡離開。從這段程式碼可以看到，會建立出 <code>tmp/cache</code>、<code>tmp/pids</code>、<code>tmp/sessions</code> 以及 <code>tmp/sockets</code> 這四個目錄。接著呼叫 <code>wrapped_app</code>，這個方法負責在指定 <code>ActiveSupport::Logger</code> 之前，建立出 Rack 應用程式。</p><p>上面 <code>start</code> 方法裡的 <code>super</code> 方法會呼叫 <a href="https://github.com/rack/rack/blob/master/lib/rack/server.rb#L228-L265"><code>Rack::Server.start</code></a>，此方法定義如下：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
def start &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, &blk
end
</pre>
</div>
<p>Rails 應用程式感興趣的是最後一行，<code>server.run</code>。這裡又遇到 <code>wrapped_app</code> 方法了，是深入介紹的時候了。</p><p><code>wrapped_app</code> 的定義：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
@wrapped_app ||= build_app app
</pre>
</div>
<p><code>app</code> 方法的定義：</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> 的預設值是 <code>config.ru</code>，而 <code>config.ru</code> 的內容：</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 <%= app_const %>
</pre>
</div>
<p><a href="https://github.com/rack/rack/blob/371cf6f3a8d390edfa901b6f963b78810270a387/lib/rack/builder.rb#L32-L46"><code>Rack::Builder.parse_file</code></a> 方法讀取 <code>config.ru</code> ，並進行解析：</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> 的 <a href="https://github.com/rack/rack/blob/371cf6f3a8d390edfa901b6f963b78810270a387/lib/rack/builder.rb#L53-L56"><code>initialize</code> 方法</a>接受區塊參數，會在 <code>Rack::Builder</code> 的實體裡執行這個區塊。Rails 啟動過程主要都在這裡發生。最先執行的是 <code>config.ru</code> 裡的這一行：</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>這個檔案通常由 <code>config.ru</code>（即 <code>rails server</code>）與 Passenger <code>require</code> 進來。這也是兩種啟動伺服器方法首次相遇的地方。在這之前都只是在設定 Rack 與 Rails 而已。</p><p>這個檔案從 <code>require</code> <code>config/application.rb</code> 開始：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
# Load the Rails application.
require File.expand_path('../application', __FILE__)
</pre>
</div>
<h4 id="啟動！-config-application-rb">1.13 <code>config/application.rb</code>
</h4><p>這個檔案 <code>require</code> <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>但只在 <code>config/boot.rb</code> 沒有被 <code>require</code> 的前提下才會進行 <code>require</code>。如此一來 <code>rails server</code> 才不會重複 <code>require</code>，但 Passenger 每次都會重新 <code>require</code> <code>config/boot.rb</code>。</p><p>有趣的事情開始了！</p><h3 id="載入-rails">2 載入 Rails</h3><p><code>config/application.rb</code> 檔案的下一行是：</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><a href="https://github.com/rails/rails/blob/master/railties/lib/rails/all.rb">View Source</a></p><p>這個檔案負責 <code>require</code> Rails 框架的各個元件：</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>這是整個 Rails 框架載入的地方，讓每個元件在應用程式裡都可以使用。每個部分怎麼載入的不深入探究，但有興趣可以自己深入研究。</p><p>現在只要記得，共用的功能像是 Rails 引擎、I18n 以及 Rails 所有的設定都是在這裡定義完成。</p><h4 id="回到-config-environment-rb">2.2 回到 <code>config/environment.rb</code>
</h4><p><code>config/application.rb</code> 的其他部分定義了 <code>Rails::Application</code> 的設定，這些設定在應用程式啟動完畢時會全部載入進來。當 <code>config/application.rb</code> 載入 Rails 完畢時，以及應用程式命名空間定義完畢時，會回到應用程式初始化的地方，也就是 <code>config/environment.rb</code>。舉個例子，若應用程式叫做 <code>Blog</code>，則會找到 <code>Rails.application.initialize!</code>，這個方法在 <code>rails/application.rb</code> 裡定義。</p><h4 id="railties-lib-rails-application-rb">2.3 <code>railties/lib/rails/application.rb</code>
</h4><p><a href="https://github.com/rails/rails/blob/master/railties/lib/rails/application.rb">View Source</a></p><p><code>initialize!</code> 方法：</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>可以看到應用程式只會初始化一次。Initializers（<code>config/initializers</code> 目錄下的設定檔）透過 <code>run_initializers</code> 方法依序執行，<code>run_initializers</code> 方法在 <a href="https://github.com/rails/rails/blob/master/railties/lib/rails/initializable.rb"><code>railties/lib/rails/initializable.rb</code></a> 裡定義：</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> 很巧妙。在這裡會遍歷所有類別的祖先，找出有回應 <code>initializers</code> 方法的類別。接著按名稱將這些類別排序，再執行它們。舉例來說，<code>Engine</code> 類別透過給每個 Engine 提供 <code>initializers</code> 方法，讓這些 Engine 都可以引用進來。</p><p>Rails::Application 類別（在 <a href="https://github.com/rails/rails/blob/master/railties/lib/rails/application.rb"><code>railties/lib/rails/application.rb</code></a> 裡定義）定義了 bootstrap、railtie、finisher 這三個 Initializers。第一個執行的 Initializer 是 bootstrap，bootstrap 將應用程式準備好（像是初始化 logger），而 finisher initializer 則是最後執行（像是把 Middleware 都建好）。而 railtie initializers 則是在 <code>Rails::Application</code> 裡定義，在 bootstrap 與 finisher 之間執行。</p><p>Initializers 都執行完畢後，回到 <code>Rack::Server</code>。</p><h4 id="載入-rails-rack-lib-rack-server-rb">2.4 Rack: lib/rack/server.rb</h4><p><a href="https://github.com/rack/rack/blob/master/lib/rack/server.rb">View Source</a></p><p>在 <a href="#%E5%95%9F%E5%8B%95%EF%BC%81-rack:-lib/rack/server.rb">1.9 小節</a> ，我們看過 <code>app</code> 是如何被定義的：</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>到了這一步，<code>app</code> 便是 Rails 應用程式本身（Middleware），接下來 Rack 會呼叫所有的 Middlewares：</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>記得 <code>wrapped_app</code> 在 <code>Server#start</code> 呼叫了 <code>build_app</code> （最後一行）：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
server.run wrapped_app, options, &blk
</pre>
</div>
<p>到這裡 <code>server.run</code> 取決於所使用的伺服器實作是那一個。假設用的是 Puma，下面是 Puma 的 <code>run</code> 方法：</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
...
DEFAULT_OPTIONS = {
:Host => '0.0.0.0',
:Port => 8080,
:Threads => '0:16',
:Verbose => 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>伺服器本身的實作不深入探究，但這是 Rails 啟動過程整個旅程的最後一站。</p><p>希望這高度抽象的綜覽能幫助你更好的了解 Rails 程式是如何執行的，進而成為一個更好的 Rails 開發者。若想了解更多的話，那就閱讀 Rails 的原始碼吧！</p>
<h3>反饋</h3>
<p>
歡迎幫忙改善指南的品質。
</p>
<p>
如發現任何錯誤之處，歡迎修正。開始貢獻前，可以先閱讀<a href="http://edgeguides.rubyonrails.org/contributing_to_ruby_on_rails.html#contributing-to-the-rails-documentation">貢獻指南：文件</a>。
</p>
<p>翻譯如有錯誤，深感抱歉，歡迎 <a href="https://github.com/docrails-tw/guides/fork">Fork</a> 修正，或至此處<a href="https://github.com/docsrails-tw/guides/issues/new">回報</a>。</p>
<p>
文章可能有未完成或過時的內容。請先檢查 <a href="http://edgeguides.rubyonrails.org">Edge Guides</a> 來確定問題在 master 是否已經修掉了。再上 master 補上缺少的文件。內容參考 <a href="ruby_on_rails_guides_guidelines.html">Ruby on Rails 指南準則</a>來了解行文風格。
</p>
<p>最後，任何關於 Ruby on Rails 文件的討論，歡迎至 <a href="http://groups.google.com/group/rubyonrails-docs">rubyonrails-docs 郵件論壇</a>。
</p>
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