Ruby on rails 项目启动流程

众所周知，我们可以通过rails s 这个命令来启动一个rails 项目，但是这条命令都干了哪些事呢？抽时间研究了下，同时感谢tomwang1013的博客。当我们输入rails s 这个命令的时候，项目会加载项目bin/rails.rb 这个文件 
#!/usr/bin/env ruby 
APP_PATH = File.expand_path('../../config/application', __FILE__) 
require_relative '../config/boot' 
require 'rails/commands' 
通过代码我们可以看到这个文件中定义了一个APP_PATH 即我们的项目文件：config/application.rb，并require了config/boot，boot文件主要是做了bundle的初始化。 
然后，我们可以看到这个时候rails/commands（railties-3.2.3/lib/rails/commands.rb）文件被load进来，由于我们传入的command参数为s，也就是server，然后我们看看commands的这个文件的源码对应的部分：

when 'server'
  # Change to the application's path if there is no config.ru file in current dir.
  # This allows us to run script/rails server from other directories, but still get
  # the main config.ru and properly set the tmp directory.
  Dir.chdir(File.expand_path('../../', APP_PATH)) unless File.exists?(File.expand_path("config.ru"))  

  require 'rails/commands/server'
  Rails::Server.new.tap { |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
  }

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在server 这个方法中我们通过server.start这行代码可以看出最终调用了start这个方法，然后查看下start 这个方法。同时这里面也打印了我们熟悉的控制台信息

def start
  url = "#{options[:SSLEnable] ? 'https' : 'http'}://#{options[:Host]}:#{options[:Port]}"
  puts "=> Booting #{ActiveSupport::Inflector.demodulize(server)}"
  puts "=> Rails #{Rails.version} application starting in #{Rails.env} on #{url}"
  puts "=> Call with -d to detach" unless options[:daemonize]
  trap(:INT) { exit }
  puts "=> Ctrl-C to shutdown server" unless options[:daemonize]
  #Create required tmp directories if not found
  %w(cache pids sessions sockets).each do |dir_to_make|
    FileUtils.mkdir_p(Rails.root.join('tmp', dir_to_make))
  end                                                                              

  puts 'server start ---'
  super
ensure
  # The '-h' option calls exit before @options is set.
  # If we call 'options' with it unset, we get double help banners.
  puts 'Exiting' unless @options && options[:daemonize]
end   

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这个start 这个方法最终会执行super 这个，super 会调用Rack::Server#start方法：代码

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  

  # 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  

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代码执行到server.run wrapped_app 这行代码。wrapped_app方法最终又会调用Rack::Server#app

def app
  @app ||= begin
    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
end     

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最终app 这个方法会加载项目config 目录下的environment.rb这个文件如：

# Load the Rails application.
require File.expand_path('../application', __FILE__)
require 'whenever'

module ProductConfig
  DYNAMIC_FIELDS = Hash.new
end

module RestConfig
  PRODUCT_SERVER = ENV["PHOTO_HOST"] || 'http://localhost:3001/'

  #CUSTOMER_SERVER = ENV["CUSTOMER_HOST"] || 'http://localhost:3001/'
  CUSTOMER_SERVER = ENV["CUSTOMER_HOST"] || 'http://localhost:3001/'

  OA_SERVER = 'http://localhost:3001/'

  #ELEPHANT_HOST = ENV["ELEPHANT_HOST"] || 'http://www.jiuyunda.net:90/'
  ELEPHANT_HOST = ENV["ELEPHANT_HOST"] || 'http://localhost:3001/'

  JXC_HOST = ENV["JXC_HOST"] || 'http://localhost:3001/'

  SETTLE_HOST = ENV["SETTLE_HOST"] || 'http://localhost:3001/'

end

# Initialize the Rails application.
Rails.application.initialize!

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Rails.application.initialize!这行代码我们可以看出 
项目的初始化完成。这个时候我们代码就回到Rack::Server#start方法的最后一行 server.run wrapped_app, options, &blk 由于我们没有设定任何参数。通过代码

def server
  @_server ||= Rack::Handler.get(options[:server]) || Rack::Handler.default(options)
end
def self.default(options = {})
  # Guess.
  if ENV.include?("PHP_FCGI_CHILDREN")
    # We already speak FastCGI
    options.delete :File
    options.delete :Port                

    Rack::Handler::FastCGI
  elsif ENV.include?("REQUEST_METHOD")
    Rack::Handler::CGI
  else
    begin
      Rack::Handler::Thin
    rescue LoadError
      Rack::Handler::WEBrick
    end
  end
end

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我们可以看到rack 为我们选定了默认的server.一般来说是WEBrick .然后WEBrick启动，打印如下信息

INFO  WEBrick 1.3.1
INFO  ruby 1.9.3
INFO  WEBrick::HTTPServer#start: pid=28371 port=3000 

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如果安装了其他application server的话打印的信息可能会不同如

Booting Puma
=> Rails 4.2.4 application starting in development on http://localhost:3000
=> Run `rails server -h` for more startup options
=> Ctrl-C to shutdown server
"assets end:false"
Puma starting in single mode...
* Version 3.4.0 (ruby 2.3.0-p0), codename: Owl Bowl Brawl
* Min threads: 0, max threads: 16
* Environment: development
* Listening on tcp://localhost:3000

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至此项目的启动已完成。