def server
  ......
  Rails::Server.new.tap do |server|
    # 关键点一
    require APP_PATH
    Dir.chdir(Rails.application.root)
    # 关键点二
    server.start
  end
end

# rails/railties/lib/rails/application.rb
def inherited(base)
  # 这里的super是指engine
  super
  base.instance
end

# rails/railties/lib/rails/railtie.rb
def instance
  @instance ||= new
end

def initialize(initial_variable_values = {}, &block)
  super()
  @initialized       = false
  @reloaders         = []
  @routes_reloader   = nil
  @app_env_config    = nil
  @ordered_railties  = nil
  @railties          = nil
  @message_verifiers = {}

  Rails.application ||= self

  # 加载lib目录到load path
  add_lib_to_load_path!

  # 4)Run config.before_configuration callbacks
  # 因为这个时候还没有调用config方法，也就是还没有进行config配置
  ActiveSupport.run_load_hooks(:before_configuration, self)

  initial_variable_values.each do |variable_name, value|
    if INITIAL_VARIABLES.include?(variable_name)
      instance_variable_set("@#{variable_name}", value)
    end
  end

  instance_eval(&block) if block_given?
end

# rails/railties/lib/rails/commands/server.rb
def start
  # 打印启动信息
  print_boot_information
  trap(:INT) { exit }
  # 创建tmp目录
  create_tmp_directories
  # 是否打印log到标准输出
  log_to_stdout if options[:log_stdout]

  # 调用rack/server的start方法
  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

# rack/lib/rack/server.rb
def start &blk
  ......

  check_pid! if options[:pid]

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

  # 关键点
  server.run wrapped_app, options, &blk
end

# rack/lib/rack/server.rb
def wrapped_app
  @wrapped_app ||= build_app app
end

# rack/lib/rack/builder.rb
def self.new_from_string(builder_script, file="(rackup)")
  # 执行了config.ru文件, 然后调用to_app
  eval "Rack::Builder.new {\n" + builder_script + "\n}.to_app",
    TOPLEVEL_BINDING, file, 0
end

# RailsDemoApp/config.ru
# This file is used by Rack-based servers to start the application.
require ::File.expand_path('../config/environment',  __FILE__)
run Rails.application

# RailsDemoApp/config/environment.rb
# Load the Rails application.
require File.expand_path('../application', __FILE__)

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

# rails/railties/lib/rails/application.rb
# Initialize the application passing the given group. By default, the
# group is :default
def initialize!(group=:default) #:nodoc:
  raise "Application has been already initialized." if @initialized
  # 执行了所有的注册的initializers
  run_initializers(group, self)
  @initialized = true
  self
end

# rails/railties/lib/rails/application.rb
# 注意这里重写了父类的该方法initializers
def initializers #:nodoc:
  Bootstrap.initializers_for(self) +
  # 这里的super得到的应该是my application的所有initializers,但是这些initializers被定义在railtie，engine，application里面，因为继承关系, 最终得到的应该是  每个gem的railtie的initializers + 每个gem的engine的initializers + my appilcation的initializers, 可以通过在rails console 中，Rails.application.send(:initializers)的结果来证实
  railties_initializers(super) +
  Finisher.initializers_for(self)
end

# rails/railties/lib/rails/engine.rb
# 这个就是启动流程中的 5) Load config/environments/ENV.rb, 特别注意这里用了before
initializer :load_environment_config, before: :load_environment_hook, group: :all do
  paths["config/environments"].existent.each do |environment|
    require environment
  end
end

# rails/railties/lib/rails/application/bootstrap.rb
initializer :bootstrap_hook, group: :all do |app|
  ActiveSupport.run_load_hooks(:before_initialize, app)
end

# rails/railties/lib/rails/engine.rb
# 这里就是config目录下的initializers执行的位置
initializer :load_config_initializers do
  config.paths["config/initializers"].existent.sort.each do |initializer|
    load_config_initializer(initializer)
  end
end

# rails/railties/lib/rails/application/finisher.rb
initializer :build_middleware_stack do
  build_middleware_stack
end

# This needs to happen before eager load so it happens
# in exactly the same point regardless of config.cache_classes
# 9) run to_prepare callbacks
initializer :run_prepare_callbacks do
  ActionDispatch::Reloader.prepare!
end

# rails/railties/lib/rails/application/finisher.rb
initializer :eager_load! do
  if config.eager_load
    ActiveSupport.run_load_hooks(:before_eager_load, self)
    config.eager_load_namespaces.each(&:eager_load!)
  end
end

# rails/railties/lib/rails/application/finisher.rb
# All initialization is done, including eager loading in production
initializer :finisher_hook do
  ActiveSupport.run_load_hooks(:after_initialize, self)
end