[elixir! #0082] application controller 應用是如何被載入和啟動的

Ljzn發表於2021-12-02
APPController

熟悉 erlang/elixir 的朋友們應該知道 application 的概念，它是一種特殊的結構，用於啟動和停止一個應用。每當我們新建一個 erlang/elixir 專案，也同時新建了一個同名的 應用。在使用依賴庫的時候，一般每個依賴庫也是一個應用，會在我們執行專案時被載入和啟動。

那麼，erlang是如何管理這些應用的呢？這就是 :application_controller 發揮作用的地方了。所有的應用的載入、啟動等狀態的儲存和變更，都要經過這個程式。

# 列出當前的全部應用狀態
> :application_controller.info()

虛應用

不是所有的應用都會啟動程式樹，有些應用即使處於 :started 狀態，也沒有啟動任何程式。姑且稱這種應用為虛應用吧。

跟蹤

利用我之前文章裡提到的 bony_trance 庫來跟蹤一下 在load 和 start 一個應用時，:application_controller 程式都做了什麼吧。

stop 應用

iex(3)> :application_controller.stop_application :play
#PID<0.44.0> RECEIVED                                                 +67.069019s
MESSAGE: {:"$gen_call", {#PID<0.199.0>, #Reference<0.2783639343.3287547907.121340>}, {:stop_application, :play}}
#PID<0.44.0> SENT TO: :code_server                                    +0.000047s
MESSAGE: {:code_call, #PID<0.44.0>, {:ensure_loaded, Logger.Translator}}
#PID<0.44.0> RECEIVED                                                 +0.004582s
MESSAGE: {:code_server, {:module, Logger.Translator}}
#PID<0.44.0> SENT TO: :code_server                                    +0.000033s
MESSAGE: {:code_call, #PID<0.44.0>, {:ensure_loaded, Logger.Utils}}
#PID<0.44.0> RECEIVED                                                 +0.002413s
MESSAGE: {:code_server, {:module, Logger.Utils}}
#PID<0.44.0> SENT TO: :code_server                                    +0.000012s
MESSAGE: {:code_call, #PID<0.44.0>, {:ensure_loaded, :calendar}}
#PID<0.44.0> RECEIVED                                                 +0.012388s
MESSAGE: {:code_server, {:module, :calendar}}

12:38:54.714 [info]  Application play exited: :stopped
:ok
#PID<0.44.0> SENT TO: Logger                                          +0.000015s
MESSAGE: {:notify, {:info, #PID<0.64.0>, {Logger, ["Application ", "play", " exited: " | ":stopped"], {{2021, 11, 28}, {12, 38, 54, 714}}, [erl_level: :notice, domain: [:otp], error_logger: %{report_cb: &:application_controller.format_log/1, tag: :info_report, type: :std_info}, file: "application_controller.erl", function: "info_exited/3", gl: #PID<0.64.0>, line: 2119, mfa: {:application_controller, :info_exited, 3}, module: :application_controller, pid: #PID<0.44.0>, report_cb: &:application_controller.format_log/2, time: 1638074334714339]}}}
#PID<0.44.0> SENT TO: #PID<0.199.0>                                   +0.000015s
MESSAGE: {#Reference<0.2783639343.3287547907.121340>, :ok}

首先向 code_server 確認一些必要的用於列印log的模組是已載入的, 這一步的結果會快取，之後不必重複詢問。如果該應用有 application 程式，則會向其傳送 :stop 訊息。最後列印出應用已停止的log。

start 及 unload 並不需要太複雜的訊息互動，與 stop 類似。

load 應用

iex(9)> :application_controller.load_application :play
#PID<0.44.0> RECEIVED                                                 +26.948851s
MESSAGE: {:"$gen_call", {#PID<0.199.0>, #Reference<0.2783639343.3287547907.121460>}, {:load_application, :play}}
#PID<0.44.0> SENT TO: :code_server                                    +0.000014s
MESSAGE: {:code_call, #PID<0.44.0>, :get_path}
#PID<0.44.0> RECEIVED                                                 +0.000210s
MESSAGE: {:code_server, [...]}
#PID<0.44.0> SENT TO: #PID<0.10.0>                                    +0.000043s
MESSAGE: {#PID<0.44.0>, {:list_dir, '.../play/_build/dev/lib/play/consolidated'}}
#PID<0.44.0> RECEIVED                                                 +0.008352s
MESSAGE: {#PID<0.10.0>, {:ok, ['Elixir.Inspect.beam', 'Elixir.IEx.Info.beam', 'Elixir.String.Chars.beam', 'Elixir.List.Chars.beam', 'Elixir.Collectable.beam', 'Elixir.Enumerable.beam']}}
#PID<0.44.0> SENT TO: #PID<0.10.0>                                    +0.000018s
MESSAGE: {#PID<0.44.0>, {:list_dir, '/Users/linjiezhang/Documents/play/_build/dev/lib/play/ebin'}}
#PID<0.44.0> RECEIVED                                                 +0.010195s
MESSAGE: {#PID<0.10.0>, {:ok, ['Elixir.MyTracer.beam', 'Elixir.Play.beam', 'play.app', 'Elixir.Demo.beam']}}
#PID<0.44.0> SENT TO: #PID<0.10.0>                                    +0.000026s
MESSAGE: {#PID<0.44.0>, {:get_file, '/Users/linjiezhang/Documents/play/_build/dev/lib/play/ebin/play.app'}}
#PID<0.44.0> RECEIVED                                                 +0.000519s
MESSAGE: {#PID<0.10.0>, {:ok, "{application,play,\n             [{applications,[kernel,stdlib,elixir,logger,syntax_tools,\n                             bony_trace]},\n              {description,\"play\"},\n              {modules,['Elixir.Demo','Elixir.MyTracer','Elixir.Play']},\n              {registered,[]},\n              {vsn,\"0.1.0\"}]}.\n", '/Users/linjiezhang/Documents/play/_build/dev/lib/play/ebin/play.app'}}
:ok
#PID<0.44.0> SENT TO: :init                                           +0.000149s
MESSAGE: {#PID<0.44.0>, {:get_argument, :play}}
#PID<0.44.0> RECEIVED                                                 +0.000102s
MESSAGE: {:init, :error}
#PID<0.44.0> SENT TO: #PID<0.199.0>                                   +0.000009s
MESSAGE: {#Reference<0.2783639343.3287547907.121460>, :ok}

在 load 應用時，AC 通過 code_server 獲取了全部應用的檔案路徑，之後拿到了此應用的確切的模組以及 .app 檔案。然後向 :init 程式請求此應用的引數。這裡就不深究這些互動的具體作用了，之後的文章裡可以詳細解讀一下 :init 程式的作用。它相當於 erlang 世界裡的創世主，擁有 PID<0.0.0> 。

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