英文:
Second Execution of func init() in GAE
问题
我有一个非常简单的代码，我的.go文件如下：
func init() {
http.HandleFunc("/", handlerMain)
log.Println("init executed")
}
func handlerMain(w http.ResponseWriter, r *http.Request) {
fmt.Fprintf(w, "TEST")
}
还有app.yaml文件：
application: newsboard
version: 1
runtime: go
api_version: go1
handlers:
- url: /.*
script: _go_app
当首次执行时，一切都正常，控制台输出如下：
INFO 2015-10-19 19:28:56,626 devappserver2.py:763] Skipping SDK update check.
INFO 2015-10-19 19:28:56,652 api_server.py:205] Starting API server at: http://localhost:56946
INFO 2015-10-19 19:28:56,655 dispatcher.py:197] Starting module "default" running at: http://localhost:8080
INFO 2015-10-19 19:28:56,658 admin_server.py:116] Starting admin server at: http://localhost:8000
2015/10/19 19:28:59 init executed
但是当我访问http://localhost:8080时，我得到以下结果：
INFO 2015-10-19 19:32:16,394 module.py:786] default: "GET / HTTP/1.1" 200 4
2015/10/19 19:32:16 init executed
所以init()函数被执行了两次。然后，每次重新加载页面时，一切都正常，控制台不再显示"init executed"。我的问题是：为什么init()会执行两次，这样做是否正常？
英文:
I have very simple code, my .go file:
func init() {
http.HandleFunc("/", handlerMain)
log.Println("init executed")
}
func handlerMain(w http.ResponseWriter, r *http.Request) {
fmt.Fprintf(w, "TEST")
}
and app.yaml:
application: newsboard
version: 1
runtime: go
api_version: go1
handlers:
- url: /.*
script: _go_app
When Executed at first everything goes fine, this is output from console
INFO 2015-10-19 19:28:56,626 devappserver2.py:763] Skipping SDK update check.
INFO 2015-10-19 19:28:56,652 api_server.py:205] Starting API server at: http://localhost:56946
INFO 2015-10-19 19:28:56,655 dispatcher.py:197] Starting module "default" running at: http://localhost:8080
INFO 2015-10-19 19:28:56,658 admin_server.py:116] Starting admin server at: http://localhost:8000
2015/10/19 19:28:59 init executed
But when I Enter http://localhost:8080 I get following:
INFO 2015-10-19 19:32:16,394 module.py:786] default: "GET / HTTP/1.1" 200 4
2015/10/19 19:32:16 init executed
So init() somehow is being executed twice. Then, Every time when I reload page everething is ok,"init executed" doesn't appear in console anymore.
My question: Why init() happens twice and is this okay?
答案1
得分: 2
这是完全没问题的。
Go AppEngine SDK会监视应用程序的代码库，每当检测到更改（例如修改了一个.go源文件），它会自动重新加载应用程序。这包括卸载代码并重新加载它-再次运行init()函数。
基本上，应用程序何时重新加载（因此执行init()函数）的控制权在SDK的“手中”。尝试修改一个.go文件，然后在浏览器中刷新：你会看到另一个init()函数的执行。
当你首次启动应用程序时，所有的代码都会被加载/编译（并执行init()函数）-这需要做以报告是否有错误。之后，SDK可能会卸载它（不确定-找不到相关文档），只有在发生请求时才会重新加载它（可能很快，也可能不会）。
英文:
It is perfectly fine.
The Go AppEngine SDK monitors the application's code base, and whenever it detects changes (e.g. you modified a .go source file) it automatically reloads your application. This includes / involves unloading your code and reloading it - running init() functions again.
Basically control over when an app is reloaded - and therefore execution of the init() functions - is in the "hands" of the SDK. Try to modify a .go file and hit refresh in the browser: you will experience another init() function execution.
When you first start your app, all the code is loaded/compiled (and init() functions get executed) - this needs to be done in order to report if there are errors. After that the SDK may unload it (not sure - couldn't find relevant docs) and only load it again if a request does happen (which may or may not come anytime soon).
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