我试图找到一个关于init()函数在Go中的作用的精确解释。我读了Effective Go说的话，但我不确定我是否完全理解了它说的话。我不太确定的句子是:
finally的意思是最后:init是在包中所有变量声明都计算了它们的初始化式之后调用的，并且只有在所有导入的包都初始化之后才会计算这些初始化式。
包中所有变量声明对其初始化式求值意味着什么?这是否意味着如果你在一个包和它的文件中声明“全局”变量，init()将不会运行，直到所有的它被评估，然后它将运行所有的init函数，然后main()当。/main_file_name运行?
我还读了Mark Summerfield的书:
如果一个包有一个或多个init()函数，它们会在主包的main()函数被调用之前自动执行。
在我的理解中，init()只在您打算运行main()时才相关，对吗?如果有人更清楚地理解init()，请随意纠正我
是的，假设你有这个:
var WhatIsThe = AnswerToLife()
func AnswerToLife() int { // 1
return 42
}
func init() { // 2
WhatIsThe = 0
}
func main() { // 3
if WhatIsThe == 0 {
fmt.Println("It's all a lie.")
}
}
AnswerToLife()保证在调用init()之前运行，并且init()保证在调用main()之前运行。
请记住，init()总是被调用，无论是否有main，所以如果你导入一个包，它有init函数，它将被执行。
此外，每个包可以有多个init()函数;它们将按照它们在文件中出现的顺序执行(当然是在所有变量初始化之后)。如果它们跨越多个文件，它们将按照词法文件名的顺序执行(正如@benc指出的那样):
init()函数似乎是按词法文件名顺序执行的。Go规范说:“构建系统被鼓励以词法文件名的顺序将属于同一个包的多个文件呈现给编译器”。似乎go build是这样工作的。
许多内部Go包使用init()来初始化表等，例如https://github.com/golang/go/blob/883bc6/src/compress/bzip2/bzip2.go#L480
补充一点(我本可以作为评论添加的，但在写这篇文章的时候，我还没有足够的声誉)
在同一个包中有多个初始化，我还没有找到任何保证的方法来知道它们将以什么顺序运行。例如，我有:
package config
- config.go
- router.go
这两个配置。去找路由器吧。Go包含init()函数，但当运行router。Go的函数先运行(这导致我的应用程序恐慌)。
如果您有多个文件，每个文件都有自己的init()函数，请注意不能保证一个文件先于另一个文件。最好使用变量赋值，就像OneToOne在他的例子中展示的那样。最好的部分是:这个变量声明将发生在包中的ALL init()函数之前。
例如
config.go:
var ConfigSuccess = configureApplication()
func init() {
doSomething()
}
func configureApplication() bool {
l4g.Info("Configuring application...")
if valid := loadCommandLineFlags(); !valid {
l4g.Critical("Failed to load Command Line Flags")
return false
}
return true
}
router.go:
func init() {
var (
rwd string
tmp string
ok bool
)
if metapath, ok := Config["fs"]["metapath"].(string); ok {
var err error
Conn, err = services.NewConnection(metapath + "/metadata.db")
if err != nil {
panic(err)
}
}
}
不管var ConfigSuccess = configureApplication()是否存在于路由器中。Go或config。go，它将在EITHER init()运行之前运行。
https://golang.org/ref/mem#tmp_4
程序初始化在单个goroutine中运行，但该goroutine可以创建其他goroutine，这些goroutine可以并发运行。 如果包p导入包q, q的初始化函数的完成发生在p的任何初始化函数开始之前。 函数main的开始。Main发生在所有init函数完成之后。
以你为其他用户设计的框架或库为例，这些用户最终会在他们的代码中有一个主函数来执行他们的应用程序。如果用户直接导入你库项目的子包，那么该子包的init将首先被调用(一次)。对于库的根包也是一样的，等等…
很多时候，您可能希望在不存在main func的情况下执行代码块，无论是否直接执行。
如果您作为虚拟库的开发人员，导入带有init函数的库的子包，它将首先被调用一次，您没有main func，但您需要确保一些变量或表将在调用其他函数之前初始化。
有一件事需要记住，不必担心，那就是: init总是在每个应用程序中执行一次。
Init执行发生:
就在“caller”包的init函数之前， 在main func函数之前， 但是在包级变量之后，var =[…]或者cost =[…]，
当你导入一个包时，它会按顺序运行所有的init函数。
我会给出一个很好的init函数的例子。它会将mime类型添加到一个名为mime的标准go库中，并且包级函数将直接使用mime标准包来获得已经在init函数中初始化的自定义mime类型:
package mime
import (
"mime"
"path/filepath"
)
var types = map[string]string{
".3dm": "x-world/x-3dmf",
".3dmf": "x-world/x-3dmf",
".7z": "application/x-7z-compressed",
".a": "application/octet-stream",
".aab": "application/x-authorware-bin",
".aam": "application/x-authorware-map",
".aas": "application/x-authorware-seg",
".abc": "text/vndabc",
".ace": "application/x-ace-compressed",
".acgi": "text/html",
".afl": "video/animaflex",
".ai": "application/postscript",
".aif": "audio/aiff",
".aifc": "audio/aiff",
".aiff": "audio/aiff",
".aim": "application/x-aim",
".aip": "text/x-audiosoft-intra",
".alz": "application/x-alz-compressed",
".ani": "application/x-navi-animation",
".aos": "application/x-nokia-9000-communicator-add-on-software",
".aps": "application/mime",
".apk": "application/vnd.android.package-archive",
".arc": "application/x-arc-compressed",
".arj": "application/arj",
".art": "image/x-jg",
".asf": "video/x-ms-asf",
".asm": "text/x-asm",
".asp": "text/asp",
".asx": "application/x-mplayer2",
".au": "audio/basic",
".avi": "video/x-msvideo",
".avs": "video/avs-video",
".bcpio": "application/x-bcpio",
".bin": "application/mac-binary",
".bmp": "image/bmp",
".boo": "application/book",
".book": "application/book",
".boz": "application/x-bzip2",
".bsh": "application/x-bsh",
".bz2": "application/x-bzip2",
".bz": "application/x-bzip",
".c++": "text/plain",
".c": "text/x-c",
".cab": "application/vnd.ms-cab-compressed",
".cat": "application/vndms-pkiseccat",
".cc": "text/x-c",
".ccad": "application/clariscad",
".cco": "application/x-cocoa",
".cdf": "application/cdf",
".cer": "application/pkix-cert",
".cha": "application/x-chat",
".chat": "application/x-chat",
".chrt": "application/vnd.kde.kchart",
".class": "application/java",
".com": "text/plain",
".conf": "text/plain",
".cpio": "application/x-cpio",
".cpp": "text/x-c",
".cpt": "application/mac-compactpro",
".crl": "application/pkcs-crl",
".crt": "application/pkix-cert",
".crx": "application/x-chrome-extension",
".csh": "text/x-scriptcsh",
".css": "text/css",
".csv": "text/csv",
".cxx": "text/plain",
".dar": "application/x-dar",
".dcr": "application/x-director",
".deb": "application/x-debian-package",
".deepv": "application/x-deepv",
".def": "text/plain",
".der": "application/x-x509-ca-cert",
".dif": "video/x-dv",
".dir": "application/x-director",
".divx": "video/divx",
".dl": "video/dl",
".dmg": "application/x-apple-diskimage",
".doc": "application/msword",
".dot": "application/msword",
".dp": "application/commonground",
".drw": "application/drafting",
".dump": "application/octet-stream",
".dv": "video/x-dv",
".dvi": "application/x-dvi",
".dwf": "drawing/x-dwf=(old)",
".dwg": "application/acad",
".dxf": "application/dxf",
".dxr": "application/x-director",
".el": "text/x-scriptelisp",
".elc": "application/x-bytecodeelisp=(compiled=elisp)",
".eml": "message/rfc822",
".env": "application/x-envoy",
".eps": "application/postscript",
".es": "application/x-esrehber",
".etx": "text/x-setext",
".evy": "application/envoy",
".exe": "application/octet-stream",
".f77": "text/x-fortran",
".f90": "text/x-fortran",
".f": "text/x-fortran",
".fdf": "application/vndfdf",
".fif": "application/fractals",
".fli": "video/fli",
".flo": "image/florian",
".flv": "video/x-flv",
".flx": "text/vndfmiflexstor",
".fmf": "video/x-atomic3d-feature",
".for": "text/x-fortran",
".fpx": "image/vndfpx",
".frl": "application/freeloader",
".funk": "audio/make",
".g3": "image/g3fax",
".g": "text/plain",
".gif": "image/gif",
".gl": "video/gl",
".gsd": "audio/x-gsm",
".gsm": "audio/x-gsm",
".gsp": "application/x-gsp",
".gss": "application/x-gss",
".gtar": "application/x-gtar",
".gz": "application/x-compressed",
".gzip": "application/x-gzip",
".h": "text/x-h",
".hdf": "application/x-hdf",
".help": "application/x-helpfile",
".hgl": "application/vndhp-hpgl",
".hh": "text/x-h",
".hlb": "text/x-script",
".hlp": "application/hlp",
".hpg": "application/vndhp-hpgl",
".hpgl": "application/vndhp-hpgl",
".hqx": "application/binhex",
".hta": "application/hta",
".htc": "text/x-component",
".htm": "text/html",
".html": "text/html",
".htmls": "text/html",
".htt": "text/webviewhtml",
".htx": "text/html",
".ice": "x-conference/x-cooltalk",
".ico": "image/x-icon",
".ics": "text/calendar",
".icz": "text/calendar",
".idc": "text/plain",
".ief": "image/ief",
".iefs": "image/ief",
".iges": "application/iges",
".igs": "application/iges",
".ima": "application/x-ima",
".imap": "application/x-httpd-imap",
".inf": "application/inf",
".ins": "application/x-internett-signup",
".ip": "application/x-ip2",
".isu": "video/x-isvideo",
".it": "audio/it",
".iv": "application/x-inventor",
".ivr": "i-world/i-vrml",
".ivy": "application/x-livescreen",
".jam": "audio/x-jam",
".jav": "text/x-java-source",
".java": "text/x-java-source",
".jcm": "application/x-java-commerce",
".jfif-tbnl": "image/jpeg",
".jfif": "image/jpeg",
".jnlp": "application/x-java-jnlp-file",
".jpe": "image/jpeg",
".jpeg": "image/jpeg",
".jpg": "image/jpeg",
".jps": "image/x-jps",
".js": "application/javascript",
".json": "application/json",
".jut": "image/jutvision",
".kar": "audio/midi",
".karbon": "application/vnd.kde.karbon",
".kfo": "application/vnd.kde.kformula",
".flw": "application/vnd.kde.kivio",
".kml": "application/vnd.google-earth.kml+xml",
".kmz": "application/vnd.google-earth.kmz",
".kon": "application/vnd.kde.kontour",
".kpr": "application/vnd.kde.kpresenter",
".kpt": "application/vnd.kde.kpresenter",
".ksp": "application/vnd.kde.kspread",
".kwd": "application/vnd.kde.kword",
".kwt": "application/vnd.kde.kword",
".ksh": "text/x-scriptksh",
".la": "audio/nspaudio",
".lam": "audio/x-liveaudio",
".latex": "application/x-latex",
".lha": "application/lha",
".lhx": "application/octet-stream",
".list": "text/plain",
".lma": "audio/nspaudio",
".log": "text/plain",
".lsp": "text/x-scriptlisp",
".lst": "text/plain",
".lsx": "text/x-la-asf",
".ltx": "application/x-latex",
".lzh": "application/octet-stream",
".lzx": "application/lzx",
".m1v": "video/mpeg",
".m2a": "audio/mpeg",
".m2v": "video/mpeg",
".m3u": "audio/x-mpegurl",
".m": "text/x-m",
".man": "application/x-troff-man",
".manifest": "text/cache-manifest",
".map": "application/x-navimap",
".mar": "text/plain",
".mbd": "application/mbedlet",
".mc$": "application/x-magic-cap-package-10",
".mcd": "application/mcad",
".mcf": "text/mcf",
".mcp": "application/netmc",
".me": "application/x-troff-me",
".mht": "message/rfc822",
".mhtml": "message/rfc822",
".mid": "application/x-midi",
".midi": "application/x-midi",
".mif": "application/x-frame",
".mime": "message/rfc822",
".mjf": "audio/x-vndaudioexplosionmjuicemediafile",
".mjpg": "video/x-motion-jpeg",
".mm": "application/base64",
".mme": "application/base64",
".mod": "audio/mod",
".moov": "video/quicktime",
".mov": "video/quicktime",
".movie": "video/x-sgi-movie",
".mp2": "audio/mpeg",
".mp3": "audio/mpeg3",
".mp4": "video/mp4",
".mpa": "audio/mpeg",
".mpc": "application/x-project",
".mpe": "video/mpeg",
".mpeg": "video/mpeg",
".mpg": "video/mpeg",
".mpga": "audio/mpeg",
".mpp": "application/vndms-project",
".mpt": "application/x-project",
".mpv": "application/x-project",
".mpx": "application/x-project",
".mrc": "application/marc",
".ms": "application/x-troff-ms",
".mv": "video/x-sgi-movie",
".my": "audio/make",
".mzz": "application/x-vndaudioexplosionmzz",
".nap": "image/naplps",
".naplps": "image/naplps",
".nc": "application/x-netcdf",
".ncm": "application/vndnokiaconfiguration-message",
".nif": "image/x-niff",
".niff": "image/x-niff",
".nix": "application/x-mix-transfer",
".nsc": "application/x-conference",
".nvd": "application/x-navidoc",
".o": "application/octet-stream",
".oda": "application/oda",
".odb": "application/vnd.oasis.opendocument.database",
".odc": "application/vnd.oasis.opendocument.chart",
".odf": "application/vnd.oasis.opendocument.formula",
".odg": "application/vnd.oasis.opendocument.graphics",
".odi": "application/vnd.oasis.opendocument.image",
".odm": "application/vnd.oasis.opendocument.text-master",
".odp": "application/vnd.oasis.opendocument.presentation",
".ods": "application/vnd.oasis.opendocument.spreadsheet",
".odt": "application/vnd.oasis.opendocument.text",
".oga": "audio/ogg",
".ogg": "audio/ogg",
".ogv": "video/ogg",
".omc": "application/x-omc",
".omcd": "application/x-omcdatamaker",
".omcr": "application/x-omcregerator",
".otc": "application/vnd.oasis.opendocument.chart-template",
".otf": "application/vnd.oasis.opendocument.formula-template",
".otg": "application/vnd.oasis.opendocument.graphics-template",
".oth": "application/vnd.oasis.opendocument.text-web",
".oti": "application/vnd.oasis.opendocument.image-template",
".otm": "application/vnd.oasis.opendocument.text-master",
".otp": "application/vnd.oasis.opendocument.presentation-template",
".ots": "application/vnd.oasis.opendocument.spreadsheet-template",
".ott": "application/vnd.oasis.opendocument.text-template",
".p10": "application/pkcs10",
".p12": "application/pkcs-12",
".p7a": "application/x-pkcs7-signature",
".p7c": "application/pkcs7-mime",
".p7m": "application/pkcs7-mime",
".p7r": "application/x-pkcs7-certreqresp",
".p7s": "application/pkcs7-signature",
".p": "text/x-pascal",
".part": "application/pro_eng",
".pas": "text/pascal",
".pbm": "image/x-portable-bitmap",
".pcl": "application/vndhp-pcl",
".pct": "image/x-pict",
".pcx": "image/x-pcx",
".pdb": "chemical/x-pdb",
".pdf": "application/pdf",
".pfunk": "audio/make",
".pgm": "image/x-portable-graymap",
".pic": "image/pict",
".pict": "image/pict",
".pkg": "application/x-newton-compatible-pkg",
".pko": "application/vndms-pkipko",
".pl": "text/x-scriptperl",
".plx": "application/x-pixclscript",
".pm4": "application/x-pagemaker",
".pm5": "application/x-pagemaker",
".pm": "text/x-scriptperl-module",
".png": "image/png",
".pnm": "application/x-portable-anymap",
".pot": "application/mspowerpoint",
".pov": "model/x-pov",
".ppa": "application/vndms-powerpoint",
".ppm": "image/x-portable-pixmap",
".pps": "application/mspowerpoint",
".ppt": "application/mspowerpoint",
".ppz": "application/mspowerpoint",
".pre": "application/x-freelance",
".prt": "application/pro_eng",
".ps": "application/postscript",
".psd": "application/octet-stream",
".pvu": "paleovu/x-pv",
".pwz": "application/vndms-powerpoint",
".py": "text/x-scriptphyton",
".pyc": "application/x-bytecodepython",
".qcp": "audio/vndqcelp",
".qd3": "x-world/x-3dmf",
".qd3d": "x-world/x-3dmf",
".qif": "image/x-quicktime",
".qt": "video/quicktime",
".qtc": "video/x-qtc",
".qti": "image/x-quicktime",
".qtif": "image/x-quicktime",
".ra": "audio/x-pn-realaudio",
".ram": "audio/x-pn-realaudio",
".rar": "application/x-rar-compressed",
".ras": "application/x-cmu-raster",
".rast": "image/cmu-raster",
".rexx": "text/x-scriptrexx",
".rf": "image/vndrn-realflash",
".rgb": "image/x-rgb",
".rm": "application/vndrn-realmedia",
".rmi": "audio/mid",
".rmm": "audio/x-pn-realaudio",
".rmp": "audio/x-pn-realaudio",
".rng": "application/ringing-tones",
".rnx": "application/vndrn-realplayer",
".roff": "application/x-troff",
".rp": "image/vndrn-realpix",
".rpm": "audio/x-pn-realaudio-plugin",
".rt": "text/vndrn-realtext",
".rtf": "text/richtext",
".rtx": "text/richtext",
".rv": "video/vndrn-realvideo",
".s": "text/x-asm",
".s3m": "audio/s3m",
".s7z": "application/x-7z-compressed",
".saveme": "application/octet-stream",
".sbk": "application/x-tbook",
".scm": "text/x-scriptscheme",
".sdml": "text/plain",
".sdp": "application/sdp",
".sdr": "application/sounder",
".sea": "application/sea",
".set": "application/set",
".sgm": "text/x-sgml",
".sgml": "text/x-sgml",
".sh": "text/x-scriptsh",
".shar": "application/x-bsh",
".shtml": "text/x-server-parsed-html",
".sid": "audio/x-psid",
".skd": "application/x-koan",
".skm": "application/x-koan",
".skp": "application/x-koan",
".skt": "application/x-koan",
".sit": "application/x-stuffit",
".sitx": "application/x-stuffitx",
".sl": "application/x-seelogo",
".smi": "application/smil",
".smil": "application/smil",
".snd": "audio/basic",
".sol": "application/solids",
".spc": "text/x-speech",
".spl": "application/futuresplash",
".spr": "application/x-sprite",
".sprite": "application/x-sprite",
".spx": "audio/ogg",
".src": "application/x-wais-source",
".ssi": "text/x-server-parsed-html",
".ssm": "application/streamingmedia",
".sst": "application/vndms-pkicertstore",
".step": "application/step",
".stl": "application/sla",
".stp": "application/step",
".sv4cpio": "application/x-sv4cpio",
".sv4crc": "application/x-sv4crc",
".svf": "image/vnddwg",
".svg": "image/svg+xml",
".svr": "application/x-world",
".swf": "application/x-shockwave-flash",
".t": "application/x-troff",
".talk": "text/x-speech",
".tar": "application/x-tar",
".tbk": "application/toolbook",
".tcl": "text/x-scripttcl",
".tcsh": "text/x-scripttcsh",
".tex": "application/x-tex",
".texi": "application/x-texinfo",
".texinfo": "application/x-texinfo",
".text": "text/plain",
".tgz": "application/gnutar",
".tif": "image/tiff",
".tiff": "image/tiff",
".tr": "application/x-troff",
".tsi": "audio/tsp-audio",
".tsp": "application/dsptype",
".tsv": "text/tab-separated-values",
".turbot": "image/florian",
".txt": "text/plain",
".uil": "text/x-uil",
".uni": "text/uri-list",
".unis": "text/uri-list",
".unv": "application/i-deas",
".uri": "text/uri-list",
".uris": "text/uri-list",
".ustar": "application/x-ustar",
".uu": "text/x-uuencode",
".uue": "text/x-uuencode",
".vcd": "application/x-cdlink",
".vcf": "text/x-vcard",
".vcard": "text/x-vcard",
".vcs": "text/x-vcalendar",
".vda": "application/vda",
".vdo": "video/vdo",
".vew": "application/groupwise",
".viv": "video/vivo",
".vivo": "video/vivo",
".vmd": "application/vocaltec-media-desc",
".vmf": "application/vocaltec-media-file",
".voc": "audio/voc",
".vos": "video/vosaic",
".vox": "audio/voxware",
".vqe": "audio/x-twinvq-plugin",
".vqf": "audio/x-twinvq",
".vql": "audio/x-twinvq-plugin",
".vrml": "application/x-vrml",
".vrt": "x-world/x-vrt",
".vsd": "application/x-visio",
".vst": "application/x-visio",
".vsw": "application/x-visio",
".w60": "application/wordperfect60",
".w61": "application/wordperfect61",
".w6w": "application/msword",
".wav": "audio/wav",
".wb1": "application/x-qpro",
".wbmp": "image/vnd.wap.wbmp",
".web": "application/vndxara",
".wiz": "application/msword",
".wk1": "application/x-123",
".wmf": "windows/metafile",
".wml": "text/vnd.wap.wml",
".wmlc": "application/vnd.wap.wmlc",
".wmls": "text/vnd.wap.wmlscript",
".wmlsc": "application/vnd.wap.wmlscriptc",
".word": "application/msword",
".wp5": "application/wordperfect",
".wp6": "application/wordperfect",
".wp": "application/wordperfect",
".wpd": "application/wordperfect",
".wq1": "application/x-lotus",
".wri": "application/mswrite",
".wrl": "application/x-world",
".wrz": "model/vrml",
".wsc": "text/scriplet",
".wsrc": "application/x-wais-source",
".wtk": "application/x-wintalk",
".x-png": "image/png",
".xbm": "image/x-xbitmap",
".xdr": "video/x-amt-demorun",
".xgz": "xgl/drawing",
".xif": "image/vndxiff",
".xl": "application/excel",
".xla": "application/excel",
".xlb": "application/excel",
".xlc": "application/excel",
".xld": "application/excel",
".xlk": "application/excel",
".xll": "application/excel",
".xlm": "application/excel",
".xls": "application/excel",
".xlt": "application/excel",
".xlv": "application/excel",
".xlw": "application/excel",
".xm": "audio/xm",
".xml": "text/xml",
".xmz": "xgl/movie",
".xpix": "application/x-vndls-xpix",
".xpm": "image/x-xpixmap",
".xsr": "video/x-amt-showrun",
".xwd": "image/x-xwd",
".xyz": "chemical/x-pdb",
".z": "application/x-compress",
".zip": "application/zip",
".zoo": "application/octet-stream",
".zsh": "text/x-scriptzsh",
".docx": "application/vnd.openxmlformats-officedocument.wordprocessingml.document",
".docm": "application/vnd.ms-word.document.macroEnabled.12",
".dotx": "application/vnd.openxmlformats-officedocument.wordprocessingml.template",
".dotm": "application/vnd.ms-word.template.macroEnabled.12",
".xlsx": "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet",
".xlsm": "application/vnd.ms-excel.sheet.macroEnabled.12",
".xltx": "application/vnd.openxmlformats-officedocument.spreadsheetml.template",
".xltm": "application/vnd.ms-excel.template.macroEnabled.12",
".xlsb": "application/vnd.ms-excel.sheet.binary.macroEnabled.12",
".xlam": "application/vnd.ms-excel.addin.macroEnabled.12",
".pptx": "application/vnd.openxmlformats-officedocument.presentationml.presentation",
".pptm": "application/vnd.ms-powerpoint.presentation.macroEnabled.12",
".ppsx": "application/vnd.openxmlformats-officedocument.presentationml.slideshow",
".ppsm": "application/vnd.ms-powerpoint.slideshow.macroEnabled.12",
".potx": "application/vnd.openxmlformats-officedocument.presentationml.template",
".potm": "application/vnd.ms-powerpoint.template.macroEnabled.12",
".ppam": "application/vnd.ms-powerpoint.addin.macroEnabled.12",
".sldx": "application/vnd.openxmlformats-officedocument.presentationml.slide",
".sldm": "application/vnd.ms-powerpoint.slide.macroEnabled.12",
".thmx": "application/vnd.ms-officetheme",
".onetoc": "application/onenote",
".onetoc2": "application/onenote",
".onetmp": "application/onenote",
".onepkg": "application/onenote",
".xpi": "application/x-xpinstall",
}
func init() {
for ext, typ := range types {
// skip errors
mime.AddExtensionType(ext, typ)
}
}
// typeByExtension returns the MIME type associated with the file extension ext.
// The extension ext should begin with a leading dot, as in ".html".
// When ext has no associated type, typeByExtension returns "".
//
// Extensions are looked up first case-sensitively, then case-insensitively.
//
// The built-in table is small but on unix it is augmented by the local
// system's mime.types file(s) if available under one or more of these
// names:
//
// /etc/mime.types
// /etc/apache2/mime.types
// /etc/apache/mime.types
//
// On Windows, MIME types are extracted from the registry.
//
// Text types have the charset parameter set to "utf-8" by default.
func TypeByExtension(fullfilename string) string {
ext := filepath.Ext(fullfilename)
typ := mime.TypeByExtension(ext)
// mime.TypeByExtension returns as text/plain; | charset=utf-8 the static .js (not always)
if ext == ".js" && (typ == "text/plain" || typ == "text/plain; charset=utf-8") {
if ext == ".js" {
typ = "application/javascript"
}
}
return typ
}
希望这对你和其他用户有所帮助，如果你有更多的问题，不要犹豫再次发帖!
Init将在任何使用它的包的地方被调用(无论是空白导入还是导入)，但只有一次。
这是一个包:
package demo
import (
"some/logs"
)
var count int
func init() {
logs.Debug(count)
}
// Do do
func Do() {
logs.Debug("dd")
}
任何包(主包或任何测试包)作为空白导入:
_ "printfcoder.com/we/models/demo"
或者使用it func导入它:
"printfcoder.com/we/models/demo"
func someFunc(){
demo.Do()
}
init将只记录一次日志0。 第一个使用它的包，它的init func将在包的init之前运行。所以:
A调用B B调用C，它们都有init func, C的init会在B之前运行，B在A之前运行。
这里是另一个例子- https://play.golang.org/p/9P-LmSkUMKY
package main
import (
"fmt"
)
func callOut() int {
fmt.Println("Outside is beinge executed")
return 1
}
var test = callOut()
func init() {
fmt.Println("Init3 is being executed")
}
func init() {
fmt.Println("Init is being executed")
}
func init() {
fmt.Println("Init2 is being executed")
}
func main() {
fmt.Println("Do your thing !")
}
上面程序的输出
$ go run init/init.go
Outside is being executed
Init3 is being executed
Init is being executed
Init2 is being executed
Do your thing !
看这张照片。：）
导入——> const——> var——> init()
如果一个包导入了其他包，则首先初始化导入的包。 当前包的常量初始化。 然后初始化当前包的变量。 最后，调用当前包的init()函数。
一个包可以有多个init函数(可以在单个文件中，也可以分布在多个文件中)，并且按照它们呈现给编译器的顺序调用它们。
一个包只能被初始化一次，即使它是从多个包中导入的。
还有一些建议:
只能有一个main()函数，但可以有很多init()函数。 你不需要显式地调用init()或main()，它们会自动调用。 Init()和main()不接受任何参数，也不返回任何东西。 Init()在main()之前运行。 如果你有很多init()，它们会按照声明的顺序运行。
init()函数何时运行?
使用Go 1.16 (Q1 2021)，您将准确地看到它运行的时间以及运行多长时间。
参见从CL(更改列表)254659提交7c58ef7，修复问题41378。
Runtime: implement GODEBUG=inittrace=1 support Setting inittrace=1 causes the runtime to emit a single line to standard error for each package with init work, summarizing the execution time and memory allocation. The emitted debug information for init functions can be used to find bottlenecks or regressions in Go startup performance. Packages with no init function work (user defined or compiler generated) are omitted. Tracing plugin inits is not supported as they can execute concurrently. This would make the implementation of tracing more complex while adding support for a very rare use case. Plugin inits can be traced separately by testing a main package importing the plugins package imports explicitly. $ GODEBUG=inittrace=1 go test init internal/bytealg @0.008 ms, 0 ms clock, 0 bytes, 0 allocs init runtime @0.059 ms, 0.026 ms clock, 0 bytes, 0 allocs init math @0.19 ms, 0.001 ms clock, 0 bytes, 0 allocs init errors @0.22 ms, 0.004 ms clock, 0 bytes, 0 allocs init strconv @0.24 ms, 0.002 ms clock, 32 bytes, 2 allocs init sync @0.28 ms, 0.003 ms clock, 16 bytes, 1 allocs init unicode @0.44 ms, 0.11 ms clock, 23328 bytes, 24 allocs ... Inspired by stapelberg@google.com who instrumented doInit in a prototype to measure init times with GDB.
