Go语言的Http 中间件实现
英文原文链接: http://www.alexedwards.net/blog/making-and-using-middleware
当你正在构建一个Web应用程序有可能要运行许多（甚至全部）的HTTP请求一些共享功能,你可能想记录每一个request，gzip压缩的每个response，或者做一些繁重的处理或者缓存检查。
实现这个共享功能的一种方法是将其设置为中间件，他可以作为一个独立的程序，在正常的handlers处理之前。根本不需要重写代码：如果你想用一个中间件，就把它加上应用中；如果你改变主意了，去掉就好了。就这么简单。
ServeMux => Middleware Handler => Application Handler
ServeMux => MiddlewareHandler => ApplicationHandler
这篇文章，我会给大家介绍怎么自己去实现一个自定义的middleware模式。以及通过使用第三方的中间件软件包的一些具体的实例。
基本原则：
在Go语言中实现和使用middleware是非常简单的。
- 使我们的中间件能搞满足 http.handlers 这个接口
- 建立一个 handlers 链，使其能够满足中间件的 handler 和 正常应用的 handler，并且能够注册到 http.ServeMux
我来解释如何实现：
首先你要知道go 的http handle，这里假设你是知道的
func messageHandler(message string) http.Handler { return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) { w.Write([]byte(message) }) } funcmessageHandler(messagestring) http.Handler { return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) { w.Write([]byte(message) }) } 这上面这个代码片段里面我们的逻辑很简单只是一个简单的 w.Write() 然后我们使用 http.HandlerFunc 适配器来转化这个闭包，并返回。
我们可以使用一个相同的方法来创建一个 handler 链。可以使用 handler 代替参数 string 传进闭包，然后把控制 handler 给传进来的 handler，并且调用 ServeHTTP() 方法。
这给了我们一个完整的模式构建中间件：
func exampleMiddleware(next http.Handler) http.Handler { return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) { // Our middleware logic goes here... next.ServeHTTP(w, r) }) } funcexampleMiddleware(nexthttp.Handler) http.Handler { return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) { // Our middleware logic goes here... next.ServeHTTP(w, r) }) } 你注意到这个中间件有一个这样的函数结构 func(http.Handler) http.Handler 。它接受一个 handler 作为参数，并且返回一个 handler。这里有两个很有用的原因：
- 因为这个函数返回一个句柄可以直接供中间件注册
- 我们可以建立任意长度的 handler 链来通过中间件的方法互相嵌套
比如：
http.Handle("/", middlewareOne(middlewareTwo(finalHandler))) http.Handle("/", middlewareOne(middlewareTwo(finalHandler))) 控制流说明：
让我们来看一个带有多个中间件的例子，并且把日志输出到控制台：
package main import ( "log" "net/http" ) func middlewareOne(next http.Handler) http.Handler { return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) { log.Println("Executing middlewareOne") next.ServeHTTP(w, r) log.Println("Executing middlewareOne again") }) } func middlewareTwo(next http.Handler) http.Handler { return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) { log.Println("Executing middlewareTwo") if r.URL.Path != "/" { return } next.ServeHTTP(w, r) log.Println("Executing middlewareTwo again") }) } func final(w http.ResponseWriter, r *http.Request) { log.Println("Executing finalHandler") w.Write([]byte("OK")) } func main() { finalHandler := http.HandlerFunc(final) http.Handle("/", middlewareOne(middlewareTwo(finalHandler))) http.ListenAndServe(":3000", nil) } package main import ( "log" "net/http" ) funcmiddlewareOne(nexthttp.Handler) http.Handler { return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) { log.Println("Executing middlewareOne") next.ServeHTTP(w, r) log.Println("Executing middlewareOne again") }) } funcmiddlewareTwo(nexthttp.Handler) http.Handler { return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) { log.Println("Executing middlewareTwo") if r.URL.Path != "/" { return } next.ServeHTTP(w, r) log.Println("Executing middlewareTwo again") }) } funcfinal(w http.ResponseWriter, r *http.Request) { log.Println("Executing finalHandler") w.Write([]byte("OK")) } funcmain() { finalHandler := http.HandlerFunc(final) http.Handle("/", middlewareOne(middlewareTwo(finalHandler))) http.ListenAndServe(":3000", nil) } 然后我们执行 go run main.go 在浏览器打开http://localhost:3000。 你会看到下面的输出。
我们能够很清楚的看到handle的流程控制。我们嵌套他们的返回顺序。我们可以通过中间件中得 return 随时停止handle链的控制。
在上面的代码中我们在 middlewareTwo function包含了retrun 语句。我们在浏览器中打开http://localhost:3000/foo，我们会看到。
2015/12/19 04:21:57 Executing middlewareOne 2015/12/19 04:21:57 Executing middlewareTwo 2015/12/19 04:21:57 Executing middlewareOne again 2015/12/19 04:21:57 Executing middlewareOne 2015/12/19 04:21:57 Executing middlewareTwo 2015/12/19 04:21:57 Executing middlewareOne again
2015/12/19 04:21:57 ExecutingmiddlewareOne 2015/12/19 04:21:57 ExecutingmiddlewareTwo 2015/12/19 04:21:57 ExecutingmiddlewareOneagain 2015/12/19 04:21:57 ExecutingmiddlewareOne 2015/12/19 04:21:57 ExecutingmiddlewareTwo 2015/12/19 04:21:57 ExecutingmiddlewareOneagain
我们实现一个真实的项目的示例：
我们实现一个判断请求是不是XMl的功能，我们要实现一个中间件。用来检查的请求体的存在。检查请求体，以确保它是XML。如果其中检查失败，我希望我们的中间件输出错误信息然后终止我们的handle处理。
package main import ( "bytes" "net/http" ) func enforceXMLHandler(next http.Handler) http.Handler { return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) { // Check for a request body if r.ContentLength == 0 { http.Error(w, http.StatusText(400), 400) return } // Check its MIME type buf := new(bytes.Buffer) buf.ReadFrom(r.Body) if http.DetectContentType(buf.Bytes()) != "text/xml; charset=utf-8" { http.Error(w, http.StatusText(415), 415) return } next.ServeHTTP(w, r) }) } func main() { finalHandler := http.HandlerFunc(final) http.Handle("/", enforceXMLHandler(finalHandler)) http.ListenAndServe(":3000", nil) } func final(w http.ResponseWriter, r *http.Request) { w.Write([]byte("OK")) } package main import ( "bytes" "net/http" ) funcenforceXMLHandler(nexthttp.Handler) http.Handler { return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) { // Check for a request body if r.ContentLength == 0 { http.Error(w, http.StatusText(400), 400) return } // Check its MIME type buf := new(bytes.Buffer) buf.ReadFrom(r.Body) if http.DetectContentType(buf.Bytes()) != "text/xml; charset=utf-8" { http.Error(w, http.StatusText(415), 415) return } next.ServeHTTP(w, r) }) } funcmain() { finalHandler := http.HandlerFunc(final) http.Handle("/", enforceXMLHandler(finalHandler)) http.ListenAndServe(":3000", nil) } funcfinal(w http.ResponseWriter, r *http.Request) { w.Write([]byte("OK")) } 为了检验我们的中间件是否实现了这个功能，我们首先创建一个XML文件。
$ cat > books.xml <?xml version="1.0"?> <books> <book> <author>H. G. Wells</author> <title>The Time Machine</title> <price>8.50</price> </book> </books>
$ cat > books.xml <?xmlversion="1.0"?> <books> <book> <author>H. G. Wells</author> <title>TheTimeMachine</title> <price>8.50</price> </book> </books>
然后通过使用cURL来进行模拟请求：
$ curl -i localhost:3000 HTTP/1.1 400 Bad Request Content-Type: text/plain; charset=utf-8 Content-Length: 12 Bad Request $ curl -i -d "This is not XML" localhost:3000 HTTP/1.1 415 Unsupported Media Type Content-Type: text/plain; charset=utf-8 Content-Length: 23 Unsupported Media Type $ curl -i -d @books.xml localhost:3000 HTTP/1.1 200 OK Date: Fri, 17 Oct 2014 13:42:10 GMT Content-Length: 2 Content-Type: text/plain; charset=utf-8 OK
$ curl -i localhost:3000 HTTP/1.1 400 BadRequest Content-Type: text/plain; charset=utf-8 Content-Length: 12 BadRequest $ curl -i -d "This is not XML" localhost:3000 HTTP/1.1 415 UnsupportedMediaType Content-Type: text/plain; charset=utf-8 Content-Length: 23 UnsupportedMediaType $ curl -i -d @books.xmllocalhost:3000 HTTP/1.1 200 OK Date: Fri, 17 Oct 2014 13:42:10 GMT Content-Length: 2 Content-Type: text/plain; charset=utf-8 OK
接下来给大家介绍一下第三方中间件的使用：
秉承不造轮子的原则，其实在Github上有很多实现了一些功能的中间件。比如这里给大家介绍2个基础验证的中间件 goji/httpauth 和Gorilla’s LoggingHandler
首先我们需要引入第三方包
$ go get github.com/goji/httpauth
$ gogetgithub.com/goji/httpauth
package main import ( "github.com/goji/httpauth" "net/http" ) func main() { finalHandler := http.HandlerFunc(final) authHandler := httpauth.SimpleBasicAuth("username", "password") http.Handle("/", authHandler(finalHandler)) http.ListenAndServe(":3000", nil) } func final(w http.ResponseWriter, r *http.Request) { w.Write([]byte("OK")) } package main import ( "github.com/goji/httpauth" "net/http" ) funcmain() { finalHandler := http.HandlerFunc(final) authHandler := httpauth.SimpleBasicAuth("username", "password") http.Handle("/", authHandler(finalHandler)) http.ListenAndServe(":3000", nil) } funcfinal(w http.ResponseWriter, r *http.Request) { w.Write([]byte("OK")) } 如果你运行这个例子，你应该得到你所期望的有效和无效的凭证响应
$ curl -i username:password@localhost:3000 HTTP/1.1 200 OK Content-Length: 2 Content-Type: text/plain; charset=utf-8 OK $ curl -i username:wrongpassword@localhost:3000 HTTP/1.1 401 Unauthorized Content-Type: text/plain; charset=utf-8 Www-Authenticate: Basic realm=""Restricted"" Content-Length: 13 Unauthorized
$ curl -i username:password@localhost:3000 HTTP/1.1 200 OK Content-Length: 2 Content-Type: text/plain; charset=utf-8 OK $ curl -i username:wrongpassword@localhost:3000 HTTP/1.1 401 Unauthorized Content-Type: text/plain; charset=utf-8 Www-Authenticate: Basicrealm=""Restricted"" Content-Length: 13 Unauthorized
Gorilla’s LoggingHandler和 Apache-style logs 有一些区别
以下是我们在其中写入日志到server.log文件一个简单的例子：
首先还是引入第三包
go get github.com/gorilla/handlers
gogetgithub.com/gorilla/handlers
package main import ( "github.com/gorilla/handlers" "net/http" "os" ) func main() { finalHandler := http.HandlerFunc(final) logFile, err := os.OpenFile("server.log", os.O_WRONLY|os.O_CREATE|os.O_APPEND, 0666) if err != nil { panic(err) } http.Handle("/", handlers.LoggingHandler(logFile, finalHandler)) http.ListenAndServe(":3000", nil) } func final(w http.ResponseWriter, r *http.Request) { w.Write([]byte("OK")) } package main import ( "github.com/gorilla/handlers" "net/http" "os" ) funcmain() { finalHandler := http.HandlerFunc(final) logFile, err := os.OpenFile("server.log", os.O_WRONLY|os.O_CREATE|os.O_APPEND, 0666) if err != nil { panic(err) } http.Handle("/", handlers.LoggingHandler(logFile, finalHandler)) http.ListenAndServe(":3000", nil) } funcfinal(w http.ResponseWriter, r *http.Request) { w.Write([]byte("OK")) } 在一个简单的情况下，这样我们的代码是相当清楚的。但是，如果我们想用LoggingHandler作为一个更大的中间件链中的一部分会发生什么？我们可以很容易地结束了一个声明，看起来像这样：
http.Handle("/", handlers.LoggingHandler(logFile, authHandler(enforceXMLHandler(finalHandler)))) http.Handle("/", handlers.LoggingHandler(logFile, authHandler(enforceXMLHandler(finalHandler)))) 不过这看起来太糟糕了。
我们可以通过创建一个构造函数打来整理一下我们给它取名为(myLoggingHandler)
和signature func(http.Handler) http.Handler .这样就会是我们的代码更加整洁和可读性：
func myLoggingHandler(h http.Handler) http.Handler { logFile, err := os.OpenFile("server.log", os.O_WRONLY|os.O_CREATE|os.O_APPEND, 0666) if err != nil { panic(err) } return handlers.LoggingHandler(logFile, h) } func main() { finalHandler := http.HandlerFunc(final) http.Handle("/", myLoggingHandler(finalHandler)) http.ListenAndServe(":3000", nil) } funcmyLoggingHandler(h http.Handler) http.Handler { logFile, err := os.OpenFile("server.log", os.O_WRONLY|os.O_CREATE|os.O_APPEND, 0666) if err != nil { panic(err) } return handlers.LoggingHandler(logFile, h) } funcmain() { finalHandler := http.HandlerFunc(final) http.Handle("/", myLoggingHandler(finalHandler)) http.ListenAndServe(":3000", nil) } $ cat server.log 127.0.0.1 - - [21/Oct/2014:18:56:43 +0100] "GET / HTTP/1.1" 200 2 127.0.0.1 - - [21/Oct/2014:18:56:36 +0100] "POST / HTTP/1.1" 200 2 127.0.0.1 - - [21/Oct/2014:18:56:43 +0100] "PUT / HTTP/1.1" 200 2
$ catserver.log 127.0.0.1 - - [21/Oct/2014:18:56:43 +0100] "GET / HTTP/1.1" 200 2 127.0.0.1 - - [21/Oct/2014:18:56:36 +0100] "POST / HTTP/1.1" 200 2 127.0.0.1 - - [21/Oct/2014:18:56:43 +0100] "PUT / HTTP/1.1" 200 2
这里还有一个比较完整结构的中间件使用的示例：
package main import ( "bytes" "github.com/goji/httpauth" "github.com/gorilla/handlers" "net/http" "os" ) func enforceXMLHandler(next http.Handler) http.Handler { return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) { if r.ContentLength == 0 { http.Error(w, http.StatusText(400), 400) return } buf := new(bytes.Buffer) buf.ReadFrom(r.Body) if http.DetectContentType(buf.Bytes()) != "text/xml; charset=utf-8" { http.Error(w, http.StatusText(415), 415) return } next.ServeHTTP(w, r) }) } func myLoggingHandler(h http.Handler) http.Handler { logFile, err := os.OpenFile("server.log", os.O_RDWR|os.O_CREATE|os.O_APPEND, 0666) if err != nil { panic(err) } return handlers.LoggingHandler(logFile, h) } func main() { indexHandler := http.HandlerFunc(index) authHandler := httpauth.SimpleBasicAuth("username", "password") http.Handle("/", myLoggingHandler(authHandler(enforceXMLHandler(indexHandler)))) http.ListenAndServe(":3000", nil) } func index(w http.ResponseWriter, r *http.Request) { w.Write([]byte("OK")) } package main import ( "bytes" "github.com/goji/httpauth" "github.com/gorilla/handlers" "net/http" "os" ) funcenforceXMLHandler(nexthttp.Handler) http.Handler { return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) { if r.ContentLength == 0 { http.Error(w, http.StatusText(400), 400) return } buf := new(bytes.Buffer) buf.ReadFrom(r.Body) if http.DetectContentType(buf.Bytes()) != "text/xml; charset=utf-8" { http.Error(w, http.StatusText(415), 415) return } next.ServeHTTP(w, r) }) } funcmyLoggingHandler(h http.Handler) http.Handler { logFile, err := os.OpenFile("server.log", os.O_RDWR|os.O_CREATE|os.O_APPEND, 0666) if err != nil { panic(err) } return handlers.LoggingHandler(logFile, h) } funcmain() { indexHandler := http.HandlerFunc(index) authHandler := httpauth.SimpleBasicAuth("username", "password") http.Handle("/", myLoggingHandler(authHandler(enforceXMLHandler(indexHandler)))) http.ListenAndServe(":3000", nil) } funcindex(w http.ResponseWriter, r *http.Request) { w.Write([]byte("OK")) } 有很多人不太喜欢中间件的设计模式，不过我还是慢喜欢的。
Go语言的Http 中间件实现
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