http 是典型的 C/S 架构,客户端向服务端发送请求(request),服务端做出应答(response)。
golang 的标准库 net/http 提供了 http 编程有关的接口,封装了内部TCP连接和报文解析的复杂琐碎的细节,使用者只需要和 http.request 和 http.ResponseWriter 两个对象交互就行。也就是说,我们只要写一个 Handler,请求会通过参数传递进来,而它要做的就是根据请求的数据做处理,把结果写到 Response 中。废话不多说,来看看 hello world 程序有多简单吧!
package main
import (
"io"
"net/http"
)
type helloHandler struct{}
func (h *helloHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
w.Write([]byte("Hello, world!"))
}
func main() {
http.Handle("/", &helloHandler{})
http.ListenAndServe(":12345", nil)
}
运行 go run hello_server.go
,我们的服务器就会监听在本地的 12345
端口,对所有的请求都会返回 hello, world!
:
正如上面程序展示的那样,我们只要实现的一个 Handler,它的接口原型是(也就是说只要实现了 ServeHTTP 方法的对象都可以作为 Handler):
type Handler interface {
ServeHTTP(ResponseWriter, *Request)
}
然后,注册到对应的路由路径上就 OK 了。
http.HandleFunc
接受两个参数:第一个参数是字符串表示的 url 路径,第二个参数是该 url 实际的处理对象。
http.ListenAndServe
监听在某个端口,启动服务,准备接受客户端的请求(第二个参数这里设置为 nil,这里也不要纠结什么意思,后面会有讲解)。每次客户端有请求的时候,把请求封装成 http.Request,调用对应的 handler 的 ServeHTTP 方法,然后把操作后的 http.ResponseWriter 解析,返回到客户端。
上面的代码没有什么问题,但是有一个不便:每次写 Handler 的时候,都要定义一个类型,然后编写对应的 ServeHTTP 方法,这个步骤对于所有 Handler 都是一样的。重复的工作总是可以抽象出来,net/http 也正这么做了,它提供了 http.HandleFunc 方法,允许直接把特定类型的函数作为 handler。上面的代码可以改成:
package main
import (
"io"
"net/http"
)
func helloHandler(w http.ResponseWriter, req *http.Request) {
io.WriteString(w, "hello, world!\n")
}
func main() {
http.HandleFunc("/", helloHandler)
http.ListenAndServe(":12345", nil)
}
其实,HandleFunc 只是一个适配器,
// The HandlerFunc type is an adapter to allow the use of
// ordinary functions as HTTP handlers. If f is a function
// with the appropriate signature, HandlerFunc(f) is a
// Handler object that calls f.
type HandlerFunc func(ResponseWriter, *Request)
// ServeHTTP calls f(w, r).
func (f HandlerFunc) ServeHTTP(w ResponseWriter, r *Request) {
f(w, r)
}
自动给 f 函数添加了 HandlerFunc 这个壳,最终调用的还是 ServerHTTP,只不过会直接使用 f(w, r)。这样封装的好处是:使用者可以专注于业务逻辑的编写,省去了很多重复的代码处理逻辑。如果只是简单的 Handler,会直接使用函数;如果是需要传递更多信息或者有复杂的操作,会使用上部分的方法。
如果需要我们自己写的话,是这样的:
package main
import (
"io"
"net/http"
)
func helloHandler(w http.ResponseWriter, req *http.Request) {
io.WriteString(w, "hello, world!\n")
}
func main() {
// 通过 HandlerFunc 把函数转换成 Handler 接口的实现对象
hh := http.HandlerFunc(helloHandler)
http.Handle("/", hh)
http.ListenAndServe(":12345", nil)
}
大部分的服务器逻辑都需要使用者编写对应的 Handler,不过有些 Handler 使用频繁,因此 net/http 提供了它们的实现。比如负责文件 hosting 的 FileServer、负责 404 的NotFoundHandler 和 负责重定向的RedirectHandler。下面这个简单的例子,把当前目录所有文件 host 到服务端:
package main
import (
"net/http"
)
func main() {
http.ListenAndServe(":12345", http.FileServer(http.Dir(".")))
}
强大吧!只要一行逻辑代码就能实现一个简单的静态文件服务器。从这里可以看出一件事:http.ListenAndServe 第二个参数就是一个 Handler 函数(请记住这一点,后面有些内容依赖于这个)。
运行这个程序,在浏览器中打开 http://127.0.0.1:12345,可以看到所有的文件,点击对应的文件还能看到它的内容。
其他两个 Handler,这里就不再举例子了,读者可以自行参考文档。
虽然上面的代码已经工作,并且能实现很多功能,但是实际开发中,HTTP 接口会有许多的 URL 和对应的 Handler。这里就要讲 net/http 的另外一个重要的概念:ServeMux。Mux 是 multiplexor 的缩写,就是多路传输的意思(请求传过来,根据某种判断,分流到后端多个不同的地方)。ServeMux 可以注册多了 URL 和 handler 的对应关系,并自动把请求转发到对应的 handler 进行处理。我们还是来看例子吧:
package main
import (
"io"
"net/http"
)
func helloHandler(w http.ResponseWriter, r *http.Request) {
io.WriteString(w, "Hello, world!\n")
}
func echoHandler(w http.ResponseWriter, r *http.Request) {
io.WriteString(w, r.URL.Path)
}
func main() {
mux := http.NewServeMux()
mux.HandleFunc("/hello", helloHandler)
mux.HandleFunc("/", echoHandler)
http.ListenAndServe(":12345", mux)
}
这个服务器的功能也很简单:如果在请求的 URL 是 /hello,就返回 hello, world!;否则就返回 URL 的路径,路径是从请求对象 http.Requests 中提取的。
这段代码和之前的代码有两点区别:
还记得我们之前说过,http.ListenAndServe 第二个参数应该是 Handler 类型的变量吗?这里为什么能传过来 ServeMux?嗯,估计你也猜到啦:ServeMux 也是是 Handler 接口的实现,也就是说它实现了 ServeHTTP 方法,我们来看一下:
type ServeMux struct {
// contains filtered or unexported fields
}
func NewServeMux() *ServeMux
func (mux *ServeMux) Handle(pattern string, handler Handler)
func (mux *ServeMux) HandleFunc(pattern string, handler func(ResponseWriter, *Request))
func (mux *ServeMux) Handler(r *Request) (h Handler, pattern string)
func (mux *ServeMux) ServeHTTP(w ResponseWriter, r *Request)
哈!果然,这里的方法我们大都很熟悉,除了 Handler() 返回某个请求的 Handler。Handle 和 HandleFunc 这两个方法 net/http 也提供了,后面我们会说明它们之间的关系。而 ServeHTTP 就是 ServeMux 的核心处理逻辑:根据传递过来的 Request,匹配之前注册的 URL 和处理函数,找到最匹配的项,进行处理。可以说 ServeMux 是个特殊的 Handler,它负责路由和调用其他后端 Handler 的处理方法。
关于ServeMux ,有几点要说明:
你可能会有疑问?我们之间为什么没有使用 ServeMux 就能实现路径功能?那是因为 net/http 在后台默认创建使用了 DefaultServeMux。
上面基本覆盖了编写 HTTP 服务端需要的所有内容。这部分就分析一下,它们的源码实现,加深理解,以后遇到疑惑也能通过源码来定位和解决。
Server
首先来看 http.ListenAndServe():
func ListenAndServe(addr string, handler Handler) error {
server := &Server{Addr: addr, Handler: handler}
return server.ListenAndServe()
}
这个函数其实也是一层封装,创建了 Server 结构,并调用它的 ListenAndServe 方法,跟进去看看:
// A Server defines parameters for running an HTTP server.
// The zero value for Server is a valid configuration.
type Server struct {
Addr string // TCP address to listen on, ":http" if empty
Handler Handler // handler to invoke, http.DefaultServeMux if nil
......
}
// ListenAndServe listens on the TCP network address srv.Addr and then
// calls Serve to handle requests on incoming connections. If
// srv.Addr is blank, ":http" is used.
func (srv *Server) ListenAndServe() error {
addr := srv.Addr
if addr == "" {
addr = ":http"
}
ln, err := net.Listen("tcp", addr)
if err != nil {
return err
}
return srv.Serve(tcpKeepAliveListener{ln.(*net.TCPListener)})
}
Server 保存了运行 HTTP 服务需要的参数,调用 net.Listen 监听在对应的 tcp 端口,tcpKeepAliveListener 设置了 TCP 的 KeepAlive 功能,最后调用 srv.Serve()方法开始真正的循环逻辑。再跟进去看 Serve 方法:
// Serve accepts incoming connections on the Listener l, creating a
// new service goroutine for each. The service goroutines read requests and
// then call srv.Handler to reply to them.
func (srv *Server) Serve(l net.Listener) error {
defer l.Close()
var tempDelay time.Duration // how long to sleep on accept failure
// 循环逻辑,接受请求并处理
for {
// 有新的连接
rw, e := l.Accept()
if e != nil {
if ne, ok := e.(net.Error); ok && ne.Temporary() {
if tempDelay == 0 {
tempDelay = 5 * time.Millisecond
} else {
tempDelay *= 2
}
if max := 1 * time.Second; tempDelay > max {
tempDelay = max
}
srv.logf("http: Accept error: %v; retrying in %v", e, tempDelay)
time.Sleep(tempDelay)
continue
}
return e
}
tempDelay = 0
// 创建 Conn 连接
c, err := srv.newConn(rw)
if err != nil {
continue
}
c.setState(c.rwc, StateNew) // before Serve can return
// 启动新的 goroutine 进行处理
go c.serve()
}
}
最上面的注释也说明了这个方法的主要功能:
func (c *conn) serve() {
origConn := c.rwc // copy it before it's set nil on Close or Hijack
...
for {
w, err := c.readRequest()
if c.lr.N != c.server.initialLimitedReaderSize() {
// If we read any bytes off the wire, we're active.
c.setState(c.rwc, StateActive)
}
...
// HTTP cannot have multiple simultaneous active requests.[*]
// Until the server replies to this request, it can't read another,
// so we might as well run the handler in this goroutine.
// [*] Not strictly true: HTTP pipelining. We could let them all process
// in parallel even if their responses need to be serialized.
serverHandler{c.server}.ServeHTTP(w, w.req)
w.finishRequest()
if w.closeAfterReply {
if w.requestBodyLimitHit {
c.closeWriteAndWait()
}
break
}
c.setState(c.rwc, StateIdle)
}
}
看到上面这段代码 serverHandler{c.server}.ServeHTTP(w, w.req)这一句了吗?它会调用最早传递给 Server 的 Handler 函数:
func (sh serverHandler) ServeHTTP(rw ResponseWriter, req *Request) {
handler := sh.srv.Handler
if handler == nil {
handler = DefaultServeMux
}
if req.RequestURI == "*" && req.Method == "OPTIONS" {
handler = globalOptionsHandler{}
}
handler.ServeHTTP(rw, req)
}
看到 DefaultServeMux 了吗?如果没有 handler 为空,就会使用它。handler.ServeHTTP(rw, req),Handler 接口都要实现 ServeHTTP 这个方法,因为这里就要被调用啦。
也就是说,无论如何,最终都会用到 ServeMux,也就是负责 URL 路由的家伙。前面也已经说过,它的 ServeHTTP 方法就是根据请求的路径,把它转交给注册的 handler 进行处理。这次,我们就在源码层面一探究竟。
ServeMux
ServeMux 会以某种方式保存 URL 和 Handlers 的对应关系,下面我们就从代码层面来解开原因:
type ServeMux struct {
mu sync.RWMutex
m map[string]muxEntry // 存放路由信息的字典!\(^o^)/
hosts bool // whether any patterns contain hostnames
}
type muxEntry struct {
explicit bool
h Handler
pattern string
}
没错,数据结构也比较直观,和我们想象的差不多,路由信息保存在字典中,接下来就看看几个重要的操作:路由信息是怎么注册的?ServeHTTP 方法到底是怎么做的?路由查找过程是怎样的?
// Handle registers the handler for the given pattern.
// If a handler already exists for pattern, Handle panics.
func (mux *ServeMux) Handle(pattern string, handler Handler) {
mux.mu.Lock()
defer mux.mu.Unlock()
// 边界情况处理
if pattern == "" {
panic("http: invalid pattern " + pattern)
}
if handler == nil {
panic("http: nil handler")
}
if mux.m[pattern].explicit {
panic("http: multiple registrations for " + pattern)
}
// 创建 `muxEntry` 并添加到路由字典中
mux.m[pattern] = muxEntry{explicit: true, h: handler, pattern: pattern}
if pattern[0] != '/' {
mux.hosts = true
}
// 这是一个很有用的小技巧,如果注册了 `/tree/`, `serveMux` 会自动添加一个 `/tree` 的路径并重定向到 `/tree/`。当然这个 `/tree` 路径会被用户显示的路由信息覆盖。
// Helpful behavior:
// If pattern is /tree/, insert an implicit permanent redirect for /tree.
// It can be overridden by an explicit registration.
n := len(pattern)
if n > 0 && pattern[n-1] == '/' && !mux.m[pattern[0:n-1]].explicit {
// If pattern contains a host name, strip it and use remaining
// path for redirect.
path := pattern
if pattern[0] != '/' {
// In pattern, at least the last character is a '/', so
// strings.Index can't be -1.
path = pattern[strings.Index(pattern, "/"):]
}
mux.m[pattern[0:n-1]] = muxEntry{h: RedirectHandler(path, StatusMovedPermanently), pattern: pattern}
}
}
路由注册没有什么特殊的地方,很简单,也符合我们的预期,注意最后一段代码对类似 /tree URL 重定向的处理。
// ServeHTTP dispatches the request to the handler whose
// pattern most closely matches the request URL.
func (mux *ServeMux) ServeHTTP(w ResponseWriter, r *Request) {
if r.RequestURI == "*" {
if r.ProtoAtLeast(1, 1) {
w.Header().Set("Connection", "close")
}
w.WriteHeader(StatusBadRequest)
return
}
h, _ := mux.Handler(r)
h.ServeHTTP(w, r)
}
好吧,ServeHTTP 也只是通过 mux.Handler(r) 找到请求对应的 handler,调用它的 ServeHTTP 方法,代码比较简单我们就显示了,它最终会调用 mux.match() 方法,我们来看一下它的实现:
// Does path match pattern?
func pathMatch(pattern, path string) bool {
if len(pattern) == 0 {
// should not happen
return false
}
n := len(pattern)
if pattern[n-1] != '/' {
return pattern == path
}
// 匹配的逻辑很简单,path 前面的字符和 pattern 一样就是匹配
return len(path) >= n && path[0:n] == pattern
}
// Find a handler on a handler map given a path string
// Most-specific (longest) pattern wins
func (mux *ServeMux) match(path string) (h Handler, pattern string) {
var n = 0
for k, v := range mux.m {
if !pathMatch(k, path) {
continue
}
// 最长匹配的逻辑在这里
if h == nil || len(k) > n {
n = len(k)
h = v.h
pattern = v.pattern
}
}
return
}
match 会遍历路由信息字典,找到所有匹配该路径最长的那个。路由部分的代码解释就到这里了,最后回答上面的一个问题:http.HandleFunc 和 ServeMux.HandlerFunc 是什么关系?
// Handle registers the handler for the given pattern
// in the DefaultServeMux.
// The documentation for ServeMux explains how patterns are matched.
func Handle(pattern string, handler Handler) { DefaultServeMux.Handle(pattern, handler) }
// HandleFunc registers the handler function for the given pattern
// in the DefaultServeMux.
// The documentation for ServeMux explains how patterns are matched.
func HandleFunc(pattern string, handler func(ResponseWriter, *Request)) {
DefaultServeMux.HandleFunc(pattern, handler)
}
原来是直接通过 DefaultServeMux 调用对应的方法,到这里上面的一切都串起来了!
Request
最后一部分,要讲讲 Handler 函数接受的两个参数:http.Request 和 http.ResponseWriter。
Request 就是封装好的客户端请求,包括 URL,method,header 等等所有信息,以及一些方便使用的方法:
// A Request represents an HTTP request received by a server
// or to be sent by a client.
//
// The field semantics differ slightly between client and server
// usage. In addition to the notes on the fields below, see the
// documentation for Request.Write and RoundTripper.
type Request struct {
// Method specifies the HTTP method (GET, POST, PUT, etc.).
// For client requests an empty string means GET.
Method string
// URL specifies either the URI being requested (for server
// requests) or the URL to access (for client requests).
//
// For server requests the URL is parsed from the URI
// supplied on the Request-Line as stored in RequestURI. For
// most requests, fields other than Path and RawQuery will be
// empty. (See RFC 2616, Section 5.1.2)
//
// For client requests, the URL's Host specifies the server to
// connect to, while the Request's Host field optionally
// specifies the Host header value to send in the HTTP
// request.
URL *url.URL
// The protocol version for incoming requests.
// Client requests always use HTTP/1.1.
Proto string // "HTTP/1.0"
ProtoMajor int // 1
ProtoMinor int // 0
// A header maps request lines to their values.
// If the header says
//
// accept-encoding: gzip, deflate
// Accept-Language: en-us
// Connection: keep-alive
//
// then
//
// Header = map[string][]string{
// "Accept-Encoding": {"gzip, deflate"},
// "Accept-Language": {"en-us"},
// "Connection": {"keep-alive"},
// }
//
// HTTP defines that header names are case-insensitive.
// The request parser implements this by canonicalizing the
// name, making the first character and any characters
// following a hyphen uppercase and the rest lowercase.
//
// For client requests certain headers are automatically
// added and may override values in Header.
//
// See the documentation for the Request.Write method.
Header Header
// Body is the request's body.
//
// For client requests a nil body means the request has no
// body, such as a GET request. The HTTP Client's Transport
// is responsible for calling the Close method.
//
// For server requests the Request Body is always non-nil
// but will return EOF immediately when no body is present.
// The Server will close the request body. The ServeHTTP
// Handler does not need to.
Body io.ReadCloser
// ContentLength records the length of the associated content.
// The value -1 indicates that the length is unknown.
// Values >= 0 indicate that the given number of bytes may
// be read from Body.
// For client requests, a value of 0 means unknown if Body is not nil.
ContentLength int64
// TransferEncoding lists the transfer encodings from outermost to
// innermost. An empty list denotes the "identity" encoding.
// TransferEncoding can usually be ignored; chunked encoding is
// automatically added and removed as necessary when sending and
// receiving requests.
TransferEncoding []string
// Close indicates whether to close the connection after
// replying to this request (for servers) or after sending
// the request (for clients).
Close bool
// For server requests Host specifies the host on which the
// URL is sought. Per RFC 2616, this is either the value of
// the "Host" header or the host name given in the URL itself.
// It may be of the form "host:port".
//
// For client requests Host optionally overrides the Host
// header to send. If empty, the Request.Write method uses
// the value of URL.Host.
Host string
// Form contains the parsed form data, including both the URL
// field's query parameters and the POST or PUT form data.
// This field is only available after ParseForm is called.
// The HTTP client ignores Form and uses Body instead.
Form url.Values
// PostForm contains the parsed form data from POST or PUT
// body parameters.
// This field is only available after ParseForm is called.
// The HTTP client ignores PostForm and uses Body instead.
PostForm url.Values
// MultipartForm is the parsed multipart form, including file uploads.
// This field is only available after ParseMultipartForm is called.
// The HTTP client ignores MultipartForm and uses Body instead.
MultipartForm *multipart.Form
...
// RemoteAddr allows HTTP servers and other software to record
// the network address that sent the request, usually for
// logging. This field is not filled in by ReadRequest and
// has no defined format. The HTTP server in this package
// sets RemoteAddr to an "IP:port" address before invoking a
// handler.
// This field is ignored by the HTTP client.
RemoteAddr string
...
}
Handler 需要知道关于请求的任何信息,都要从这个对象中获取,一般不会直接修改这个对象(除非你非常清楚自己在做什么)!
ResponseWriter
ResponseWriter 是一个接口,定义了三个方法:
// A ResponseWriter interface is used by an HTTP handler to
// construct an HTTP response.
type ResponseWriter interface {
// Header returns the header map that will be sent by WriteHeader.
// Changing the header after a call to WriteHeader (or Write) has
// no effect.
Header() Header
// Write writes the data to the connection as part of an HTTP reply.
// If WriteHeader has not yet been called, Write calls WriteHeader(http.StatusOK)
// before writing the data. If the Header does not contain a
// Content-Type line, Write adds a Content-Type set to the result of passing
// the initial 512 bytes of written data to DetectContentType.
Write([]byte) (int, error)
// WriteHeader sends an HTTP response header with status code.
// If WriteHeader is not called explicitly, the first call to Write
// will trigger an implicit WriteHeader(http.StatusOK).
// Thus explicit calls to WriteHeader are mainly used to
// send error codes.
WriteHeader(int)
}
实际上传递给 Handler 的对象是:
// A response represents the server side of an HTTP response.
type response struct {
conn *conn
req *Request // request for this response
wroteHeader bool // reply header has been (logically) written
wroteContinue bool // 100 Continue response was written
w *bufio.Writer // buffers output in chunks to chunkWriter
cw chunkWriter
sw *switchWriter // of the bufio.Writer, for return to putBufioWriter
// handlerHeader is the Header that Handlers get access to,
// which may be retained and mutated even after WriteHeader.
// handlerHeader is copied into cw.header at WriteHeader
// time, and privately mutated thereafter.
handlerHeader Header
...
status int // status code passed to WriteHeader
...
}
它当然实现了上面提到的三个方法,具体代码就不放到这里了,感兴趣的可以自己去看。
虽然 net/http 提供的各种功能已经满足基本需求了,但是很多时候还不够方便,比如:
alice
alice 的功能很简单——把多个 handler 串联起来,有请求过来的时候,逐个通过这个 handler 进行处理。
alice.New(Middleware1, Middleware2, Middleware3).Then(App)
Gorilla Mux
Gorilla 提供了很多网络有关的组件, Mux 就是其中一个,负责 HTTP 的路由功能。这个组件弥补了上面提到的 ServeMux 的一些缺陷,支持的功能有:
r := mux.NewRouter()
r.HandleFunc("/products/{key}", ProductHandler)
r.HandleFunc("/articles/{category}/", ArticlesCategoryHandler)
r.HandleFunc("/articles/{category}/{id:[0-9]+}", ArticleHandler)
httprouter
httprouter 和 mux 一样,也是扩展了自带 ServeMux 功能的路由库。它的主要特点是速度快、内存使用少、可扩展性高(使用 radix tree 数据结构进行路由匹配,路由项很多的时候速度也很快)。
package main
import (
"fmt"
"github.com/julienschmidt/httprouter"
"net/http"
"log"
)
func Index(w http.ResponseWriter, r *http.Request, _ httprouter.Params) {
fmt.Fprint(w, "Welcome!\n")
}
func Hello(w http.ResponseWriter, r *http.Request, ps httprouter.Params) {
fmt.Fprintf(w, "hello, %s!\n", ps.ByName("name"))
}
func main() {
router := httprouter.New()
router.GET("/", Index)
router.GET("/hello/:name", Hello)
log.Fatal(http.ListenAndServe(":8080", router))
}
negroni
http middleware 库,支持嵌套的中间件,能够和其他路由库兼容。同时它也自带了不少 middleware 可以使用,比如Recovery、Logger、Static。
router := mux.NewRouter()
router.HandleFunc("/", HomeHandler)
n := negroni.New(Middleware1, Middleware2)
// Or use a middleware with the Use() function
n.Use(Middleware3)
// router goes last
n.UseHandler(router)
http.ListenAndServe(":3001", n)