Merging other languages
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James Miranda
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en/03.4.md
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en/03.4.md
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# 3.4 Go的http包详解
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前面小节介绍了Go怎么样实现了Web工作模式的一个流程，这一小节，我们将详细地解剖一下http包，看它到底是怎样实现整个过程的。
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Go的http有两个核心功能：Conn、ServeMux
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## Conn的goroutine
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与我们一般编写的http服务器不同, Go为了实现高并发和高性能, 使用了goroutines来处理Conn的读写事件, 这样每个请求都能保持独立，相互不会阻塞，可以高效的响应网络事件。这是Go高效的保证。
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Go在等待客户端请求里面是这样写的：
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c, err := srv.newConn(rw)
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if err != nil {
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continue
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}
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go c.serve()
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这里我们可以看到客户端的每次请求都会创建一个Conn，这个Conn里面保存了该次请求的信息，然后再传递到对应的handler，该handler中便可以读取到相应的header信息，这样保证了每个请求的独立性。
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## ServeMux的自定义
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我们前面小节讲述conn.server的时候，其实内部是调用了http包默认的路由器，通过路由器把本次请求的信息传递到了后端的处理函数。那么这个路由器是怎么实现的呢？
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它的结构如下：
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type ServeMux struct {
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mu sync.RWMutex //锁，由于请求涉及到并发处理，因此这里需要一个锁机制
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m map[string]muxEntry // 路由规则，一个string对应一个mux实体，这里的string就是注册的路由表达式
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hosts bool // 是否在任意的规则中带有host信息
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}
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下面看一下muxEntry
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type muxEntry struct {
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explicit bool // 是否精确匹配
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h Handler // 这个路由表达式对应哪个handler
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pattern string //匹配字符串
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}
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接着看一下Handler的定义
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type Handler interface {
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ServeHTTP(ResponseWriter, *Request) // 路由实现器
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}
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Handler是一个接口，但是前一小节中的`sayhelloName`函数并没有实现ServeHTTP这个接口，为什么能添加呢？原来在http包里面还定义了一个类型`HandlerFunc`,我们定义的函数`sayhelloName`就是这个HandlerFunc调用之后的结果，这个类型默认就实现了ServeHTTP这个接口，即我们调用了HandlerFunc(f),强制类型转换f成为HandlerFunc类型，这样f就拥有了ServeHTTP方法。
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type HandlerFunc func(ResponseWriter, *Request)
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// ServeHTTP calls f(w, r).
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func (f HandlerFunc) ServeHTTP(w ResponseWriter, r *Request) {
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f(w, r)
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}
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路由器里面存储好了相应的路由规则之后，那么具体的请求又是怎么分发的呢？请看下面的代码，默认的路由器实现了`ServeHTTP`：
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func (mux *ServeMux) ServeHTTP(w ResponseWriter, r *Request) {
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if r.RequestURI == "*" {
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w.Header().Set("Connection", "close")
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w.WriteHeader(StatusBadRequest)
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return
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}
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h, _ := mux.Handler(r)
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h.ServeHTTP(w, r)
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}
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如上所示路由器接收到请求之后，如果是`*`那么关闭链接，不然调用`mux.Handler(r)`返回对应设置路由的处理Handler，然后执行`h.ServeHTTP(w, r)`
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也就是调用对应路由的handler的ServerHTTP接口，那么mux.Handler(r)怎么处理的呢？
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func (mux *ServeMux) Handler(r *Request) (h Handler, pattern string) {
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if r.Method != "CONNECT" {
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if p := cleanPath(r.URL.Path); p != r.URL.Path {
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_, pattern = mux.handler(r.Host, p)
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return RedirectHandler(p, StatusMovedPermanently), pattern
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}
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}
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return mux.handler(r.Host, r.URL.Path)
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}
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func (mux *ServeMux) handler(host, path string) (h Handler, pattern string) {
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mux.mu.RLock()
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defer mux.mu.RUnlock()
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// Host-specific pattern takes precedence over generic ones
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if mux.hosts {
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h, pattern = mux.match(host + path)
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}
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if h == nil {
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h, pattern = mux.match(path)
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}
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if h == nil {
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h, pattern = NotFoundHandler(), ""
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}
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return
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}
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原来他是根据用户请求的URL和路由器里面存储的map去匹配的，当匹配到之后返回存储的handler，调用这个handler的ServeHTTP接口就可以执行到相应的函数了。
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通过上面这个介绍，我们了解了整个路由过程，Go其实支持外部实现的路由器 `ListenAndServe`的第二个参数就是用以配置外部路由器的，它是一个Handler接口，即外部路由器只要实现了Handler接口就可以,我们可以在自己实现的路由器的ServeHTTP里面实现自定义路由功能。
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如下代码所示，我们自己实现了一个简易的路由器
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package main
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import (
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"fmt"
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"net/http"
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)
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type MyMux struct {
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}
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func (p *MyMux) ServeHTTP(w http.ResponseWriter, r *http.Request) {
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if r.URL.Path == "/" {
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sayhelloName(w, r)
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return
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}
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http.NotFound(w, r)
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return
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}
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func sayhelloName(w http.ResponseWriter, r *http.Request) {
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fmt.Fprintf(w, "Hello myroute!")
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}
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func main() {
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mux := &MyMux{}
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http.ListenAndServe(":9090", mux)
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}
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## Go代码的执行流程
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通过对http包的分析之后，现在让我们来梳理一下整个的代码执行过程。
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- 首先调用Http.HandleFunc
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按顺序做了几件事：
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1 调用了DefaultServeMux的HandleFunc
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2 调用了DefaultServeMux的Handle
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3 往DefaultServeMux的map[string]muxEntry中增加对应的handler和路由规则
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- 其次调用http.ListenAndServe(":9090", nil)
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按顺序做了几件事情：
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1 实例化Server
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2 调用Server的ListenAndServe()
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3 调用net.Listen("tcp", addr)监听端口
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4 启动一个for循环，在循环体中Accept请求
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5 对每个请求实例化一个Conn，并且开启一个goroutine为这个请求进行服务go c.serve()
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6 读取每个请求的内容w, err := c.readRequest()
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7 判断handler是否为空，如果没有设置handler（这个例子就没有设置handler），handler就设置为DefaultServeMux
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8 调用handler的ServeHttp
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9 在这个例子中，下面就进入到DefaultServeMux.ServeHttp
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10 根据request选择handler，并且进入到这个handler的ServeHTTP
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mux.handler(r).ServeHTTP(w, r)
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11 选择handler：
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A 判断是否有路由能满足这个request（循环遍历ServerMux的muxEntry）
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B 如果有路由满足，调用这个路由handler的ServeHttp
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C 如果没有路由满足，调用NotFoundHandler的ServeHttp
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## links
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* [目录](<preface.md>)
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* 上一节: [Go如何使得web工作](<03.3.md>)
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* 下一节: [小结](<03.5.md>)
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# 3.4 Get into http package
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In previous sections, we learned about the work flow of the web and talked a little bit about Go's `http` package. In this section, we are going to learn about two core functions in the `http` package: Conn and ServeMux.
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## goroutine in Conn
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Unlike normal HTTP servers, Go uses goroutines for every job initiated by Conn in order to achieve high concurrency and performance, so every job is independent.
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Go uses the following code to wait for new connections from clients.
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c, err := srv.newConn(rw)
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if err != nil {
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continue
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}
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go c.serve()
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As you can see, it creates a new goroutine for every connection, and passes the handler that is able to read data from the request to the goroutine.
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## Customized ServeMux
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We used Go's default router in previous sections when discussing conn.server, with the router passing request data to a back-end handler.
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The struct of the default router:
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type ServeMux struct {
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mu sync.RWMutex // because of concurrency, we have to use a mutex here
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m map[string]muxEntry // router rules, every string mapping to a handler
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}
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The struct of muxEntry:
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type muxEntry struct {
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explicit bool // exact match or not
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h Handler
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}
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The interface of Handler:
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type Handler interface {
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ServeHTTP(ResponseWriter, *Request) // routing implementer
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}
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`Handler` is an interface, but if the function `sayhelloName` didn't implement this interface, then how did we add it as handler? The answer lies in another type called `HandlerFunc` in the `http` package. We called `HandlerFunc` to define our `sayhelloName` method, so `sayhelloName` implemented `Handler` at the same time. It's like we're calling `HandlerFunc(f)`, and the function `f` is force converted to type `HandlerFunc`.
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type HandlerFunc func(ResponseWriter, *Request)
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// ServeHTTP calls f(w, r).
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func (f HandlerFunc) ServeHTTP(w ResponseWriter, r *Request) {
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f(w, r)
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}
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How does the router call handlers after we set the router rules?
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The router calls `mux.handler.ServeHTTP(w, r)` when it receives requests. In other words, it calls the `ServeHTTP` interface of the handlers which have implemented it.
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Now, let's see how `mux.handler` works.
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func (mux *ServeMux) handler(r *Request) Handler {
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mux.mu.RLock()
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defer mux.mu.RUnlock()
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// Host-specific pattern takes precedence over generic ones
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h := mux.match(r.Host + r.URL.Path)
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if h == nil {
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h = mux.match(r.URL.Path)
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}
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if h == nil {
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h = NotFoundHandler()
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}
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return h
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}
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The router uses the request's URL as a key to find the corresponding handler saved in the map, then calls handler.ServeHTTP to execute functions to handle the data.
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You should understand the default router's work flow by now, and Go actually supports customized routers. The second argument of `ListenAndServe` is for configuring customized routers. It's an interface of `Handler`. Therefore, any router that implements the `Handler` interface can be used.
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The following example shows how to implement a simple router.
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package main
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import (
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"fmt"
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"net/http"
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)
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type MyMux struct {
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}
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func (p *MyMux) ServeHTTP(w http.ResponseWriter, r *http.Request) {
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if r.URL.Path == "/" {
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sayhelloName(w, r)
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return
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}
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http.NotFound(w, r)
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return
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}
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func sayhelloName(w http.ResponseWriter, r *http.Request) {
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fmt.Fprintf(w, "Hello myroute!")
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}
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func main() {
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mux := &MyMux{}
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http.ListenAndServe(":9090", mux)
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}
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## Go code execution flow
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Let's take a look at the whole execution flow.
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- Call `http.HandleFunc`
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1. Call HandleFunc of DefaultServeMux
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2. Call Handle of DefaultServeMux
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3. Add router rules to map[string]muxEntry of DefaultServeMux
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- Call `http.ListenAndServe(":9090", nil)`
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1. Instantiate Server
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2. Call ListenAndServe method of Server
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3. Call net.Listen("tcp", addr) to listen to port
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4. Start a loop and accept requests in the loop body
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5. Instantiate a Conn and start a goroutine for every request: `go c.serve()`
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6. Read request data: `w, err := c.readRequest()`
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7. Check whether handler is empty or not, if it's empty then use DefaultServeMux
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8. Call ServeHTTP of handler
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9. Execute code in DefaultServeMux in this case
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10. Choose handler by URL and execute code in that handler function: `mux.handler.ServeHTTP(w, r)`
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11. How to choose handler:
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A. Check router rules for this URL
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B. Call ServeHTTP in that handler if there is one
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C. Call ServeHTTP of NotFoundHandler otherwise
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## Links
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- [Directory](preface.md)
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- Previous section: [How Go works with web](03.3.md)
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- Next section: [Summary](03.5.md)
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