httprouter 源码刨析

作者：王博

2021 年 12 月 29 日
本文字数：3989 字
阅读完需：约 13 分钟

首先第一个问题，我们为什么需要 httprouter？

也就是为什么需要所谓的 web 框架，如果没有使用 httprouter 而是使用的官方的框架会有什么问题？

其实看过 go 官方提供的 http 代码的同学都知道，go 官方提供了DefaultServeMux的路由规则，但是其路由规则先对简单，同时官方大大也提供了接口的方式允许我们自己定制路由规则

//只需要实现这个接口，就可以定制自己的路由规则，将实现通过ListenAndServe传入进来，type Handler interface {	ServeHTTP(ResponseWriter, *Request)}func ListenAndServe(addr string, handler Handler) error {	server := &Server{Addr: addr, Handler: handler}	return server.ListenAndServe()}
func (sh serverHandler) ServeHTTP(rw ResponseWriter, req *Request) {	handler := sh.srv.Handler//如果有定制，使用定制的，否则使用默认的	if handler == nil {		handler = DefaultServeMux	}	if req.RequestURI == "*" && req.Method == "OPTIONS" {		handler = globalOptionsHandler{}	}	handler.ServeHTTP(rw, req)}

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当然，在 httprouter 里面也有一句很有意思的代码，就是保证自己定义的 Router 实现了 Handler 接口

var _ http.Handler = New()func New() *Router {	return &Router{		RedirectTrailingSlash:  true,		RedirectFixedPath:      true,		HandleMethodNotAllowed: true,		HandleOPTIONS:          true,	}}

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从上面可以看到核心代码就是 ServeHTTP 方法，但是在看这个方法之前，我们先看一下 Router 的数据结构定义

type Router struct {	trees map[string]*node
	paramsPool sync.Pool	maxParams  uint16
	// If enabled, adds the matched route path onto the http.Request context	// before invoking the handler.	// The matched route path is only added to handlers of routes that were	// registered when this option was enabled.  // 如果是true，会把匹配到的path在调用handler之前加到context里  // 只有当这个值是true的时候，匹配到的path才会被加到handler的routes里面	SaveMatchedRoutePath bool
	// Enables automatic redirection if the current route can't be matched but a	// handler for the path with (without) the trailing slash exists.	// For example if /foo/ is requested but a route only exists for /foo, the	// client is redirected to /foo with http status code 301 for GET requests	// and 308 for all other request methods.  // 作用是如果后面多了个下划线，也可以匹配的到，但是对于GET会返回301，其他的会返回308	RedirectTrailingSlash bool
	// If enabled, the router tries to fix the current request path, if no	// handle is registered for it.	// First superfluous path elements like ../ or // are removed.	// Afterwards the router does a case-insensitive lookup of the cleaned path.	// If a handle can be found for this route, the router makes a redirection	// to the corrected path with status code 301 for GET requests and 308 for	// all other request methods.	// For example /FOO and /..//Foo could be redirected to /foo.	// RedirectTrailingSlash is independent of this option.  // 如果匹配不上，尝试修复path，比如删除/ ../之类的，但是对于GET会返回301，其他的会返回308	RedirectFixedPath bool
	// If enabled, the router checks if another method is allowed for the	// current route, if the current request can not be routed.	// If this is the case, the request is answered with 'Method Not Allowed'	// and HTTP status code 405.	// If no other Method is allowed, the request is delegated to the NotFound	// handler.  // 如果没有找到对应的http方法，也会调用某个方法	HandleMethodNotAllowed bool
	// If enabled, the router automatically replies to OPTIONS requests.	// Custom OPTIONS handlers take priority over automatic replies.  // 如果是true，那么router自动回复OPTIONS的request  // 用户自定义的比上面提到的自动回复的函数优先级更高	HandleOPTIONS bool
	// An optional http.Handler that is called on automatic OPTIONS requests.	// The handler is only called if HandleOPTIONS is true and no OPTIONS	// handler for the specific path was set.	// The "Allowed" header is set before calling the handler.	GlobalOPTIONS http.Handler
	// Cached value of global (*) allowed methods	globalAllowed string
	// Configurable http.Handler which is called when no matching route is	// found. If it is not set, http.NotFound is used.  // 路由没有匹配上时调用这个 handler 。  // 如果没有定义这个 handler ，就会返回标准库中的 http.NotFound 。	NotFound http.Handler
	// Configurable http.Handler which is called when a request	// cannot be routed and HandleMethodNotAllowed is true.	// If it is not set, http.Error with http.StatusMethodNotAllowed is used.	// The "Allow" header with allowed request methods is set before the handler	// is called.  // 当一个请求是不被允许的，并且上面的 HandleMethodNotAllowed 设置为 ture 的时候，  // 如果这个参数没有设置，将使用状态为 with http.StatusMethodNotAllowed 的 http.Error  // 在 handler 被调用以前，为允许请求的方法设置 "Allow" header	MethodNotAllowed http.Handler
	// Function to handle panics recovered from http handlers.	// It should be used to generate a error page and return the http error code	// 500 (Internal Server Error).	// The handler can be used to keep your server from crashing because of	// unrecovered panics.  // panic recover的，相当于其他框架的recover middleware	PanicHandler func(http.ResponseWriter, *http.Request, interface{})}

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下面我们来分析一下 ServeHTTP 做了哪些事

func (r *Router) ServeHTTP(w http.ResponseWriter, req *http.Request) {	// panic处理模块，相当于一个recover middleware  if r.PanicHandler != nil {		defer r.recv(w, req)	}	//获取path	path := req.URL.Path	//根据HTTP方法找到节点（httprouter的路由存储结构为一颗Radix树，根节点连接不同的请求方法，  //每个方法的子节点负责存储自己的路径）	if root := r.trees[req.Method]; root != nil {    // 如果路由匹配上了，从基数树中将路由取出		if handle, ps, tsr := root.getValue(path, r.getParams); handle != nil {			if ps != nil {        //调用处理函数				handle(w, req, *ps)				r.putParams(ps)			} else {				handle(w, req, nil)			}			return		} else if req.Method != http.MethodConnect && path != "/" {      //如果路由没有匹配上，请求方法不是CONNECT并且路径不是/      //在前面提到过，Get方法会返回301状态码，其他的会返回308			// Moved Permanently, request with GET method			code := http.StatusMovedPermanently			if req.Method != http.MethodGet {				// Permanent Redirect, request with same method				code = http.StatusPermanentRedirect			}
      // 尝试删除末尾的/，然后进行请求转发      // tsr 返回值是一个 bool 值，用来判断是否需要重定向, getValue 返回来的       // RedirectTrailingSlash我们上面也有提到过			if tsr && r.RedirectTrailingSlash {				if len(path) > 1 && path[len(path)-1] == '/' {					req.URL.Path = path[:len(path)-1]				} else {					req.URL.Path = path + "/"				}				http.Redirect(w, req, req.URL.String(), code)				return			}
			// Try to fix the request path      // RedirectFixedPath同样上面提到过，这里尝试做路径fix，然后进行请求转发			if r.RedirectFixedPath {				fixedPath, found := root.findCaseInsensitivePath(					CleanPath(path),					r.RedirectTrailingSlash,				)				if found {					req.URL.Path = fixedPath					http.Redirect(w, req, req.URL.String(), code)					return				}			}		}	}
  // 如果没有任何路由匹配，请求方式又是 OPTIONS， 并且允许响应 OPTIONS  // 就会给 Header 设置一个 Allow 头，返回去	if req.Method == http.MethodOptions && r.HandleOPTIONS {		// Handle OPTIONS requests		if allow := r.allowed(path, http.MethodOptions); allow != "" {			w.Header().Set("Allow", allow)			if r.GlobalOPTIONS != nil {				r.GlobalOPTIONS.ServeHTTP(w, req)			}			return		}	} else if r.HandleMethodNotAllowed { // Handle 405    //如果自定义了MethodNotAllowed方法，则会调用自定义的，否则会返回error		if allow := r.allowed(path, req.Method); allow != "" {			w.Header().Set("Allow", allow)			if r.MethodNotAllowed != nil {				r.MethodNotAllowed.ServeHTTP(w, req)			} else {				http.Error(w,					http.StatusText(http.StatusMethodNotAllowed),					http.StatusMethodNotAllowed,				)			}			return		}	}
	// Handle 404  // 如果自定义了NotFound，则会调用自定义的方法，否则会返回http.NotFound	if r.NotFound != nil {		r.NotFound.ServeHTTP(w, req)	} else {		http.NotFound(w, req)	}}