上一篇里讨论了 go 官方库里提供的 http 服务框架,使用者需要关心的是 Server 的 handler 域。当 Server 调用 Serve 函数时 Server.Handler 为 nil,则默认使用 http.DefaultServeMux 作为 handler。
DefaultServeMux
来看一下它的定义和描述:
// ServeMux is an HTTP request multiplexer.
// It matches the URL of each incoming request against a list of registered
// patterns and calls the handler for the pattern that
// most closely matches the URL.
简单的说,它就是一个路由分发器。
路由注册
type ServeMux struct {
mu sync.RWMutex
m map[string]muxEntry
//路由规则,一个string对应一个mux实例对象,map的key就是注册的路由表达式(string类型的)
hosts bool // whether any patterns contain hostnames
}
var defaultServeMux ServeMux
var DefaultServeMux = &defaultServeMux
type muxEntry struct { // 代表着一个 路由-处理函数 组合
explicit bool //表示 patern 是否已经被明确注册过了
h Handler
pattern string //路由表达式
}
之前提到过,Server.Handler 需要有路由功能,并且可以执行路由对应的处理函数。当注册路由时,调用mux.Handle
:
func (mux *ServeMux) Handle(pattern string, handler Handler) {
mux.mu.Lock()
defer mux.mu.Unlock()
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)
}
if mux.m == nil {
mux.m = make(map[string]muxEntry)
}
mux.m[pattern] = muxEntry{explicit: true, h: handler, pattern: pattern}
if pattern[0] != '/' {
mux.hosts = true
}
// 以下是很有用的功能:当pattern == “/tree/”时,
// 会插入一条永久的重定向到“/tree”,注意最后的斜杠。
// 当然前提是在这之前没有“/tree”这条路由
n := len(pattern)
if n > 0 && pattern[n-1] == '/' && !mux.m[pattern[0:n-1]].explicit {
//如果包含host,
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, "/"):]
}
url := &url.URL{Path: path}
mux.m[pattern[0:n-1]] = muxEntry{h: RedirectHandler(url.String(), StatusMovedPermanently), pattern: pattern}
}
}
代码挺多,其实主要就做了一件事,向DefaultServeMux
的map[string]muxEntry
中增加对应的路由规则和handler
。注意这里每条路由并没有包含我们常说的 GET、POST 等等区别,主要有两个原因:一是为了简洁,很多开发者偏好不同的处理方法,官方库只提供最基本的功能;二是不直接和请求方法绑定起来便于写 RESTful API。
但是这里还要注意路径结尾的/
,这时候该路径为一个子树,如果能完全匹配到其子路由,那么也能匹配到这个子树,不过路由越长,优先级越大;如果不能完全匹配到其子路由,会匹配到这个子树的路由。比如有一个根路由/
、/example/
和 /example/1
,那么访问/example/2
时,会匹配到/example/
,访问/nothing
会匹配到/
。
处理路由请求
注册好路由,并且没有使用别的 handler 时,DefaultServerMux 的 ServeHTTP 就会在接收到 request 时被调用。
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)
}
ServeHTTP 主要从之前注册好的路由表中获取对应的 handler:
func (mux *ServeMux) ServeHTTP(w ResponseWriter, r *Request) {
...
h, _ := mux.Handler(r) // 匹配和 request 最接近的路由,拿到对应的 handler
h.ServeHTTP(w, r)
}
func (mux *ServeMux) Handler(r *Request) (h Handler, pattern string) {
...
host := stripHostPort(r.Host)
path := cleanPath(r.URL.Path)
if path != r.URL.Path {
_, pattern = mux.handler(host, path)
url := *r.URL
url.Path = path
return RedirectHandler(url.String(), StatusMovedPermanently), pattern
//注意这里的重定向 handler
}
return mux.handler(host, r.URL.Path)
}
func (mux *ServeMux) handler(host, path string) (h Handler, pattern string) {
mux.mu.RLock()
defer mux.mu.RUnlock()
// Host-specific pattern takes precedence over generic ones
if mux.hosts {
h, pattern = mux.match(host + path)
}
if h == nil {
h, pattern = mux.match(path) // match 做的是字符串匹配的工作
}
if h == nil {
h, pattern = NotFoundHandler(), ""
}
return
}
在没有找到匹配的路由时,返回 NotFoundHandler, 默认只是写入 404 not found,但通常我们会自定义它,然后返回一个专门的好看的 404 页面。
如果需要重定向,则会通过返回的 redirectHandler 调用 Redirect:
func Redirect(w ResponseWriter, r *Request, url string, code int) {
if u, err := parseURL(url); err == nil {
// If url was relative, make absolute by
// combining with request path.
// The browser would probably do this for us,
// but doing it ourselves is more reliable.
// NOTE(rsc): RFC 2616 says that the Location
// line must be an absolute URI, like
// "http://www.google.com/redirect/",
// not a path like "/redirect/".
// Unfortunately, we don't know what to
// put in the host name section to get the
// client to connect to us again, so we can't
// know the right absolute URI to send back.
// Because of this problem, no one pays attention
// to the RFC; they all send back just a new path.
// So do we.
if u.Scheme == "" && u.Host == "" {
oldpath := r.URL.Path
if oldpath == "" { // should not happen, but avoid a crash if it does
oldpath = "/"
}
// no leading http://server
if url == "" || url[0] != '/' {
// make relative path absolute
olddir, _ := path.Split(oldpath)
url = olddir + url
}
var query string
if i := strings.Index(url, "?"); i != -1 {
url, query = url[:i], url[i:]
}
// clean up but preserve trailing slash
trailing := strings.HasSuffix(url, "/")
url = path.Clean(url)
if trailing && !strings.HasSuffix(url, "/") {
url += "/"
}
url += query
}
}
w.Header().Set("Location", hexEscapeNonASCII(url))
w.WriteHeader(code)
// RFC 2616 recommends that a short note "SHOULD" be included in the
// response because older user agents may not understand 301/307.
// Shouldn't send the response for POST or HEAD; that leaves GET.
if r.Method == "GET" {
note := "<a href=\"" + htmlEscape(url) + "\">" + statusText[code] + "</a>.\n"
fmt.Fprintln(w, note)
}
}
可以看到,DefaultServerMux 只有一个最基本的路由功能,是一个最简单的 HTTP 服务框架。可是这通常不能满足我们的需求,于是我们可以根据我们自己的需要自定义一个简单通用的 HTTP Server 框架。
网友评论