136 lines
4.6 KiB
Markdown
136 lines
4.6 KiB
Markdown
# 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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