一文讲清go的http请求处理过程

1、服务启动

首先启动一个http服务器：

func ListenAndServe(addr string, handler Handler) error {
	server := &Server{Addr: addr, Handler: handler}
	return server.ListenAndServe()
}

这里有两个比较重要入参：地址和处理http请求的handler。这个handler是一个包含方法：ServeHTTP的interface。
这为我们自定义处理http请求的方法创造了可能。任何实现了ServeHTTP这个方法的结构体均可作为当前http请求的处理方。

type Handler interface {
	ServeHTTP(ResponseWriter, *Request)
}

在这个函数中，创建了一个http的server，并为这个server指定了两个非常重要的参数：监听地址和http请求处理的方法ServeHTTP。随后server开始监听http请求并服务http请求：

func (srv *Server) ListenAndServe() error {
	if srv.shuttingDown() {
		return ErrServerClosed
	}
	addr := srv.Addr
	if addr == "" {
		addr = ":http"
	}
	ln, err := net.Listen("tcp", addr)
	if err != nil {
		return err
	}
	return srv.Serve(ln)
}

http协议还是依托的tcp协议，所以走的那一套和linux下socket编程那一套本质上是一样的。下面从listen开始分析整个连接建立和服务请求的全过程。

2、TCP连接建立

首先调用了net包下的Listen方法进行服务的监听。

func Listen(network, address string) (Listener, error) {
	var lc ListenConfig
	return lc.Listen(context.Background(), network, address)
}

调用ListenConfig结构体的Listen方法，监听tcp对应的address端口。

// Listen announces on the local network address.
//
// See func Listen for a description of the network and address
// parameters.
func (lc *ListenConfig) Listen(ctx context.Context, network, address string) (Listener, error) {
	addrs, err := DefaultResolver.resolveAddrList(ctx, "listen", network, address, nil)
	if err != nil {
		return nil, &OpError{Op: "listen", Net: network, Source: nil, Addr: nil, Err: err}
	}
	sl := &sysListener{
		ListenConfig: *lc,
		network:      network,
		address:      address,
	}
	var l Listener
	la := addrs.first(isIPv4)
	switch la := la.(type) {
	case *TCPAddr:
		l, err = sl.listenTCP(ctx, la)
	case *UnixAddr:
		l, err = sl.listenUnix(ctx, la)
	default:
		return nil, &OpError{Op: "listen", Net: sl.network, Source: nil, Addr: la, Err: &AddrError{Err: "unexpected address type", Addr: address}}
	}
	if err != nil {
		return nil, &OpError{Op: "listen", Net: sl.network, Source: nil, Addr: la, Err: err} // l is non-nil interface containing nil pointer
	}
	return l, nil
}

这个函数返回的是一个Listener这样的interface，可以看到这里面的一些诸如Accept 的方法和之前unix中的socket编程很一致。

// A Listener is a generic network listener for stream-oriented protocols.
//
// Multiple goroutines may invoke methods on a Listener simultaneously.
type Listener interface {
	// Accept waits for and returns the next connection to the listener.
	Accept() (Conn, error)

	// Close closes the listener.
	// Any blocked Accept operations will be unblocked and return errors.
	Close() error

	// Addr returns the listener's network address.
	Addr() Addr
}

根据传入的tcp这个参数，可以确定Listen返回的Listener实例是这里返回的listenTCP实例。

	case *TCPAddr:
		l, err = sl.listenTCP(ctx, la)

TCPListener类型：

// TCPListener is a TCP network listener. Clients should typically
// use variables of type Listener instead of assuming TCP.
type TCPListener struct {
	fd *netFD
	lc ListenConfig
}

这里先记住这个Listener实例，后面会有很多地方用到。
在ListenAndServe函数的最后一行，新建出来的server开始为这个Listener服务：

	srv.Serve(ln)

3、HTTP连接建立

现在进入之前创建server的Serve方法，为了分析方便，只截取重要部分的代码分析：

func (srv *Server) Serve(l net.Listener) error {
	if fn := testHookServerServe; fn != nil {
		fn(srv, l) // call hook with unwrapped listener
	}

	origListener := l
	l = &onceCloseListener{Listener: l}
	defer l.Close()

	if err := srv.setupHTTP2_Serve(); err != nil {
		return err
	}

	if !srv.trackListener(&l, true) {
		return ErrServerClosed
	}
	defer srv.trackListener(&l, false)

	baseCtx := context.Background()
	if srv.BaseContext != nil {
		baseCtx = srv.BaseContext(origListener)
		if baseCtx == nil {
			panic("BaseContext returned a nil context")
		}
	}

	var tempDelay time.Duration // how long to sleep on accept failure

	ctx := context.WithValue(baseCtx, ServerContextKey, srv)
	for {
		rw, err := l.Accept()
		if err != nil {
			if srv.shuttingDown() {
				return ErrServerClosed
			}
			if ne, ok := err.(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", err, tempDelay)
				time.Sleep(tempDelay)
				continue
			}
			return err
		}

这里会议一下入参l的实例，是一个TCPListener。进入for循环以后，调用了它的Accept方法。这里划重点提示一下：和linux中的socket编程，先listen某个端口，在这个连接上监听连接请求。然后再accept返回一个新建的tcp连接来处理tcp请求有类似之处。去看一下TCPListener的Accept方法：

// Accept implements the Accept method in the Listener interface; it
// waits for the next call and returns a generic Conn.
func (l *TCPListener) Accept() (Conn, error) {
	if !l.ok() {
		return nil, syscall.EINVAL
	}
	c, err := l.accept()
	if err != nil {
		return nil, &OpError{Op: "accept", Net: l.fd.net, Source: nil, Addr: l.fd.laddr, Err: err}
	}
	return c, nil
}

该方法最终调用accept方法返回了一个TCPConn：

func (ln *TCPListener) accept() (*TCPConn, error) {
	fd, err := ln.fd.accept()
	if err != nil {
		return nil, err
	}
	return newTCPConn(fd, ln.lc.KeepAlive, nil), nil
}

// TCPConn is an implementation of the Conn interface for TCP network
// connections.
type TCPConn struct {
	conn
}

划重点：这个conn结构体是net包下的：

type conn struct {
	fd *netFD
}

TCPConn结构体有一个匿名成员conn，所以TCPConn会继承conn的方法，通过分析源码，我们可以发现，Conn这个interface：

// Conn is a generic stream-oriented network connection.
//
// Multiple goroutines may invoke methods on a Conn simultaneously.
type Conn interface {
	Read(b []byte) (n int, err error)
	Write(b []byte) (n int, err error)
	Close() error
	LocalAddr() Addr
	RemoteAddr() Addr
	SetDeadline(t time.Time) error
	SetReadDeadline(t time.Time) error
	SetWriteDeadline(t time.Time) error
}

很多的实现，是在conn这个结构体中。conn中有大家熟悉的套接字fd。
这里再总结一下：TCPListener调用自己的方法Accept以后，返回的是一个TCPConn结构体，该结构体继承了conn结构体的方法，实现了interface Conn中的方法。简单点说：Accept方法后返回了一个tcp连接。它的实例是TCPConn，在这个连接上可以调用方法Read和Write进行tcp连接上的双向读与写。这与socket网络编程里面Accept类似，新建一个fd，来专门处理连接上的数据传输。
在Accept结束以后：

		tempDelay = 0
		c := srv.newConn(rw)
		c.setState(c.rwc, StateNew, runHooks) // before Serve can return
		go c.serve(connCtx)

server用刚才Accept返回的TCPConn，初始化了一个conn

// Create new connection from rwc.
func (srv *Server) newConn(rwc net.Conn) *conn {
	c := &conn{
		server: srv,
		rwc:    rwc,
	}
	if debugServerConnections {
		c.rwc = newLoggingConn("server", c.rwc)
	}
	return c
}

注意这里的conn与前文的conn不是一类型。这里conn是http包下的conn，代表了http意义上的连接。只截取比较重要的几个字段：

// A conn represents the server side of an HTTP connection.
type conn struct {
	server *Server //当前http连接所对应的server
	rwc net.Conn //当前http连接所用的网络连接，也就是tcp连接。
}

最后go了一个协程来处理这个tcp连接的请求：

		tempDelay = 0
		c := srv.newConn(rw)
		c.setState(c.rwc, StateNew, runHooks) // before Serve can return
		go c.serve(connCtx)

其实这个http包下的连接两个重要的参数：
1、server，标识了这个conn属于哪个server。
2、net.Conn，标识当前http连接用的哪个tcp连接。

4、连接小结

1、首先是Listen返回一个Listener的interface，该interface的实例是TCPListener，实际上是一个tcp连接的监听实例。
2、server调用Serve方法，以这个Listener为入参，启动服务于这个Listener。
3、调用这个Listener的Accept方法，也就是实例TCPListener的的Accept方法，返回一个Conn的interface，它的实例是TCPConn。也就是在监听这个端口地址上，建立了一个tcp连接。
4、用这个tcp连接作为入参，新建了一个http包下的连接。包含两个主要参数分别标识了当前http的conn所属的server以及所用到的网络连接，也就是tcp连接。
5、大致流程：先listen，然后再accept。再有连接请求的过来的情况下，建立tcp连接。用tcp连接，构建http意义上的连接。
6、从本质上看，在accept建立一条新的tcp连接后，才会继续往下执行，初始化新的http意义下的conn，所以go c.serve实际上处理的是一个tcp连接。也就是一个协程处理一个对应的tcp连接，以及这个tcp连接上的所有http请求。对于客户端来说，尽量做到tcp连接复用，因为客户端的每个tcp连接请求都会消耗服务端的一个fd和一个协程来处理建立的tcp连接。

5、HTTP请求处理

至此，进入http请求处理阶段。

// Serve a new connection.
func (c *conn) serve(ctx context.Context) {
	c.remoteAddr = c.rwc.RemoteAddr().String()
	ctx = context.WithValue(ctx, LocalAddrContextKey, c.rwc.LocalAddr())
	var inFlightResponse *response

在进入for循环之前有几个关键的初始化：

	c.r = &connReader{conn: c}
	c.bufr = newBufioReader(c.r)
	c.bufw = newBufioWriterSize(checkConnErrorWriter{c}, 4<<10)

首先是初始化了，一个connReader，它的conn成员赋值c。而这个c，就是之前创建的http意义上的连接conn。new了一个bufioReader，也就是读取缓冲区的reader

func newBufioReader(r io.Reader) *bufio.Reader {
	if v := bufioReaderPool.Get(); v != nil {
		br := v.(*bufio.Reader)
		br.Reset(r)
		return br
	}
	// Note: if this reader size is ever changed, update
	// TestHandlerBodyClose's assumptions.
	return bufio.NewReader(r)
}

这个形参r是你interface，他的实例是connReader：

// connReader is the io.Reader wrapper used by *conn. It combines a
// selectively-activated io.LimitedReader (to bound request header
// read sizes) with support for selectively keeping an io.Reader.Read
// call blocked in a background goroutine to wait for activity and
// trigger a CloseNotifier channel.
type connReader struct {
	conn *conn

	mu      sync.Mutex // guards following
	hasByte bool
	byteBuf [1]byte
	cond    *sync.Cond
	inRead  bool
	aborted bool  // set true before conn.rwc deadline is set to past
	remain  int64 // bytes remaining
}

// NewReader returns a new Reader whose buffer has the default size.
func NewReader(rd io.Reader) *Reader {
	return NewReaderSize(rd, defaultBufSize)
}

// NewReaderSize returns a new Reader whose buffer has at least the specified
// size. If the argument io.Reader is already a Reader with large enough
// size, it returns the underlying Reader.
func NewReaderSize(rd io.Reader, size int) *Reader {
	// Is it already a Reader?
	b, ok := rd.(*Reader)
	if ok && len(b.buf) >= size {
		return b
	}
	if size < minReadBufferSize {
		size = minReadBufferSize
	}
	r := new(Reader)
	r.reset(make([]byte, size), rd)
	return r
}
func (b *Reader) reset(buf []byte, r io.Reader) {
	*b = Reader{
		buf:          buf,
		rd:           r,
		lastByte:     -1,
		lastRuneSize: -1,
	}
}

最后经过一系列的初始化，conn成员bufr的成员rd最终的值就是c.r，类型connReader。这里要记一下，因为后面readRequest会用到这里。

挑重点看：

	for {
		w, err := c.readRequest(ctx)
		if c.r.remain != c.server.initialReadLimitSize() {
			// If we read any bytes off the wire, we're active.
			c.setState(c.rwc, StateActive, runHooks)
		}
		if err != nil {
			const errorHeaders = "\r\nContent-Type: text/plain; charset=utf-8\r\nConnection: close\r\n\r\n"

			switch {
			case err == errTooLarge:
				// Their HTTP client may or may not be
				// able to read this if we're
				// responding to them and hanging up
				// while they're still writing their
				// request. Undefined behavior.
				const publicErr = "431 Request Header Fields Too Large"
				fmt.Fprintf(c.rwc, "HTTP/1.1 "+publicErr+errorHeaders+publicErr)
				c.closeWriteAndWait()
				return

			case isUnsupportedTEError(err):
				// Respond as per RFC 7230 Section 3.3.1 which says,
				//      A server that receives a request message with a
				//      transfer coding it does not understand SHOULD
				//      respond with 501 (Unimplemented).
				code := StatusNotImplemented

				// We purposefully aren't echoing back the transfer-encoding's value,
				// so as to mitigate the risk of cross side scripting by an attacker.
				fmt.Fprintf(c.rwc, "HTTP/1.1 %d %s%sUnsupported transfer encoding", code, StatusText(code), errorHeaders)
				return

			case isCommonNetReadError(err):
				return // don't reply

			default:
				if v, ok := err.(statusError); ok {
					fmt.Fprintf(c.rwc, "HTTP/1.1 %d %s: %s%s%d %s: %s", v.code, StatusText(v.code), v.text, errorHeaders, v.code, StatusText(v.code), v.text)
					return
				}
				publicErr := "400 Bad Request"
				fmt.Fprintf(c.rwc, "HTTP/1.1 "+publicErr+errorHeaders+publicErr)
				return
			}
		}

		// Expect 100 Continue support
		req := w.req
		if req.expectsContinue() {
			if req.ProtoAtLeast(1, 1) && req.ContentLength != 0 {
				// Wrap the Body reader with one that replies on the connection
				req.Body = &expectContinueReader{readCloser: req.Body, resp: w}
				w.canWriteContinue.Store(true)
			}
		} else if req.Header.get("Expect") != "" {
			w.sendExpectationFailed()
			return
		}

		c.curReq.Store(w)

		if requestBodyRemains(req.Body) {
			registerOnHitEOF(req.Body, w.conn.r.startBackgroundRead)
		} else {
			w.conn.r.startBackgroundRead()
		}

		// 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.
		// But we're not going to implement HTTP pipelining because it
		// was never deployed in the wild and the answer is HTTP/2.
		inFlightResponse = w
		serverHandler{c.server}.ServeHTTP(w, w.req)
		inFlightResponse = nil
		w.cancelCtx()
		if c.hijacked() {
			return
		}
		w.finishRequest()
		c.rwc.SetWriteDeadline(time.Time{})
		if !w.shouldReuseConnection() {
			if w.requestBodyLimitHit || w.closedRequestBodyEarly() {
				c.closeWriteAndWait()
			}
			return
		}
		c.setState(c.rwc, StateIdle, runHooks)
		c.curReq.Store(nil)

		if !w.conn.server.doKeepAlives() {
			// We're in shutdown mode. We might've replied
			// to the user without "Connection: close" and
			// they might think they can send another
			// request, but such is life with HTTP/1.1.
			return
		}

		if d := c.server.idleTimeout(); d != 0 {
			c.rwc.SetReadDeadline(time.Now().Add(d))
		} else {
			c.rwc.SetReadDeadline(time.Time{})
		}

		// Wait for the connection to become readable again before trying to
		// read the next request. This prevents a ReadHeaderTimeout or
		// ReadTimeout from starting until the first bytes of the next request
		// have been received.
		if _, err := c.bufr.Peek(4); err != nil {
			return
		}

		c.rwc.SetReadDeadline(time.Time{})
	}
}

首先是读取连接上的http请求：

w, err := c.readRequest(ctx)

这里进入细看：

// Read next request from connection.
func (c *conn) readRequest(ctx context.Context) (w *response, err error) {
	if c.hijacked() {
		return nil, ErrHijacked
	}

	var (
		wholeReqDeadline time.Time // or zero if none
		hdrDeadline      time.Time // or zero if none
	)
	t0 := time.Now()
	if d := c.server.readHeaderTimeout(); d > 0 {
		hdrDeadline = t0.Add(d)
	}
	if d := c.server.ReadTimeout; d > 0 {
		wholeReqDeadline = t0.Add(d)
	}
	c.rwc.SetReadDeadline(hdrDeadline)
	if d := c.server.WriteTimeout; d > 0 {
		defer func() {
			c.rwc.SetWriteDeadline(time.Now().Add(d))
		}()
	}

	c.r.setReadLimit(c.server.initialReadLimitSize())
	if c.lastMethod == "POST" {
		// RFC 7230 section 3 tolerance for old buggy clients.
		peek, _ := c.bufr.Peek(4) // ReadRequest will get err below
		c.bufr.Discard(numLeadingCRorLF(peek))
	}
	req, err := readRequest(c.bufr)
	if err != nil {
		if c.r.hitReadLimit() {
			return nil, errTooLarge
		}
		return nil, err
	}

	if !http1ServerSupportsRequest(req) {
		return nil, statusError{StatusHTTPVersionNotSupported, "unsupported protocol version"}
	}

	c.lastMethod = req.Method
	c.r.setInfiniteReadLimit()

	hosts, haveHost := req.Header["Host"]
	isH2Upgrade := req.isH2Upgrade()
	if req.ProtoAtLeast(1, 1) && (!haveHost || len(hosts) == 0) && !isH2Upgrade && req.Method != "CONNECT" {
		return nil, badRequestError("missing required Host header")
	}
	if len(hosts) == 1 && !httpguts.ValidHostHeader(hosts[0]) {
		return nil, badRequestError("malformed Host header")
	}
	for k, vv := range req.Header {
		if !httpguts.ValidHeaderFieldName(k) {
			return nil, badRequestError("invalid header name")
		}
		for _, v := range vv {
			if !httpguts.ValidHeaderFieldValue(v) {
				return nil, badRequestError("invalid header value")
			}
		}
	}
	delete(req.Header, "Host")

	ctx, cancelCtx := context.WithCancel(ctx)
	req.ctx = ctx
	req.RemoteAddr = c.remoteAddr
	req.TLS = c.tlsState
	if body, ok := req.Body.(*body); ok {
		body.doEarlyClose = true
	}

	// Adjust the read deadline if necessary.
	if !hdrDeadline.Equal(wholeReqDeadline) {
		c.rwc.SetReadDeadline(wholeReqDeadline)
	}

	w = &response{
		conn:          c,
		cancelCtx:     cancelCtx,
		req:           req,
		reqBody:       req.Body,
		handlerHeader: make(Header),
		contentLength: -1,
		closeNotifyCh: make(chan bool, 1),

		// We populate these ahead of time so we're not
		// reading from req.Header after their Handler starts
		// and maybe mutates it (Issue 14940)
		wants10KeepAlive: req.wantsHttp10KeepAlive(),
		wantsClose:       req.wantsClose(),
	}
	if isH2Upgrade {
		w.closeAfterReply = true
	}
	w.cw.res = w
	w.w = newBufioWriterSize(&w.cw, bufferBeforeChunkingSize)
	return w, nil
}

继续挑重点：

req, err := readRequest(c.bufr)

从连接上读取数据，并解析http协议放在Request这样的一个结构体中。核心流程见注释：

func readRequest(b *bufio.Reader) (req *Request, err error) {
	tp := newTextprotoReader(b)//生成一个protoReader
	defer putTextprotoReader(tp)

	req = new(Request)

	// First line: GET /index.html HTTP/1.0
	var s string
	if s, err = tp.ReadLine(); err != nil {//读第一行
		return nil, err
	}
	defer func() {
		if err == io.EOF {
			err = io.ErrUnexpectedEOF
		}
	}()

	var ok bool
	req.Method, req.RequestURI, req.Proto, ok = parseRequestLine(s)//解析获取method URL proto
	if !ok {
		return nil, badStringError("malformed HTTP request", s)
	}
	if !validMethod(req.Method) {
		return nil, badStringError("invalid method", req.Method)
	}
	rawurl := req.RequestURI
	if req.ProtoMajor, req.ProtoMinor, ok = ParseHTTPVersion(req.Proto); !ok {
		return nil, badStringError("malformed HTTP version", req.Proto)
	}
	justAuthority := req.Method == "CONNECT" && !strings.HasPrefix(rawurl, "/")
	if justAuthority {
		rawurl = "http://" + rawurl
	}

	if req.URL, err = url.ParseRequestURI(rawurl); err != nil {
		return nil, err
	}

	if justAuthority {
		// Strip the bogus "http://" back off.
		req.URL.Scheme = ""
	}

	// Subsequent lines: Key: value.
	mimeHeader, err := tp.ReadMIMEHeader()//获取header
	if err != nil {
		return nil, err
	}
	req.Header = Header(mimeHeader)
	if len(req.Header["Host"]) > 1 {
		return nil, fmt.Errorf("too many Host headers")
	}
	req.Host = req.URL.Host
	if req.Host == "" {
		req.Host = req.Header.get("Host")
	}

	fixPragmaCacheControl(req.Header)

	req.Close = shouldClose(req.ProtoMajor, req.ProtoMinor, req.Header, false)

	err = readTransfer(req, b)//body赋值等。
	if err != nil {
		return nil, err
	}

	if req.isH2Upgrade() {
		// Because it's neither chunked, nor declared:
		req.ContentLength = -1
		req.Close = true
	}
	return req, nil
}

此时通过解析获得了req，并存在http包下Request这样的结构体中。
此后用这个req初始化了一个response结构体：

	w = &response{
		conn:          c,
		cancelCtx:     cancelCtx,
		req:           req,
		reqBody:       req.Body,
		handlerHeader: make(Header),
		contentLength: -1,
		closeNotifyCh: make(chan bool, 1),

		// We populate these ahead of time so we're not
		// reading from req.Header after their Handler starts
		// and maybe mutates it (Issue 14940)
		wants10KeepAlive: req.wantsHttp10KeepAlive(),
		wantsClose:       req.wantsClose(),
	}

这个response有几个重要的信息需要提一下：
1、conn，标识当前response所属的http连接。
2、req，解析出来的对应的req请求。
3、reqBody，对应请求过来的数据包提。
继续挑重点：

		inFlightResponse = w
		serverHandler{c.server}.ServeHTTP(w, w.req)
		inFlightResponse = nil
		w.cancelCtx()
		if c.hijacked() {
			return
		}

这个ServeHTTP记得之前说过，是一个interface。任何实现了此方法的实例均可传递给server，来实现自定义的http请求处理方法。这里仅分析go http库自带的handler。

// serverHandler delegates to either the server's Handler or
// DefaultServeMux and also handles "OPTIONS *" requests.
type serverHandler struct {
	srv *Server
}

func (sh serverHandler) ServeHTTP(rw ResponseWriter, req *Request) {
	handler := sh.srv.Handler
	if handler == nil {
		handler = DefaultServeMux
	}
	if !sh.srv.DisableGeneralOptionsHandler && req.RequestURI == "*" && req.Method == "OPTIONS" {
		handler = globalOptionsHandler{}
	}

	if req.URL != nil && strings.Contains(req.URL.RawQuery, ";") {
		var allowQuerySemicolonsInUse atomic.Bool
		req = req.WithContext(context.WithValue(req.Context(), silenceSemWarnContextKey, func() {
			allowQuerySemicolonsInUse.Store(true)
		}))
		defer func() {
			if !allowQuerySemicolonsInUse.Load() {
				sh.srv.logf("http: URL query contains semicolon, which is no longer a supported separator; parts of the query may be stripped when parsed; see golang.org/issue/25192")
			}
		}()
	}

	handler.ServeHTTP(rw, req)
}

这里调用了serverHandler的ServeHTTP。在函数里面做了一个判断，如果当前server的handler是nil，则使用默认的DefaultServeMux，否则使用自定义的handler。回头看sever启动，ListenAndServe的入参即可知道：

func ListenAndServe(addr string, handler Handler) error {
	server := &Server{Addr: addr, Handler: handler}
	return server.ListenAndServe()
}

6、响应写回

方便起见，以DefaultServeMux为例分析。

type Handler interface {
	ServeHTTP(ResponseWriter, *Request)
}

这是一个接口。根据前面的分析，它的实例是DefaultServeMux。入参是

type ResponseWriter interface {
	Header() Header
	Write([]byte) (int, error)
	WriteHeader(statusCode int)
}

这样的interface。根据代码，它的实例是前面readRequest返回的response。这个response的核心内容参看前文。去看defaltServeMux的ServeHTTP实现：

// DefaultServeMux is the default ServeMux used by Serve.
var DefaultServeMux = &defaultServeMux

var defaultServeMux ServeMux

它的ServeHTTP 方法：

// 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)
}

http库自带的handler是根据request中的url构建的map去寻找处理路由的。

// If there is no registered handler that applies to the request,
// Handler returns a “page not found” handler and an empty pattern.
func (mux *ServeMux) Handler(r *Request) (h Handler, pattern string) {

	// CONNECT requests are not canonicalized.
	if r.Method == "CONNECT" {
		// If r.URL.Path is /tree and its handler is not registered,
		// the /tree -> /tree/ redirect applies to CONNECT requests
		// but the path canonicalization does not.
		if u, ok := mux.redirectToPathSlash(r.URL.Host, r.URL.Path, r.URL); ok {
			return RedirectHandler(u.String(), StatusMovedPermanently), u.Path
		}

		return mux.handler(r.Host, r.URL.Path)
	}

	// All other requests have any port stripped and path cleaned
	// before passing to mux.handler.
	host := stripHostPort(r.Host)
	path := cleanPath(r.URL.Path)

	// If the given path is /tree and its handler is not registered,
	// redirect for /tree/.
	if u, ok := mux.redirectToPathSlash(host, path, r.URL); ok {
		return RedirectHandler(u.String(), StatusMovedPermanently), u.Path
	}

	if path != r.URL.Path {
		_, pattern = mux.handler(host, path)
		u := &url.URL{Path: path, RawQuery: r.URL.RawQuery}
		return RedirectHandler(u.String(), StatusMovedPermanently), pattern
	}

	return mux.handler(host, r.URL.Path)
}

// handler is the main implementation of Handler.
// The path is known to be in canonical form, except for CONNECT methods.
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)
	}
	if h == nil {
		h, pattern = NotFoundHandler(), ""
	}
	return
}

以NotFoundHanlder为例：

// NotFoundHandler returns a simple request handler
// that replies to each request with a “404 page not found” reply.
func NotFoundHandler() Handler { return HandlerFunc(NotFound) }

看下HandlerFunc

type HandlerFunc func(ResponseWriter, *Request)

// ServeHTTP calls f(w, r).
func (f HandlerFunc) ServeHTTP(w ResponseWriter, r *Request) {
	f(w, r)
}

实现了ServeHTTP方法，

// NotFound replies to the request with an HTTP 404 not found error.
func NotFound(w ResponseWriter, r *Request) { Error(w, "404 page not found", StatusNotFound) }

// The error message should be plain text.
func Error(w ResponseWriter, error string, code int) {
	w.Header().Set("Content-Type", "text/plain; charset=utf-8")
	w.Header().Set("X-Content-Type-Options", "nosniff")
	w.WriteHeader(code)
	fmt.Fprintln(w, error)
}

最终调用response实例的Header Set方法和WriteHeader方法写入header。以及调用fmt.Fprintln写入数据数据。fmt.Frpintln的第一个参数入参是interface，

// Implementations must not retain p.
type Writer interface {
	Write(p []byte) (n int, err error)
}

从里可以看到它调用的Write方法最终是实例response的Write方法：

// It returns the number of bytes written and any write error encountered.
func Fprintln(w io.Writer, a ...any) (n int, err error) {
	p := newPrinter()
	p.doPrintln(a)
	n, err = w.Write(p.buf)
	p.free()
	return
}

至此，业务端的header与数据写入完成。
完成当前request请求的http响应：

func (w *response) finishRequest() {
	w.handlerDone.Store(true)

	if !w.wroteHeader {
		w.WriteHeader(StatusOK)
	}

	w.w.Flush()
	putBufioWriter(w.w)
	w.cw.close()//写入EOF结束标识符
	w.conn.bufw.Flush()

	w.conn.r.abortPendingRead()

	// Close the body (regardless of w.closeAfterReply) so we can
	// re-use its bufio.Reader later safely.
	w.reqBody.Close()

	if w.req.MultipartForm != nil {
		w.req.MultipartForm.RemoveAll()
	}
}

在close中写入\r\n标识当前数据包的结束：

func (cw *chunkWriter) close() {
	if !cw.wroteHeader {
		cw.writeHeader(nil)
	}
	if cw.chunking {
		bw := cw.res.conn.bufw // conn's bufio writer
		// zero chunk to mark EOF
		bw.WriteString("0\r\n")
		if trailers := cw.res.finalTrailers(); trailers != nil {
			trailers.Write(bw) // the writer handles noting errors
		}
		// final blank line after the trailers (whether
		// present or not)
		bw.WriteString("\r\n")
	}
}

7、小结

本文包括了go处理http请求的全过程：
1、tcp连接建立
2、request请求解析
3、业务端处理request的handler
4、业务端处理的回包信息如何写回给客户端。
本文只是介绍了整个流程。中间具体细节比如tcp连接管理、各种读写buffer的管理等需要进一步深挖。
业务handler简单介绍了go自带的路由注册管理。常用web框架，如gin的路由注册等后续介绍。