构建一个HTTP服务器的简单例子
package main
import (
"net/http"
)
func SayHello(w http.ResponseWriter, req *http.Request) {
w.Write([]byte("Hello"))
}
func main() {
http.HandleFunc("/hello", SayHello)
http.ListenAndServe(":8001", nil)
}
当我们访问 地址IP：8001/hello 就会得到“hello”输出在浏览器上。
我们分析下这是怎么实现的。先看下接口函数ListenAndServe
func ListenAndServe(addr string, handler Handler) error {
server := &Server{Addr: addr, Handler: handler}
return server.ListenAndServe()
}
实际上是生成一个Server对象，然后执行Server对象的函数ListenAndServe()。在分析ListenAndServe()之前我们先看下Handler，实际上Handler是个接口
type Handler interface {
ServeHTTP(ResponseWriter, *Request)
}
在看Request和ResponseWriter
描述请求头的Request
type Request struct {
Method string
URL *url.URL
Proto string // "HTTP/1.0"
ProtoMajor int // 1
ProtoMinor int // 0
Header Header
Body io.ReadCloser
ContentLength int64
TransferEncoding []string
Close bool
Host string
Form url.Values
PostForm url.Values
MultipartForm *multipart.Form
Trailer Header
RemoteAddr string
RequestURI string
TLS *tls.ConnectionState
Cancel <-chan struct{}
}
描述应答的ResponseWriter
type ResponseWriter interface {
// Header returns the header map that will be sent by
// WriteHeader. Changing the header after a call to
// WriteHeader (or Write) has no effect unless the modified
// headers were declared as trailers by setting the
// "Trailer" header before the call to WriteHeader (see example).
// To suppress implicit response headers, set their value to nil.
Header() Header
// Write writes the data to the connection as part of an HTTP reply.
// If WriteHeader has not yet been called, Write calls WriteHeader(http.StatusOK)
// before writing the data. If the Header does not contain a
// Content-Type line, Write adds a Content-Type set to the result of passing
// the initial 512 bytes of written data to DetectContentType.
Write([]byte) (int, error)
// WriteHeader sends an HTTP response header with status code.
// If WriteHeader is not called explicitly, the first call to Write
// will trigger an implicit WriteHeader(http.StatusOK).
// Thus explicit calls to WriteHeader are mainly used to
// send error codes.
WriteHeader(int)
}
现在我们回到Server类的ListenAndServe()函数
func (srv *Server) ListenAndServe() error {
addr := srv.Addr
if addr == "" {
addr = ":http"
}
ln, err := net.Listen("tcp", addr)
if err != nil {
return err
}
return srv.Serve(tcpKeepAliveListener{ln.(*net.TCPListener)})
}
实际上这个函数只是执行TCP地址监听，然后执行Server函数，我们看下该函数干了什；函数头有解释，大概意思就是监听一个地址，然后接受连接，然后生成一个goroutine读取这个新的连接请求，而是调用handler去处理这个请求
// Serve accepts incoming connections on the Listener l, creating a
// new service goroutine for each. The service goroutines read requests and
// then call srv.Handler to reply to them.
// Serve always returns a non-nil error.
func (srv *Server) Serve(l net.Listener) error {
defer l.Close()
if fn := testHookServerServe; fn != nil {
fn(srv, l)
}
var tempDelay time.Duration // how long to sleep on accept failure
if err := srv.setupHTTP2(); err != nil {
return err
}
for {
rw, e := l.Accept()
if e != nil {
if ne, ok := e.(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", e, tempDelay)
time.Sleep(tempDelay)
continue
}
return e
}
tempDelay = 0
c := srv.newConn(rw)
c.setState(c.rwc, StateNew) // before Serve can return
go c.serve()
}
}
handler是啥？我们一开始就说明了，实际上它是一个接口，具体怎么实现由客户定义。但是我们的例子中没有传入这样一个handler。那这个handler怎么来呢？我们继续分析，接受客户端连接后，执行
c := srv.newConn(rw) c.setState(c.rwc, StateNew) // before Serve can return go c.serve()
生成一个连接对象，然后执行连接对象的函数server，我们接着分析该函数：
// Serve a new connection.
func (c *conn) serve() {
c.remoteAddr = c.rwc.RemoteAddr().String()
defer func() {
if err := recover(); err != nil {
const size = 64 << 10
buf := make([]byte, size)
buf = buf[:runtime.Stack(buf, false)]
c.server.logf("http: panic serving %v: %v\n%s", c.remoteAddr, err, buf)
}
if !c.hijacked() {
c.close()
c.setState(c.rwc, StateClosed)
}
}()
if tlsConn, ok := c.rwc.(*tls.Conn); ok {
if d := c.server.ReadTimeout; d != 0 {
c.rwc.SetReadDeadline(time.Now().Add(d))
}
if d := c.server.WriteTimeout; d != 0 {
c.rwc.SetWriteDeadline(time.Now().Add(d))
}
if err := tlsConn.Handshake(); err != nil {
c.server.logf("http: TLS handshake error from %s: %v", c.rwc.RemoteAddr(), err)
return
}
c.tlsState = new(tls.ConnectionState)
*c.tlsState = tlsConn.ConnectionState()
if proto := c.tlsState.NegotiatedProtocol; validNPN(proto) {
if fn := c.server.TLSNextProto[proto]; fn != nil {
h := initNPNRequest{tlsConn, serverHandler{c.server}}
fn(c.server, tlsConn, h)
}
return
}
}
c.r = &connReader{r: c.rwc}
c.bufr = newBufioReader(c.r)
c.bufw = newBufioWriterSize(checkConnErrorWriter{c}, 4<<10)
for {
w, err := c.readRequest()
if c.r.remain != c.server.initialReadLimitSize() {
// If we read any bytes off the wire, we're active.
c.setState(c.rwc, StateActive)
}
if err != nil {
if 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.
io.WriteString(c.rwc, "HTTP/1.1 431 Request Header Fields Too Large\r\nContent-Type: text/plain\r\nConnection: close\r\n\r\n431 Request Header Fields Too Large")
c.closeWriteAndWait()
return
}
if err == io.EOF {
return // don't reply
}
if neterr, ok := err.(net.Error); ok && neterr.Timeout() {
return // don't reply
}
var publicErr string
if v, ok := err.(badRequestError); ok {
publicErr = ": " + string(v)
}
io.WriteString(c.rwc, "HTTP/1.1 400 Bad Request\r\nContent-Type: text/plain\r\nConnection: close\r\n\r\n400 Bad Request"+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}
}
} else if req.Header.get("Expect") != "" {
w.sendExpectationFailed()
return
}
// 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.
serverHandler{c.server}.ServeHTTP(w, w.req)
if c.hijacked() {
return
}
w.finishRequest()
if !w.shouldReuseConnection() {
if w.requestBodyLimitHit || w.closedRequestBodyEarly() {
c.closeWriteAndWait()
}
return
}
c.setState(c.rwc, StateIdle)
}
}
该函数开始尝试TLS握手，如果失败，超时则不是TLS连接，则直接读取http请求：
w, err := c.readRequest()
接着生成一个服务对象，执行服务对象的服务函数：
serverHandler{c.server}.ServeHTTP(w, w.req)
我们看下该函数：
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)
}
意思很明显，如果handler为空，则赋值一个DefaultServeMux，也就是默认的Handler，然后执行该默认Handler的服务函数来处理请求：
handler.ServeHTTP(rw, req)
下篇我们将分析该默认Handler.
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