The following tutorial column will introduce you to the http client of Go. I hope it will be helpful to friends who need it!                                                                                                                                                

http client

go encapsulates the http client, and it is very convenient to request remote data. Let’s take a look at how to implement it at the bottom of the source code.

resp, err := http.Get("https://baidu.com") if err != nil {
    fmt.Printf("发起请求失败：%v", err)
    return }defer resp.Body.Close() io.Copy(os.Stdout, resp.Body)

The general process of the request

1. Construct the request object according to the request conditions

2. All client requests , will be processed by client.do()

func (c *Client) do(req *Request) (retres *Response, reterr error)

2.1 request request is processed by client.send()

func (c *Client) send(req *Request, deadline time.Time) (resp *Response, didTimeout func() bool, err error)resp, didTimeout, err = send(req, c.transport(), deadline)//默认传DefaultTransport

3.send function

func send(ireq *Request, rt RoundTripper, deadline time.Time) (resp *Response, didTimeout func() bool, err error) {
    resp, err = rt.RoundTrip(req) }

4. The RoundTrip method of DefaultTransport is actually the RoundTrip method of Transport

func (t *Transport) roundTrip(req *Request) (*Response, error) {
    treq := &transportRequest{Request: req, trace: trace} //封装新的request
    cm, err := t.connectMethodForRequest(treq)
    pconn, err := t.getConn(treq, cm) //使用连接池技术，获取连接对象*persistConn，
    resp, err = pconn.roundTrip(treq) //使用连接对象获取response}

5. Use connection pool technology to obtain the connection object *persistConn

func (t *Transport) getConn(treq *transportRequest, cm connectMethod) (pc *persistConn, err error) {
    w := &wantConn{ //构建连接对象
        cm:         cm,
        key:        cm.key(),
        ctx:        ctx,
        ready:      make(chan struct{}, 1),
        beforeDial: testHookPrePendingDial,
        afterDial:  testHookPostPendingDial,
    }
    if delivered := t.queueForIdleConn(w); delivered {//从连接池获取符合的连接对象，有就返回
        pc := w.pc        
        return pc, nil
    }    
    t.queueForDial(w)//发起连接

    select {
    case <-w.ready:    //连接准备好，就返回连接对象    
        return w.pc, w.err}

5.1 Transport.queueForDial initiates connection

func (t *Transport) queueForDial(w *wantConn) {
    go t.dialConnFor(w)}

5.2 Initiates dialing dialConnFor

func (t *Transport) dialConnFor(w *wantConn) {
    pc, err := t.dialConn(w.ctx, w.cm) //发起拨号，返回连接对象
    delivered := w.tryDeliver(pc, err)}

5.3 Initiate dialing

func (t *Transport) dialConn(ctx context.Context, cm connectMethod) (pconn *persistConn, err error) {
    pconn = &persistConn{ //构建连接对象
        t:             t,
        cacheKey:      cm.key(),
        reqch:         make(chan requestAndChan, 1),
        writech:       make(chan writeRequest, 1),
        closech:       make(chan struct{}),
        writeErrCh:    make(chan error, 1),
        writeLoopDone: make(chan struct{}),
    }
    conn, err := t.dial(ctx, "tcp", cm.addr()) //tcp连接，获取到net.conn对象

    pconn.br = bufio.NewReaderSize(pconn, t.readBufferSize())//可以从conn读
    pconn.bw = bufio.NewWriterSize(persistConnWriter{pconn}, t.writeBufferSize())//写到conn

    go pconn.readLoop()//开启读协程
    go pconn.writeLoop()//开启写协程
    return pconn, nil}

5.4 Reading the coroutine, although it is a for loop, the requested response is read in one go. If it is not closed, the coroutine will be leaked

func (pc *persistConn) readLoop() {
    alive := true
    for alive {
        rc := <-pc.reqch //读取request,写入的地方在步骤6

        resp, err = pc.readResponse(rc, trace) //返回response
        //response的body是否可写，服务器code101才可写，所以正常这个是false
        bodyWritable := resp.bodyIsWritable()

        //response.Close设置循环结束，退出协程
        if resp.Close || rc.req.Close || resp.StatusCode <= 199 || bodyWritable {                    alive = false
        }          

        //把response写入通道，在步骤6会读取这个通道
        select {
        case rc.ch <- responseAndError{res: resp}:
        case <-rc.callerGone:
            return
        }
        //循环结束的一些情况
        select {
        case bodyEOF := <-waitForBodyRead: //读完body也会自动结束            
        case <-rc.req.Cancel:
        case <-rc.req.Context().Done():
        case <-pc.closech:
            alive = false
            pc.t.CancelRequest(rc.req)
        }
    }

5.4.1 pc.readResponse Get response

func (pc *persistConn) readResponse(rc requestAndChan, trace *httptrace.ClientTrace) (resp *Response, err error) {
    for{
        resp, err = ReadResponse(pc.br, rc.req) //获取response
    }}

5.4.2 ReadResponse Read response

func ReadResponse(r *bufio.Reader, req *Request) (*Response, error) {
    tp := textproto.NewReader(r) //可以处理HTTP, NNTP, SMTP协议的内容，方便读取
    resp := &Response{
        Request: req,
    }    
    line, err := tp.ReadLine()//读取第一行，获取协议，状态码
    resp.Proto = line[:i]
    resp.Status = strings.TrimLeft(line[i+1:], " ")

    mimeHeader, err := tp.ReadMIMEHeader()//读取header头
    resp.Header = Header(mimeHeader)}

5.5 Writing coroutine

func (pc *persistConn) writeLoop() {
    for {
        select {
        case wr := <-pc.writech:
            startBytesWritten := pc.nwrite
            err := wr.req.Request.write(pc.bw, pc.isProxy, wr.req.extra, pc.waitForContinue(wr.continueCh))            
    }}

6. Use the connection object *persistConn to get the response

func (pc *persistConn) roundTrip(req *transportRequest) (resp *Response, err error) {
    var continueCh chan struct{}
    resc := make(chan responseAndError) //response通道

    pc.writech <- writeRequest{req, writeErrCh, continueCh}//written by roundTrip; read by writeLoop   

    pc.reqch <- requestAndChan{ //written by roundTrip; read by readLoop
        req:        req.Request,
        ch:         resc,
        addedGzip:  requestedGzip,
        continueCh: continueCh,
        callerGone: gone,
    }
    for { //监听这些通道
        testHookWaitResLoop()
        select {
        case err := <-writeErrCh:            
        case <-pc.closech:            
        case re := <-resc: //监听 response通道，返回response         
            return re.res, nil
        }
    }}

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