Gin 框架源码学习（二） -- 服务启动

本篇主要介绍gin服务启动过程的源码 (gin版本v1.7.0)

Run() 启动入口

我们的程序都是通过调用Run函数来启动gin的实例，下面来看一下Run的源码：

func (engine *Engine) Run(addr ...string) (err error) {
    defer func() { debugPrintError(err) }()
    // 解析服务地址
    address := resolveAddress(addr)
    debugPrint("Listening and serving HTTP on %s\n", address)
    // 此处会block
    err = http.ListenAndServe(address, engine)
    return
}

该方法其实是对http.ListenAndServe的简单封装，以下逻辑就进入net/http包中了，继续往下看：

func ListenAndServe(addr string, handler Handler) error {
    // 配置tcp的监听地址和监听到来后的处理函数
    server := &Server{Addr: addr, Handler: handler}
    // 继续调用。。。
    return server.ListenAndServe()
}

// Handler定义
type Handler interface {
    // 请求到来后的处理逻辑，gin框架会自己实现
    ServeHTTP(ResponseWriter, *Request)
}

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

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

    // 带值的上下文
    ctx := context.WithValue(baseCtx, ServerContextKey, srv)
    // 死循环，监听&处理请求
    for {
        // 请求过来了
        rw, err := l.Accept()
        if err != nil {
            select {
            case <-srv.getDoneChan():
                return ErrServerClosed
            default:
            }
            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
        }
        connCtx := ctx
        if cc := srv.ConnContext; cc != nil {
            connCtx = cc(connCtx, rw)
            if connCtx == nil {
                panic("ConnContext returned nil")
            }
        }
        tempDelay = 0
        // 创建一个新连接，一个conn对象，代表服务端的http连接，里边包含该次请求的数据
        c := srv.newConn(rw)
        c.setState(c.rwc, StateNew) // before Serve can return
        // 开启一个goroutine处理请求，将上下文传递进去，高并发的保障
        go c.serve(connCtx)
    }
}

// serve函数只摘取最重要的一行
func (c *conn) serve(ctx context.Context) {

    ...
    // 这里就会调用gin框架实现的ServeHTTP逻辑
    serverHandler{c.server}.ServeHTTP(w, w.req)

    ...
}

ServeHTTP 逻辑

func (engine *Engine) ServeHTTP(w http.ResponseWriter, req *http.Request) {
    // 从对象池子里获取一个Context对象，池子里有可以直接拿来用，没有则创建一个新的对象返回
    c := engine.pool.Get().(*Context)
    // 把请求的相关数据都写入Context
    c.writermem.reset(w)
    c.Request = req
    c.reset()

    // 请求处理逻辑
    engine.handleHTTPRequest(c)
    // 使用完毕，放回池子，复用
    engine.pool.Put(c)
}

func (engine *Engine) handleHTTPRequest(c *Context) {
    httpMethod := c.Request.Method
    rPath := c.Request.URL.Path
    unescape := false
    if engine.UseRawPath && len(c.Request.URL.RawPath) > 0 {
        rPath = c.Request.URL.RawPath
        unescape = engine.UnescapePathValues
    }

    if engine.RemoveExtraSlash {
        rPath = cleanPath(rPath)
    }

    // Find root of the tree for the given HTTP method
    t := engine.trees
    // 遍历路由数组，根据请求方法找出对应的那棵树
    for i, tl := 0, len(t); i < tl; i++ {
        if t[i].method != httpMethod {
            continue
        }
        root := t[i].root
        // 从树中找到路由对应的函数处理链和参数
        // Find route in tree
        value := root.getValue(rPath, c.params, unescape)
        if value.params != nil {
            c.Params = *value.params
        }
        if value.handlers != nil { // 找到了
            c.handlers = value.handlers
            c.fullPath = value.fullPath
            // 按顺序调用处理函数链
            c.Next()
            c.writermem.WriteHeaderNow()
            return
        }
        if httpMethod != "CONNECT" && rPath != "/" {
            if tsr && engine.RedirectTrailingSlash {
                redirectTrailingSlash(c)
                return
            }
            if engine.RedirectFixedPath && redirectFixedPath(c, root, engine.RedirectFixedPath) {
                return
            }
        }
        break
    }

    if engine.HandleMethodNotAllowed {
        for _, tree := range engine.trees {
            if tree.method == httpMethod {
                continue
            }
            if value := tree.root.getValue(rPath, nil, unescape); value.handlers != nil {
                c.handlers = engine.allNoMethod
                serveError(c, http.StatusMethodNotAllowed, default405Body)
                return
            }
        }
    }
    c.handlers = engine.allNoRoute
    serveError(c, http.StatusNotFound, default404Body)
}

// 看一下Next方法，很简单
func (c *Context) Next() {
    // 初始化的时候是-1
    c.index++
    for c.index < int8(len(c.handlers)) {
        c.handlers[c.index](c)
        c.index++
    }
}