1. Gin简介

前面通过两篇文章分享了Golang HTTP编程的路由分发、请求/响应处理。

• Golang HTTP编程及源码解析-路由分发
• Golang HTTP编程及源码解析-请求/响应处理

可以看出来Golang原生HTTP编程在路由分组、动态路由及参数读取/验证、构造String/Data/JSON/HTML响应的方法等存在优化的空间。

Gin是一个用Golang编写的高性能Web框架。

• 基于前缀树的路由，快速且支持动态路由
• 支持中间件及路由分组，将具有同一特性的路由划入统一组别、设置相同的中间件。
  • 比如需要登录的一批接口接入登录权限认证中间件、而不需要登录一批接口则不需要接入
• …

2. 快速使用

基于gin@v1.8.1，基本使用如下

func main() {
    // Creates a new blank Engine instance without any middleware attached
    engine := gin.New()
    // Global middleware
    // Logger middleware will write the logs to gin.DefaultWriter even if you set with GIN_MODE=release.
    // By default gin.DefaultWriter = os.Stdout
    engine.Use(gin.Logger())
    // Recovery middleware recovers from any panics and writes a 500 if there was one.
    engine.Use(gin.Recovery())
    v1Group := engine.Group("app/v1", accessHandler())
    v1Group.GET("user/info", userInfoLogic())
    engine.Run(":8019")
}

终端运行go run main.go，输出如下

$ go run main.go
[GIN-debug] [WARNING] Running in "debug" mode. Switch to "release" mode in production.
 - using env:   export GIN_MODE=release
 - using code:  gin.SetMode(gin.ReleaseMode)

[GIN-debug] GET    /app/v1/user/info         --> main.userInfoLogic.func1 (4 handlers)
[GIN-debug] [WARNING] You trusted all proxies, this is NOT safe. We recommend you to set a value.
Please check https://pkg.go.dev/github.com/gin-gonic/gin#readme-don-t-trust-all-proxies for details.
[GIN-debug] Listening and serving HTTP on :8019

通过打印可以看出注册了GET方法的路由/app/v1/user/info，对应处理函数为main.userInfoLogic，

总共包括四个处理函数，按顺序为gin.Logger()、gin.Recovery() 、accessHandler()以及userInfoLogic

最终在端口8019启动了HTTP监听服务。

2.1 创建Engine并使用gin.Logger()和 gin.Recovery() 两个全局中间件，对engine下的所有路由都生效

通过代码及注释，gin.Logger()和 gin.Recovery()放到了Engine.RouterGroup.Handlers切片中。

// Use attaches a global middleware to the router. i.e. the middleware attached through Use() will be
// included in the handlers chain for every single request. Even 404, 405, static files...
// For example, this is the right place for a logger or error management middleware.
func (engine *Engine) Use(middleware ...HandlerFunc) IRoutes {
    engine.RouterGroup.Use(middleware...)
    engine.rebuild404Handlers()
    engine.rebuild405Handlers()
    return engine
}
// Use adds middleware to the group, see example code in GitHub.
func (group *RouterGroup) Use(middleware ...HandlerFunc) IRoutes {
    group.Handlers = append(group.Handlers, middleware...)
    return group.returnObj()
}

2.2 创建路由分组v1Group，且该分组使用了accessHandler()，accessHandler()对v1Group分组路由均生效

// Group creates a new router group. You should add all the routes that have common middlewares or the same path prefix.
// For example, all the routes that use a common middleware for authorization could be grouped.
func (group *RouterGroup) Group(relativePath string, handlers ...HandlerFunc) *RouterGroup {
    return &RouterGroup{
        Handlers: group.combineHandlers(handlers),
        basePath: group.calculateAbsolutePath(relativePath),
        engine:   group.engine,
    }
}

从代码可以看出，返回了新的gin.RouterGroup，并且

v1Group.Handlers = append(group.Handlers, handlers)，此时gin.RouterGroup.Handlers为[gin.Logger(),gin.Recovery(),accessHandler()]

同时v1Group.basePath = "app/v1"

从代码同时可以得出，支持分组嵌套分组。即在v1Group都基础上在创建分组，比如v1Group.Group("north")

2.3 在v1Group下注册路由user/info，该路由的处理函数是userInfoLogic，方法为GET

func (group *RouterGroup) handle(httpMethod, relativePath string, handlers HandlersChain) IRoutes {
    absolutePath := group.calculateAbsolutePath(relativePath) // 计算出完整路由
    handlers = group.combineHandlers(handlers) // 将新处理函数拼接到原来的末尾
    group.engine.addRoute(httpMethod, absolutePath, handlers) // 路由加入到前缀树
    return group.returnObj()
}
func (engine *Engine) addRoute(method, path string, handlers HandlersChain) {
    ...
    root := engine.trees.get(method)
    if root == nil {
        root = new(node)
        root.fullPath = "/"
        engine.trees = append(engine.trees, methodTree{method: method, root: root})
    }
    root.addRoute(path, handlers)
    ...
}

将分组v1Group的路由前缀和当前user/info计算得到完整路由，即app/v1/user/info

合并处理函数，此时handlers = [gin.Logger(),gin.Recovery(),accessHandler(),userInfoLogic()]

最后将路由及处理函数按http method分组，加入到不同路由树中。

2.4 通过 engine.Run(":8019") 在启动HTTP服务

// Run attaches the router to a http.Server and starts listening and serving HTTP requests.
// It is a shortcut for http.ListenAndServe(addr, router)
// Note: this method will block the calling goroutine indefinitely unless an error happens.
func (engine *Engine) Run(addr ...string) (err error) {
    ...
    address := resolveAddress(addr)
    debugPrint("Listening and serving HTTP on %s\n", address)
    err = http.ListenAndServe(address, engine.Handler())
    return
}

这里调用http.ListenAndServe启动HTTP监听服务，Engine实现了http.Handler接口，如果有客户端请求，会调用到Engine.ServeHTTP函数。

3. 路由过程

// gin.go
func (engine *Engine) handleHTTPRequest(c *Context) {
    httpMethod := c.Request.Method
    rPath := c.Request.URL.Path
    ...
    // Find root of the tree for the given HTTP method
    t := engine.trees
    for i, tl := 0, len(t); i < tl; i++ {
        if t[i].method != httpMethod {
            continue
        }
        root := t[i].root
        // Find route in tree
        value := root.getValue(rPath, c.params, c.skippedNodes, unescape)
        if value.params != nil {
            c.Params = *value.params
        }
        if value.handlers != nil {
            c.handlers = value.handlers
            c.fullPath = value.fullPath
            c.Next()
            c.writermem.WriteHeaderNow()
            return
        }
        ...
        break
    }
}
// context.go
func (c *Context) Next() {
    c.index++
    for c.index < int8(len(c.handlers)) {
        c.handlers[c.index](c)
        c.index++
    }
}

从上面代码可以看出，通过http method找到对应的路由树，再根据URL从路由树中查找对应的节点，

获取到处理函数切片，通过c.Next按通过顺序执行处理函数。

对于请求GET /app/v1/user/info，将依次执行[gin.Logger(),gin.Recovery(),accessHandler(),userInfoLogic()]

4. 请求/响应参数处理

func accessHandler() func(*gin.Context) {
    return func(c *gin.Context) {
        // 不允许crul访问
        if strings.Contains(c.GetHeader("user-agent"), "curl") {
            c.JSON(http.StatusBadRequest, "cant't not visited by curl")
            c.Abort() // 直接退出，避免执行后续处理函数
        }
    }
}
func userInfoLogic() func(*gin.Context) {
    return func(c *gin.Context) {
        id := c.Query("id")
        c.JSON(http.StatusOK, map[string]interface{}{"id": id, "name": "bob", "age": 18})
    }
}

v1Group的通用处理函数accessHandler，达到v1Group下注册的路由无法用curl访问的效果。

通过c.Query("id") 获取URL查询参数，

通过以下代码可以看出，第一次获取URL查询时会缓存所有URL查询参数，这减少了内存的分配，节省了计算资源。

因为每次调用url.ParseQuery都会重新申请缓存，重复解析URL。

func (c *Context) Query(key string) (value string) {
    value, _ = c.GetQuery(key)
    return
}
func (c *Context) initQueryCache() {
    if c.queryCache == nil {
        if c.Request != nil {
            c.queryCache = c.Request.URL.Query()
        } else {
            c.queryCache = url.Values{}
        }
    }
}
func (c *Context) GetQueryArray(key string) (values []string, ok bool) {
    c.initQueryCache()
    values, ok = c.queryCache[key]
    return
}

通过c.JSON返回Content-Type为application/json的响应体，

这也是Gin对原生net/http编程的一个优化，对常用的响应类型进行封装，方便使用者使用。

当然，Gin对请求/响应参数的处理还有其它很多细微的优化，这里就不详细说明了。

5. 总结

Gin使用Map来实现路由匹配，而Gin使用路由树来实现路由匹配，支持动态路由，内存占用小且路由匹配快。

同时Gin使用缓存来优化请求参数的处理过程，提供了通用的响应参数处理等，方便用户使用。