go web framework gin 启动流程分析

最主要的package : gin

最主要的struct: Engine

Engine 是整个framework的实例，它包含了muxer, middleware, configuration settings. 通过New() 或者Default()来创建一个Engine 的实例。

type Engine struct {
    RouterGroup

    // Enables automatic redirection if the current route can't be matched but a
    // handler for the path with (without) the trailing slash exists.
    // For example if /foo/ is requested but a route only exists for /foo, the
    // client is redirected to /foo with http status code 301 for GET requests
    // and 307 for all other request methods.
    RedirectTrailingSlash bool

    // If enabled, the router tries to fix the current request path, if no
    // handle is registered for it.
    // First superfluous path elements like ../ or // are removed.
    // Afterwards the router does a case-insensitive lookup of the cleaned path.
    // If a handle can be found for this route, the router makes a redirection
    // to the corrected path with status code 301 for GET requests and 307 for
    // all other request methods.
    // For example /FOO and /..//Foo could be redirected to /foo.
    // RedirectTrailingSlash is independent of this option.
    RedirectFixedPath bool

    // If enabled, the router checks if another method is allowed for the
    // current route, if the current request can not be routed.
    // If this is the case, the request is answered with 'Method Not Allowed'
    // and HTTP status code 405.
    // If no other Method is allowed, the request is delegated to the NotFound
    // handler.
    HandleMethodNotAllowed bool
    ForwardedByClientIP    bool

    // #726 #755 If enabled, it will thrust some headers starting with
    // 'X-AppEngine...' for better integration with that PaaS.
    AppEngine bool

    // If enabled, the url.RawPath will be used to find parameters.
    UseRawPath bool

    // If true, the path value will be unescaped.
    // If UseRawPath is false (by default), the UnescapePathValues effectively is true,
    // as url.Path gonna be used, which is already unescaped.
    UnescapePathValues bool

    // Value of 'maxMemory' param that is given to http.Request's ParseMultipartForm
    // method call.
    MaxMultipartMemory int64

    delims           render.Delims
    secureJsonPrefix string
    HTMLRender       render.HTMLRender
    FuncMap          template.FuncMap
    allNoRoute       HandlersChain
    allNoMethod      HandlersChain
    noRoute          HandlersChain
    noMethod         HandlersChain
    pool             sync.Pool
    trees            methodTrees
}

获取一个gin Engin的实例：r := gin.Default()

查看gin.Default()方法的实现：

// Default returns an Engine instance with the Logger and Recovery middleware already attached.
func Default() *Engine {
	debugPrintWARNINGDefault()
	engine := New()  #实例化一个engine
	engine.Use(Logger(), Recovery())
	return engine
}

　　查看New()的实现：

// New returns a new blank Engine instance without any middleware attached.
// By default the configuration is:
// - RedirectTrailingSlash:  true
// - RedirectFixedPath:      false
// - HandleMethodNotAllowed: false
// - ForwardedByClientIP:    true
// - UseRawPath:             false
// - UnescapePathValues:     true
func New() *Engine {
	debugPrintWARNINGNew()
	engine := &Engine{
		RouterGroup: RouterGroup{
			Handlers: nil,
			basePath: "/",
			root:     true,
		},
		FuncMap:                template.FuncMap{},
		RedirectTrailingSlash:  true,
		RedirectFixedPath:      false,
		HandleMethodNotAllowed: false,
		ForwardedByClientIP:    true,
		AppEngine:              defaultAppEngine,
		UseRawPath:             false,
		UnescapePathValues:     true,
		MaxMultipartMemory:     defaultMultipartMemory,
		trees:                  make(methodTrees, 0, 9),
		delims:                 render.Delims{Left: "{{", Right: "}}"},
		secureJsonPrefix:       "while(1);",
	}
	engine.RouterGroup.engine = engine
	engine.pool.New = func() interface{} { //这里定义了engine.pool.New方法，所以在Get()的时候，如果为nil,那么直接执行New 方法
		return engine.allocateContext()
	}
	return engine
}

　如果engine.pool.New被执行，那么将返回一个*Context类型的指针。

func (engine *Engine) allocateContext() *Context {
	return &Context{engine: engine}
}

然后调用r.Run(":8080"), 在本地网卡上监听8080端口，等待连接。实际上调用的是：http.ListenAndServe(address, engine)

查看http.ListenAndServe方法：

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

　它且将addr 和handler作为参数实例化了一个Server， 然后调用server.ListenAndServe()方法等待连接。

查看Handler的定义：

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

　它是一个interface, 那么也就是说它作为参数的话，那么这个struct必须实现ServeHTTP(ResponseWriter, *Request) 这个方法。

在gin.go中查看ServeHTTP方法的定义：

// ServeHTTP conforms to the http.Handler interface.
func (engine *Engine) ServeHTTP(w http.ResponseWriter, req *http.Request) {
	c := engine.pool.Get().(*Context) 返回的是New()里面定义的func,然后执行。
	c.writermem.reset(w)
	c.Request = req
	c.reset()

	engine.handleHTTPRequest(c)

	engine.pool.Put(c)
}

　这里的engine.pool.Get().(*Context) 方法，实际上执行的是上面的New方法，所以得到的是一个Context类型的指针。

分析到这里，也就是说，如果有一个HTTP请求到来，那么会先触发ServeHTTP方法得到一个Context请求上下文，然后调用engine.handleHTTPRequest(C)，处理这个请求，同时将处理的内容写入到http.ResponseWriter中。

关于engine.handleHTTPRequest()如何实现的，参考gin 路由表的设计

处理完以后为什么还要执行engine.pool.Put(c)呢？

因为：engine.pool需要一个buffer来存储内容，当下一个请求到达的时候，就可以直接来使用，避免又调用一次New函数，节省了内存。