github.com/gin-gonic/gin 是 golang 的 web 框架，其用字典树做路由匹配、支持中间件，本文介绍其源码实现。

快速使用

package main

import (
	"github.com/gin-gonic/gin"
	"net/http"
)

func main() {
	r := gin.Default()
	r.GET("/ping", func(c *gin.Context) {
		c.JSON(http.StatusOK, gin.H{
			"message": "pong",
		})
	})
	r.Run()
}

// curl localhost:8000/ping 则返回 PONG

默认使用 encoding/json 库，当 go build -tags=jsoniter . 时会用 github.com/json-iterator/go 库。

在 Gin examples repository 可以找到官方使用示例。

返回响应

func Error500(ctx *gin.Context, err error) {
	ctx.AbortWithStatusJSON(http.StatusInternalServerError, gin.H{"msg": err.Error()})
}

func Error400(ctx *gin.Context, err error) {
	ctx.AbortWithStatusJSON(http.StatusBadRequest, gin.H{"msg": err.Error()})
}

路由匹配

path

package main

import (
	"github.com/gin-gonic/gin"
	"net/http"
)

func main() {
	router := gin.Default()

	// 可以用:匹配路径
	 This handler will match /user/john or /user/ but will not match /user
	router.GET("/user/:name", func(c *gin.Context) {
		name := c.Param("name")
		c.String(http.StatusOK, "Hello %s", name)
	})

	// 可以用:匹配路径，可以用*做可选路径匹配（匹配的结果带/）
	 However, this one will match /user/john/ and also /user/john/send
	 If no other routers match /user/john, it will redirect to /user/john/
	router.GET("/user/:name/*action", func(c *gin.Context) {
		name := c.Param("name")
		action := c.Param("action")
		message := name + " is " + action
		c.String(http.StatusOK, message)
	})

	 可以用完整路径匹配
	 For each matched request Context will hold the route definition
	router.POST("/user/:name/*action", func(c *gin.Context) {
		b := c.FullPath() == "/user/:name/*action" // true
		c.String(http.StatusOK, "%t, %v", b, c.FullPath())
	})

	// 如果没有:或*	则表示路由组，可保证路由树解析优先级高于:路径匹配
	// This handler will add a new router for /user/groups.
	// Exact routes are resolved before param routes, regardless of the order they were defined.
	// Routes starting with /user/groups are never interpreted as /user/:name/... routes
	router.GET("/user/groups", func(c *gin.Context) {
		c.String(http.StatusOK, "The available groups are [...]")
	})

	router.Run(":8080")
}

可以用*来匹配变长 url，例如需求为（定义了一个路由 /a/:name/d ，真实请求的url为 /a/b/c/d ，怎么能让参数name匹配成 b/c），示例如下：

query

package main

import (
	"github.com/gin-gonic/gin"
	"net/http"
)

func main() {
	router := gin.Default()

	// Query string parameters are parsed using the existing underlying request object.
	// The request responds to an url matching:  /welcome?firstname=Jane&lastname=Doe
	router.GET("/welcome", func(c *gin.Context) {
		firstname := c.DefaultQuery("firstname", "Guest")
		lastname := c.Query("lastname") // shortcut for c.Request.URL.Query().Get("lastname")

		c.String(http.StatusOK, "Hello %s %s", firstname, lastname)
	})
	router.Run(":8080")
}

Multipart/Urlencoded Form

package main

import (
	"github.com/gin-gonic/gin"
	"net/http"
)

func main() {
	router := gin.Default()

	router.POST("/form_post", func(c *gin.Context) {
		message := c.PostForm("message")
		nick := c.DefaultPostForm("nick", "anonymous")

		c.JSON(http.StatusOK, gin.H{
			"status":  "posted",
			"message": message,
			"nick":    nick,
		})
	})
	router.Run(":8080")
}

解析请求

MultipartFrom

// ParseMultipartForm 解析 multipart/form-data 类型的 Content-Type，其将收到的文件先存储在内存中，若超限则存储在磁盘中
// ParseMultipartForm parses a request body as multipart/form-data.
// The whole request body is parsed and up to a total of maxMemory bytes of
// its file parts are stored in memory, with the remainder stored on
// disk in temporary files.
// ParseMultipartForm calls ParseForm if necessary.
// If ParseForm returns an error, ParseMultipartForm returns it but also
// continues parsing the request body.
// After one call to ParseMultipartForm, subsequent calls have no effect.
func (r *Request) ParseMultipartForm(maxMemory int64) error {
	if r.MultipartForm == multipartByReader {
		return errors.New("http: multipart handled by MultipartReader")
	}
	var parseFormErr error
	if r.Form == nil {
		// Let errors in ParseForm fall through, and just
		// return it at the end.
		parseFormErr = r.ParseForm()
	}
	if r.MultipartForm != nil {
		return nil
	}

	mr, err := r.multipartReader(false)
	if err != nil {
		return err
	}

	f, err := mr.ReadForm(maxMemory)
	if err != nil {
		return err
	}

	if r.PostForm == nil {
		r.PostForm = make(url.Values)
	}
	for k, v := range f.Value {
		r.Form[k] = append(r.Form[k], v...)
		// r.PostForm should also be populated. See Issue 9305.
		r.PostForm[k] = append(r.PostForm[k], v...)
	}

	r.MultipartForm = f

	return parseFormErr
}

用 postman 传递的方式如下：

在 gin 解析参数时，可用 multipartForm.Value["k"] 方式接收，得到 []string：

解析的参数定义如下：

// Form is a parsed multipart form.
// Its File parts are stored either in memory or on disk,
// and are accessible via the *FileHeader's Open method.
// Its Value parts are stored as strings.
// Both are keyed by field name.
type Form struct {
	Value map[string][]string
	File  map[string][]*FileHeader
}

// A FileHeader describes a file part of a multipart request.
type FileHeader struct {
	Filename string
	Header   textproto.MIMEHeader
	Size     int64

	content []byte
	tmpfile string
}

MiddleWare

gin 的 middleware 和 处理函数，都是 HandlerFunc 类型。

// HandlerFunc defines the handler used by gin middleware as return value.
type HandlerFunc func(*Context)

// HandlersChain defines a HandlerFunc slice.
type HandlersChain []HandlerFunc

gin 有各种中间件，是通过数组实现的。