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echo 源码分析之数据绑定过程

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2021-05-10 17:58

我们知道http的参数传递的形式有很多，header、path、query、body，body( json,form)等等，针对各种形式的参数，通过bind方式来解析是比较清晰的方式，但是echo的bind 方式也是在从不完完善到逐渐完善的过程中，实践中你会发现，不同版本的echo，会出现诡异的结果，我将以下面的例子，针对v3.3.10、v4.1.17、v4.2.1三个版本的echo进行分析。

package main
import (  "fmt"  "net/http"
  "github.com/labstack/echo/v4")
type User struct {  Name  string `json:"name" xml:"name` //param:"name" query:"name" form:"name"   //curl -XGET http://localhost:1323/users/Joe\?email\=joe_email  Email string `json:"email" form:"email" query:"email"`}
func main() {  e := echo.New()  e.GET("/users/:name", func(c echo.Context) error {    u := new(User)    u.Name = c.Param("name")    if err := c.Bind(u); err != nil {      return c.JSON(http.StatusBadRequest, nil)    }    return c.JSON(http.StatusOK, u)  })  fmt.Println("--------------------")  e.GET("/users/:name/share/:id", func(c echo.Context) error {    u := new(User)    //u.Name = c.Param("name")    if err := c.Bind(u); err != nil {      return c.JSON(http.StatusBadRequest, nil)    }    return c.JSON(http.StatusOK, u)  })  fmt.Println("--------------------")  e.GET("/users/names", func(c echo.Context) error {    u := new(User)    if err := c.Bind(u); err != nil {      return c.JSON(http.StatusBadRequest, nil)    }    return c.JSON(http.StatusOK, u)  })  fmt.Println("--------------------")  e.GET("/users/names/*", func(c echo.Context) error {    u := new(User)    if err := c.Bind(u); err != nil {      return c.JSON(http.StatusBadRequest, nil)    }    return c.JSON(http.StatusOK, u)  })  fmt.Println(e.Start(":1336"))}

如果我们引用

 "github.com/labstack/echo"

默认版本是v3.3.10

如果引用

 "github.com/labstack/echo/v4"

默认是最新版v4.2.1，但是v4.2.1和v4.1.17版本差异比较大，所以分析上述三个版本。

首先看下路由注册的过程

e.GET("/users/:name", func(c echo.Context) error {    u := new(User)    u.Name = c.Param("name")    if err := c.Bind(u); err != nil {      return c.JSON(http.StatusBadRequest, nil)    }    return c.JSON(http.StatusOK, u)  })
func (e *Echo) Add(method, path string, handler HandlerFunc, middleware ...MiddlewareFunc) *Route {  name := handlerName(handler)  e.router.Add(method, path, func(c Context) error {    h := handler    // Chain middleware    for i := len(middleware) - 1; i >= 0; i-- {      h = middleware[i](h)    }    return h(c)  })  r := &Route{    Method: method,    Path:   path,    Name:   name,  }  e.router.routes[method+path] = r  return r}
func (r *Router) Add(method, path string, h HandlerFunc) {  // Validate path  if path == "" {    panic("echo: path cannot be empty")  }  if path[0] != '/' {    path = "/" + path  }  pnames := []string{} // Param names  ppath := path        // Pristine path
  for i, l := 0, len(path); i < l; i++ {    if path[i] == ':' {      j := i + 1
      r.insert(method, path[:i], nil, skind, "", nil)      for ; i < l && path[i] != '/'; i++ {      }
      pnames = append(pnames, path[j:i])      path = path[:j] + path[i:]      i, l = j, len(path)
      if i == l {        r.insert(method, path[:i], h, pkind, ppath, pnames)        return      }      r.insert(method, path[:i], nil, pkind, "", nil)    } else if path[i] == '*' {      r.insert(method, path[:i], nil, skind, "", nil)      pnames = append(pnames, "*")      r.insert(method, path[:i+1], h, akind, ppath, pnames)      return    }  }
  r.insert(method, path, h, skind, ppath, pnames)}

这里可以看到，在路由注册构建前缀树的过程中会把路由解析规整为三个类型，路径参数类型（：），精确匹配路由（/），正则匹配路由（*）

同时针对路径参数类型（：），会将路径中的参数名字保存在变量pnames里面，最终存在router的tree上

func (r *Router) insert(method, path string, h HandlerFunc, t kind, ppath string, pnames []string) {  // Adjust max param  l := len(pnames)  if *r.echo.maxParam < l {    *r.echo.maxParam = l  }
  cn := r.tree // Current node as root  if cn == nil {    panic("echo: invalid method")  }  search := path
  for {    sl := len(search)    pl := len(cn.prefix)    l := 0
    // LCP    max := pl    if sl < max {      max = sl    }    for ; l < max && search[l] == cn.prefix[l]; l++ {    }
    if l == 0 {      // At root node      cn.label = search[0]      cn.prefix = search      if h != nil {        cn.kind = t        cn.addHandler(method, h)        cn.ppath = ppath        cn.pnames = pnames      }    } else if l < pl {      // Split node      n := newNode(cn.kind, cn.prefix[l:], cn, cn.children, cn.methodHandler, cn.ppath, cn.pnames)
      // Reset parent node      cn.kind = skind      cn.label = cn.prefix[0]      cn.prefix = cn.prefix[:l]      cn.children = nil      cn.methodHandler = new(methodHandler)      cn.ppath = ""      cn.pnames = nil
      cn.addChild(n)
      if l == sl {        // At parent node        cn.kind = t        cn.addHandler(method, h)        cn.ppath = ppath        cn.pnames = pnames      } else {        // Create child node        n = newNode(t, search[l:], cn, nil, new(methodHandler), ppath, pnames)        n.addHandler(method, h)        cn.addChild(n)      }    } else if l < sl {      search = search[l:]      c := cn.findChildWithLabel(search[0])      if c != nil {        // Go deeper        cn = c        continue      }      // Create child node      n := newNode(t, search, cn, nil, new(methodHandler), ppath, pnames)      n.addHandler(method, h)      cn.addChild(n)    } else {      // Node already exists      if h != nil {        cn.addHandler(method, h)        cn.ppath = ppath        if len(cn.pnames) == 0 { // Issue #729          cn.pnames = pnames        }      }    }    return  }}

接着我们看下，请求到来的时候，参数匹配的过程


func (e *Echo) ServeHTTP(w http.ResponseWriter, r *http.Request) {  // Acquire context  c := e.pool.Get().(*context)  c.Reset(r, w)
  h := NotFoundHandler
  if e.premiddleware == nil {    e.router.Find(r.Method, getPath(r), c)    h = c.Handler()    for i := len(e.middleware) - 1; i >= 0; i-- {      h = e.middleware[i](h)    }  } else {    h = func(c Context) error {      e.router.Find(r.Method, getPath(r), c)      h := c.Handler()      for i := len(e.middleware) - 1; i >= 0; i-- {        h = e.middleware[i](h)      }      return h(c)    }    for i := len(e.premiddleware) - 1; i >= 0; i-- {      h = e.premiddleware[i](h)    }  }
  // Execute chain  if err := h(c); err != nil {    e.HTTPErrorHandler(err, c)  }
  // Release context  e.pool.Put(c)}

其实就是到router中通过最长前缀匹配算法进行匹配

e.router.Find(r.Method, getPath(r), c)

其中getPath函数定义如下

func getPath(r *http.Request) string {  path := r.URL.RawPath  if path == "" {    path = r.URL.Path  }  return path}

find是路径匹配的过程


func (r *Router) Find(method, path string, c Context) {  ctx := c.(*context)  ctx.path = path  cn := r.tree // Current node as root
  var (    search  = path    child   *node         // Child node    n       int           // Param counter    nk      kind          // Next kind    nn      *node         // Next node    ns      string        // Next search    pvalues = ctx.pvalues // Use the internal slice so the interface can keep the illusion of a dynamic slice  )
  // Search order static > param > any  for {    if search == "" {      break    }
    pl := 0 // Prefix length    l := 0  // LCP length
    if cn.label != ':' {      sl := len(search)      pl = len(cn.prefix)
      // LCP      max := pl      if sl < max {        max = sl      }      for ; l < max && search[l] == cn.prefix[l]; l++ {      }    }
    if l == pl {      // Continue search      search = search[l:]    } else {      cn = nn      search = ns      if nk == pkind {        goto Param      } else if nk == akind {        goto Any      }      // Not found      return    }
    if search == "" {      break    }
    // Static node    if child = cn.findChild(search[0], skind); child != nil {      // Save next      if cn.prefix[len(cn.prefix)-1] == '/' { // Issue #623        nk = pkind        nn = cn        ns = search      }      cn = child      continue    }
    // Param node  Param:    if child = cn.findChildByKind(pkind); child != nil {      // Issue #378      if len(pvalues) == n {        continue      }
      // Save next      if cn.prefix[len(cn.prefix)-1] == '/' { // Issue #623        nk = akind        nn = cn        ns = search      }
      cn = child      i, l := 0, len(search)      for ; i < l && search[i] != '/'; i++ {      }      pvalues[n] = search[:i]      n++      search = search[i:]      continue    }
    // Any node  Any:    if cn = cn.findChildByKind(akind); cn == nil {      if nn != nil {        cn = nn        nn = cn.parent // Next (Issue #954)        search = ns        if nk == pkind {          goto Param        } else if nk == akind {          goto Any        }      }      // Not found      return    }    pvalues[len(cn.pnames)-1] = search    break  }
  ctx.handler = cn.findHandler(method)  ctx.path = cn.ppath  ctx.pnames = cn.pnames
  // NOTE: Slow zone...  if ctx.handler == nil {    ctx.handler = cn.checkMethodNotAllowed()
    // Dig further for any, might have an empty value for *, e.g.    // serving a directory. Issue #207.    if cn = cn.findChildByKind(akind); cn == nil {      return    }    if h := cn.findHandler(method); h != nil {      ctx.handler = h    } else {      ctx.handler = cn.checkMethodNotAllowed()    }    ctx.path = cn.ppath    ctx.pnames = cn.pnames    pvalues[len(cn.pnames)-1] = ""  }
  return}

可以看到，将匹配到的值和路径参数名一一对应保存到了pvalues里面，最终是保存再来ctx里面

可以看到匹配过程中，会根据路径参数类型来进行处理

func (n *node) findChild(l byte, t kind) *node {  for _, c := range n.children {    if c.label == l && c.kind == t {      return c    }  }  return nil}

有没有简单直接的方法来查看我们最终路由注册后pnames的存储结果和请求路径匹配过程中pvalues的参数匹配结果呢？可以在echo中，加下面几行代码，进行打印

func (e *Echo) Add(method, path string, handler HandlerFunc, middleware ...MiddlewareFunc) *Route {  e.router.routes[method+path] = r  printTree(e.router.tree)  return r}

其中printTree的定义如下

func printTree(tree *node) {  v1, err1 := json.Marshal(struct {    Kind          kind    Label         byte    Prefix        string    Parent        *node    Children      children    ChildrenNum   int    Ppath         string    Pnames        []string    MethodHandler *methodHandler  }{    Kind:          tree.kind,    Label:         tree.label,    Prefix:        tree.prefix,    Parent:        tree.parent,    Children:      tree.children,    ChildrenNum:   len(tree.children),    Ppath:         tree.ppath,    Pnames:        tree.pnames,    MethodHandler: tree.methodHandler,  })  fmt.Println(string(v1), err1)  for i, v := range tree.children {    fmt.Println(i)    printTree(v)  }}

可以看到我们的路由注册结果

{"Kind":0,"Label":47,"Prefix":"/users/","Parent":null,"Children":[{}],"ChildrenNum":1,"Ppath":"","Pnames":null,"MethodHandler":{}} <nil>0{"Kind":1,"Label":58,"Prefix":":","Parent":{},"Children":null,"ChildrenNum":0,"Ppath":"/users/:name","Pnames":["name"],"MethodHandler":{}} <nil>--------------------{"Kind":0,"Label":47,"Prefix":"/users/","Parent":null,"Children":[{}],"ChildrenNum":1,"Ppath":"","Pnames":null,"MethodHandler":{}} <nil>0{"Kind":1,"Label":58,"Prefix":":","Parent":{},"Children":[{}],"ChildrenNum":1,"Ppath":"/users/:name","Pnames":["name"],"MethodHandler":{}} <nil>0{"Kind":0,"Label":47,"Prefix":"/share/","Parent":{},"Children":[{}],"ChildrenNum":1,"Ppath":"","Pnames":null,"MethodHandler":{}} <nil>0{"Kind":1,"Label":58,"Prefix":":","Parent":{},"Children":null,"ChildrenNum":0,"Ppath":"/users/:name/share/:id","Pnames":["name","id"],"MethodHandler":{}} <nil>--------------------{"Kind":0,"Label":47,"Prefix":"/users/","Parent":null,"Children":[{},{}],"ChildrenNum":2,"Ppath":"","Pnames":null,"MethodHandler":{}} <nil>0{"Kind":1,"Label":58,"Prefix":":","Parent":{},"Children":[{}],"ChildrenNum":1,"Ppath":"/users/:name","Pnames":["name"],"MethodHandler":{}} <nil>0{"Kind":0,"Label":47,"Prefix":"/share/","Parent":{},"Children":[{}],"ChildrenNum":1,"Ppath":"","Pnames":null,"MethodHandler":{}} <nil>0{"Kind":1,"Label":58,"Prefix":":","Parent":{},"Children":null,"ChildrenNum":0,"Ppath":"/users/:name/share/:id","Pnames":["name","id"],"MethodHandler":{}} <nil>1{"Kind":0,"Label":110,"Prefix":"names","Parent":{},"Children":null,"ChildrenNum":0,"Ppath":"/users/names","Pnames":[],"MethodHandler":{}} <nil>--------------------{"Kind":0,"Label":47,"Prefix":"/users/","Parent":null,"Children":[{},{}],"ChildrenNum":2,"Ppath":"","Pnames":null,"MethodHandler":{}} <nil>0{"Kind":1,"Label":58,"Prefix":":","Parent":{},"Children":[{}],"ChildrenNum":1,"Ppath":"/users/:name","Pnames":["name"],"MethodHandler":{}} <nil>0{"Kind":0,"Label":47,"Prefix":"/share/","Parent":{},"Children":[{}],"ChildrenNum":1,"Ppath":"","Pnames":null,"MethodHandler":{}} <nil>0{"Kind":1,"Label":58,"Prefix":":","Parent":{},"Children":null,"ChildrenNum":0,"Ppath":"/users/:name/share/:id","Pnames":["name","id"],"MethodHandler":{}} <nil>1{"Kind":0,"Label":110,"Prefix":"names","Parent":{},"Children":[{}],"ChildrenNum":1,"Ppath":"/users/names","Pnames":[],"MethodHandler":{}} <nil>0{"Kind":0,"Label":47,"Prefix":"/","Parent":{},"Children":[{}],"ChildrenNum":1,"Ppath":"","Pnames":null,"MethodHandler":{}} <nil>0{"Kind":2,"Label":42,"Prefix":"*","Parent":{},"Children":null,"ChildrenNum":0,"Ppath":"/users/names/*","Pnames":["*"],"MethodHandler":{}} <nil>

如何看参数匹配结果呢？同样处理


func (e *Echo) ServeHTTP(w http.ResponseWriter, r *http.Request) {  e.router.Find(r.Method, getPath(r), c)}

在find函数里加下面代码


func (r *Router) Find(method, path string, c Context) {  v, err := json.Marshal(struct {    //Request *http.Request    //Response *Response    Path    string    Pnames  []string    Pvalues []string    Query   url.Values    //Handler HandlerFunc    Store Map  }{    //Request: ctx.request,    //Response: ctx.response,    Path:    ctx.path,    Pnames:  ctx.pnames,    Pvalues: ctx.pvalues,    Query:   ctx.query,    //Handler: ctx.handler,    Store: ctx.store,  })  fmt.Println(string(v), err)  return}

//{"Path":"/users/:name","Pnames":["name"],"Pvalues":["Joe",""],"Query":null,"Store":null} <nil>//{"Path":"/users/:name/share/:id","Pnames":["name","id"],"Pvalues":["Joe","1"],"Query":null,"Store":null} <nil>

这时候我们切换不同版本的echo，可以看到不同的结果

% curl -XGET http://localhost:1336/users/Joe/share\?email\=joe_email{"message":"Not Found"}% curl -XGET http://localhost:1336/users/Joe/share/1\?email\=joe_email{"name":"Joe","email":"joe_email"}

首先看v3.3.10的实现

func (b *DefaultBinder) Bind(i interface{}, c Context) (err error) {  if req.ContentLength == 0 {        if err = b.bindData(i, c.QueryParams(), "query"); err != nil {        }    }
    ctype := req.Header.Get(HeaderContentType)  switch {  case strings.HasPrefix(ctype, MIMEApplicationJSON):    if err = json.NewDecoder(req.Body).Decode(i); err != nil {
        }    }}

这个实现是有问题的，因为即使是get请求，ContentLength也不会是0


% curl -i -XGET http://localhost:1336/users/Joe/share/1\?email\=joe_emailHTTP/1.1 200 OKContent-Type: application/json; charset=UTF-8Date: Tue, 30 Mar 2021 03:40:22 GMTContent-Length: 35{"name":"","email":"joe_email"}

针对contentlength=0的情况，调用了bindData方法


func (b *DefaultBinder) bindData(ptr interface{}, data map[string][]string, tag string) error {  typ := reflect.TypeOf(ptr).Elem()  val := reflect.ValueOf(ptr).Elem()    for i := 0; i < typ.NumField(); i++ {        inputFieldName := typeField.Tag.Get(tag)
              // If tag is nil, we inspect if the field is a struct.      if _, ok := bindUnmarshaler(structField); !ok && structFieldKind == reflect.Struct {        if err := b.bindData(structField.Addr().Interface(), data, tag); err != nil {                }            }
            inputValue, exists := data[inputFieldName]
    }}

里面其实是应用了反射，对结构体的值进行了修改，可以看到，如果header里contentlength不为零，路径里的参数根本匹配不上

其中QueryParams()返回的是url里面的值

QueryParams() url.Values

我们升级到v4.1.17看看

% go get -u github.com/labstack/echo/v4@v4.1.17go: finding module for package github.com/labstack/echo代码里引用的地方也要由"github.com/labstack/echo"改成"github.com/labstack/echo/v4"否则会go: found github.com/labstack/echo in github.com/labstack/echo v3.3.10+incompatible

路径参数绑定成功了 

 % curl -i -XGET http://localhost:1336/users/Joe/share/1\?email\=joe_emailHTTP/1.1 200 OKContent-Type: application/json; charset=UTF-8Date: Tue, 30 Mar 2021 05:21:10 GMTContent-Length: 35
{"name":"Joe","email":"joe_email"}

我们发现参数绑定方法已经重写了

// Bind implements the `Binder#Bind` function.func (b *DefaultBinder) Bind(i interface{}, c Context) (err error) {  req := c.Request()
  names := c.ParamNames()  values := c.ParamValues()  params := map[string][]string{}  for i, name := range names {    params[name] = []string{values[i]}  }  if err := b.bindData(i, params, "param"); err != nil {    return NewHTTPError(http.StatusBadRequest, err.Error()).SetInternal(err)  }  if err = b.bindData(i, c.QueryParams(), "query"); err != nil {    return NewHTTPError(http.StatusBadRequest, err.Error()).SetInternal(err)  }  if req.ContentLength == 0 {    return  }  ctype := req.Header.Get(HeaderContentType)  switch {  case strings.HasPrefix(ctype, MIMEApplicationJSON):    if err = json.NewDecoder(req.Body).Decode(i); err != nil {      if ute, ok := err.(*json.UnmarshalTypeError); ok {        return NewHTTPError(http.StatusBadRequest, fmt.Sprintf("Unmarshal type error: expected=%v, got=%v, field=%v, offset=%v", ute.Type, ute.Value, ute.Field, ute.Offset)).SetInternal(err)      } else if se, ok := err.(*json.SyntaxError); ok {        return NewHTTPError(http.StatusBadRequest, fmt.Sprintf("Syntax error: offset=%v, error=%v", se.Offset, se.Error())).SetInternal(err)      }      return NewHTTPError(http.StatusBadRequest, err.Error()).SetInternal(err)    }  }  return}

依次会对路径参数param，query参数query，以及body进行绑定，body绑定依赖http的header

下面是bindData函数

func (b *DefaultBinder) bindData(ptr interface{}, data map[string][]string, tag string) error {  if ptr == nil || len(data) == 0 {    return nil  }  typ := reflect.TypeOf(ptr).Elem()  val := reflect.ValueOf(ptr).Elem()
  // Map  if typ.Kind() == reflect.Map {    for k, v := range data {      val.SetMapIndex(reflect.ValueOf(k), reflect.ValueOf(v[0]))    }    return nil  }
  // !struct  if typ.Kind() != reflect.Struct {    return errors.New("binding element must be a struct")  }
  for i := 0; i < typ.NumField(); i++ {    typeField := typ.Field(i)    structField := val.Field(i)    if !structField.CanSet() {      continue    }    structFieldKind := structField.Kind()    inputFieldName := typeField.Tag.Get(tag)
    if inputFieldName == "" {      inputFieldName = typeField.Name //在4.2.1中删除了这个字段      // If tag is nil, we inspect if the field is a struct.      if _, ok := structField.Addr().Interface().(BindUnmarshaler); !ok && structFieldKind == reflect.Struct {        if err := b.bindData(structField.Addr().Interface(), data, tag); err != nil {          return err        }        continue      }    }
    inputValue, exists := data[inputFieldName]    if !exists {      // Go json.Unmarshal supports case insensitive binding.  However the      // url params are bound case sensitive which is inconsistent.  To      // fix this we must check all of the map values in a      // case-insensitive search.      for k, v := range data {        if strings.EqualFold(k, inputFieldName) {          inputValue = v          exists = true          break        }      }    }
    if !exists {      continue    }
    // Call this first, in case we're dealing with an alias to an array type    if ok, err := unmarshalField(typeField.Type.Kind(), inputValue[0], structField); ok {      if err != nil {        return err      }      continue    }
    numElems := len(inputValue)    if structFieldKind == reflect.Slice && numElems > 0 {      sliceOf := structField.Type().Elem().Kind()      slice := reflect.MakeSlice(structField.Type(), numElems, numElems)      for j := 0; j < numElems; j++ {        if err := setWithProperType(sliceOf, inputValue[j], slice.Index(j)); err != nil {          return err        }      }      val.Field(i).Set(slice)    } else if err := setWithProperType(typeField.Type.Kind(), inputValue[0], structField); err != nil {      return err
    }  }  return nil}

可以看到，在匹配路径参数的过程中，如果结构体的tag里没有param，会选取结构体的参数名，到路径参数里去取对应的value

    inputFieldName := typeField.Tag.Get(tag)
    if inputFieldName == "" {      inputFieldName = typeField.Name //在4.2.1中删除了这个字段      // If tag is nil, we inspect if the field is a struct.      if _, ok := structField.Addr().Interface().(BindUnmarshaler); !ok && structFieldKind == reflect.Struct {        if err := b.bindData(structField.Addr().Interface(), data, tag); err != nil {          return err        }        continue      }    }

也就是说，struct的tag即使不正确，也是可能匹配到正确参数的

最后看看v4.2.1的实现

go get -u github.com/labstack/echo/v4
func (b *DefaultBinder) Bind(i interface{}, c Context) (err error) {  if err := b.BindPathParams(c, i); err != nil {    return err  }      if c.Request().Method == http.MethodGet || c.Request().Method == http.MethodDelete {    if err = b.BindQueryParams(c, i); err != nil {      return err    }  }  return b.BindBody(c, i)}

func (b *DefaultBinder) BindPathParams(c Context, i interface{}) error {  names := c.ParamNames()  values := c.ParamValues()  params := map[string][]string{}  for i, name := range names {    params[name] = []string{values[i]}  }  if err := b.bindData(i, params, "param"); err != nil {    return NewHTTPError(http.StatusBadRequest, err.Error()).SetInternal(err)  }  return nil}

func (b *DefaultBinder) BindQueryParams(c Context, i interface{}) error {  if err := b.bindData(i, c.QueryParams(), "query"); err != nil {    return NewHTTPError(http.StatusBadRequest, err.Error()).SetInternal(err)  }  return nil}

func (b *DefaultBinder) BindBody(c Context, i interface{}) (err error) {  req := c.Request()  if req.ContentLength == 0 {    return  }
  ctype := req.Header.Get(HeaderContentType)  switch {  case strings.HasPrefix(ctype, MIMEApplicationJSON):    if err = json.NewDecoder(req.Body).Decode(i); err != nil {        }    }}

func (b *DefaultBinder) bindData(destination interface{}, data map[string][]string, tag string) error {  if destination == nil || len(data) == 0 {    return nil  }  typ := reflect.TypeOf(destination).Elem()  val := reflect.ValueOf(destination).Elem()
  // Map  if typ.Kind() == reflect.Map {    for k, v := range data {      val.SetMapIndex(reflect.ValueOf(k), reflect.ValueOf(v[0]))    }    return nil  }
  // !struct  if typ.Kind() != reflect.Struct {    return errors.New("binding element must be a struct")  }
  for i := 0; i < typ.NumField(); i++ {    typeField := typ.Field(i)    structField := val.Field(i)    if !structField.CanSet() {      continue    }    structFieldKind := structField.Kind()    inputFieldName := typeField.Tag.Get(tag)
    if inputFieldName == "" {      // If tag is nil, we inspect if the field is a not BindUnmarshaler struct and try to bind data into it (might contains fields with tags).      // structs that implement BindUnmarshaler are binded only when they have explicit tag      if _, ok := structField.Addr().Interface().(BindUnmarshaler); !ok && structFieldKind == reflect.Struct {        if err := b.bindData(structField.Addr().Interface(), data, tag); err != nil {          return err        }      }      // does not have explicit tag and is not an ordinary struct - so move to next field      continue  //注意从哪部移动出来了，所以，没有tag就不继续了    }
    inputValue, exists := data[inputFieldName]    if !exists {      // Go json.Unmarshal supports case insensitive binding.  However the      // url params are bound case sensitive which is inconsistent.  To      // fix this we must check all of the map values in a      // case-insensitive search.      for k, v := range data {        if strings.EqualFold(k, inputFieldName) {          inputValue = v          exists = true          break        }      }    }
    if !exists {      continue    }
    // Call this first, in case we're dealing with an alias to an array type    if ok, err := unmarshalField(typeField.Type.Kind(), inputValue[0], structField); ok {      if err != nil {        return err      }      continue    }
    numElems := len(inputValue)    if structFieldKind == reflect.Slice && numElems > 0 {      sliceOf := structField.Type().Elem().Kind()      slice := reflect.MakeSlice(structField.Type(), numElems, numElems)      for j := 0; j < numElems; j++ {        if err := setWithProperType(sliceOf, inputValue[j], slice.Index(j)); err != nil {          return err        }      }      val.Field(i).Set(slice)    } else if err := setWithProperType(typeField.Type.Kind(), inputValue[0], structField); err != nil {      return err
    }  }  return nil}

可以看到，匹配过程更严格了，严格要求按照结构体tag定义来匹配

 inputFieldName := typeField.Tag.Get(tag)
    if inputFieldName == "" {      // If tag is nil, we inspect if the field is a not BindUnmarshaler struct and try to bind data into it (might contains fields with tags).      // structs that implement BindUnmarshaler are binded only when they have explicit tag      if _, ok := structField.Addr().Interface().(BindUnmarshaler); !ok && structFieldKind == reflect.Struct {        if err := b.bindData(structField.Addr().Interface(), data, tag); err != nil {          return err        }      }      // does not have explicit tag and is not an ordinary struct - so move to next field      continue  //注意从哪部移动出来了，所以，没有tag就不继续了    }

好处是什么呢？可以处理同名参数

这两个版本可以具体diff一下看看改动

 vimdiff ~/go/pkg/mod/github.com/labstack/echo/v4@v4.1.17/bind.go ~/go/pkg/mod/github.com/labstack/echo/v4@v4.2.1/bind.go

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福利

我为大家整理了一份从入门到进阶的Go学习资料礼包，包含学习建议：入门看什么，进阶看什么。关注公众号 「polarisxu」，回复 ebook 获取；还可以回复「进群」，和数万 Gopher 交流学习。

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