如何将Postman授权POST转为VBA JSON POST并解决invalid_grant错误

我用Postman向Google发送授权请求获取access token，在Authorization选项卡填完所有必填信息并点击Get New Access Token按钮后，成功拿到了返回的access token，说明数据有效且Google项目配置正确。现在要把这个请求转成VBA代码。

查看Postman日志能看到发送给Google的数据和返回的响应。我写了如下VBA代码复现请求（redirect_uri已经设为Google项目中批准的本地地址）：

Dim sClient_ID As String
Dim sClient_Secret As String
Dim sGrant_Type As String
Dim sCode As String
Dim sRedirect_URI As String
sClient_ID = "{the client id that google provided to me}"
sClient_Secret = "{the client secret that google provided to me}"
sGrant_Type = "authorization_code"
sCode = "4/0AWtgzh75ZFps55t9vPx-gm_rm8W_uyWQbZwBF1qTLthpUuPjaAXBr9iywT-RweVvagcGPg"
sRedirect_URI = "https://localhost:8080"

' build the json string which will be sent to get the google access token

Dim a As New Scripting.Dictionary

a.Add "grant_type", sGrant_Type
a.Add "code", sCode
a.Add "redirect_uri", sRedirect_URI
a.Add "client_id", sClient_ID
a.Add "client_secret", sClient_Secret

Dim Json_Get_Access_Token As String
Json_Get_Access_Token = JsonConverter.ConvertToJson(a, Whitespace:=" ")

' send the json string via POST
Set httpCall = CreateObject("MSXML2.ServerXMLHTTP")
Dim sTokenURL As String
sTokenURL = "https://oauth2.googleapis.com/token"
httpCall.Open "POST", sTokenURL, False
httpCall.setRequestHeader "Content-Type", "application/json;charset=UTF-8"
httpCall.Send Json_Get_Access_Token
Dim sReturnToken As String
sReturnToken = httpCall.responseText

运行代码后，sReturnToken返回错误：

{
  "error": "invalid_grant",
  "error_description": "Bad Request"
}

请问哪里配置错了？

出现invalid_grant错误，结合你的场景，主要排查以下几个点：

1. 请求格式不匹配 #

Google OAuth2的token端点不接受JSON格式的请求体，需要用application/x-www-form-urlencoded（表单键值对）格式。Postman在发送请求时，默认会把Authorization选项卡的参数转成表单格式发送，而你的VBA代码用了JSON格式，这是核心问题。

修改VBA代码，把请求体改成表单格式：

Dim sClient_ID As String
Dim sClient_Secret As String
Dim sGrant_Type As String
Dim sCode As String
Dim sRedirect_URI As String
sClient_ID = "{the client id that google provided to me}"
sClient_Secret = "{the client secret that google provided to me}"
sGrant_Type = "authorization_code"
sCode = "4/0AWtgzh75ZFps55t9vPx-gm_rm8W_uyWQbZwBF1qTLthpUuPjaAXBr9iywT-RweVvagcGPg"
sRedirect_URI = "https://localhost:8080"

' 构建表单格式请求体，需URL编码避免特殊字符问题
Dim formData As String
formData = "grant_type=" & URLEncode(sGrant_Type) & _
           "&code=" & URLEncode(sCode) & _
           "&redirect_uri=" & URLEncode(sRedirect_URI) & _
           "&client_id=" & URLEncode(sClient_ID) & _
           "&client_secret=" & URLEncode(sClient_Secret)

' 发送POST请求
Set httpCall = CreateObject("MSXML2.ServerXMLHTTP")
Dim sTokenURL As String
sTokenURL = "https://oauth2.googleapis.com/token"
httpCall.Open "POST", sTokenURL, False
httpCall.setRequestHeader "Content-Type", "application/x-www-form-urlencoded"
httpCall.Send formData
Dim sReturnToken As String
sReturnToken = httpCall.responseText

添加URL编码函数处理特殊字符：

Function URLEncode(str As String) As String
    Dim objURL As Object
    Set objURL = CreateObject("MSXML2.XMLHTTP")
    URLEncode = objURL.encodeURIComponent(str)
    Set objURL = Nothing
End Function

2. Authorization Code已过期或被使用 #

Google的Authorization Code是一次性的，只能使用一次。如果Postman已经用过这个code获取过token，VBA再用同一个code就会返回invalid_grant。

• 解决方法：重新生成新的Authorization Code后再用VBA请求。

3. Redirect URI完全匹配问题 #

确保VBA中的sRedirect_URI和Postman使用的、Google项目中配置的完全一致，包括协议（http/https）、端口、路径，不能有任何差异（比如末尾是否带斜杠）。

4. 系统时间偏差过大 #

如果本地系统时间和Google服务器时间偏差超过5分钟，也会触发invalid_grant错误。检查并同步本地系统时间即可。

内容的提问来源于stack exchange，提问作者Jonathan Small