为什么Google OAuth 2.0代码令牌交换会返回“ invalid_request”？

Question

I have successfully got the authorization_code . And getting "Error" : "invalid_request" in response for authorization_code-token exchange .
Here is my java code to get the Google OAuth token in exchange to authentication_code:
(Made use of HttpComponents for HTTP requests)

String urlString = "https://accounts.google.com/o/oauth2/token";
String client_id = "<my_client_id>";
String client_secret = "<my_client_secret>";
String redirect_uri = "<my_redirect_url>";
String grant_type = "authorization_code";
HttpParams params = new BasicHttpParams();
params.setParameter("code", code);
params.setParameter("client_id", client_id);
params.setParameter("client_secret", client_secret);
params.setParameter("redirect_uri", redirect_uri);
params.setParameter("grant_type", grant_type);
HttpPost post = new HttpPost(urlString);
post.addHeader("Content-Type", "application/x-www-form-urlencoded");
post.setParams(params);
DefaultHttpClient httpClient = new DefaultHttpClient();
try {
    HttpResponse response = httpClient.execute(post);
    HttpEntity entity = response.getEntity();
    System.out.println(response.toString());
    DataInputStream in = new DataInputStream(entity.getContent());
    String line;
    while ((line = in.readLine()) != null) {
        System.out.println(line);
    }
}

Getting the following Error in response:

HTTP/1.1 400 Bad Request [Cache-Control: no-cache, no-store, max-age=0, must-revalidate, Pragma: no-cache, Expires: Fri, 01 Jan 1990 00:00:00 GMT, Date: Thu, 21 Feb 2013 11:39:04 GMT, Content-Type: application/json, X-Content-Type-Options: nosniff, X-Frame-Options: SAMEORIGIN, X-XSS-Protection: 1; mode=block, Server: GSE, Transfer-Encoding: chunked]
{
  "error" : "invalid_request"
}

Is there any way to know what exactly caused the error?
Or could you find any fault with the request?

Answer 1

Though I don't have personal experience with the Apache library, it looks like that the parameters are sent as query parameters instead of posted form parameters. According to this HttpClient Quick Start , it should look like:

HttpPost httpPost = new HttpPost(urlString);
List <NameValuePair> nvps = new ArrayList <NameValuePair>();
nvps.add(new BasicNameValuePair("code", code));
nvps.add(new BasicNameValuePair("client_id", client_id));
nvps.add(new BasicNameValuePair("client_secret", client_secret));
nvps.add(new BasicNameValuePair("redirect_uri", redirect_uri));
nvps.add(new BasicNameValuePair("grant_type", grant_type));
httpPost.setEntity(new UrlEncodedFormEntity(nvps));
HttpResponse response = httpclient.execute(httpPost);