百度获取访问令牌 - 客户端凭据流

Question

我想使用客户端凭据方法来获取访问令牌 我需要发送这样的请求：

https://openapi.baidu.com/oauth/2.0/token?
grant_type=client_credentials&
client_id=SOME_CLIENT_ID
client_secret= A_CLIENT_SECRET

client_id: API密钥
client_secret:密钥

但是当执行到httpResponse = httpclient.execute(httpRequest)时，它会导致错误......，我无法弄清楚发生了什么......

baidu参考：here

public class MainActivity extends Activity {


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    TextView tv = new TextView(this);
    String result = null;
    try {
        result = sendRequest();
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

    tv.setText(result);
    setContentView(tv);  

}

private String sendRequest() throws IOException{
    Log.i("sendRequest", "in"); 
    String httpUrl = "https://openapi.baidu.com/oauth/2.0/token?";      
    HttpClient httpclient = new DefaultHttpClient();
    HttpPost httpRequest = new HttpPost(httpUrl);    

    List<NameValuePair> params = new ArrayList<NameValuePair>(3);
    params.add(new BasicNameValuePair("grant_type", "client_credentials"));
    params.add(new BasicNameValuePair("client_id", "MY_ID"));
    params.add(new BasicNameValuePair("client_secret", "MY_SECRET"));
    httpRequest.setEntity(new UrlEncodedFormEntity(params));    

    HttpResponse httpResponse = null;

    try{      

        httpResponse = httpclient.execute(httpRequest);

    }catch(ClientProtocolException e){

    }catch(IOException e){

    }

    if (httpResponse.getStatusLine().getStatusCode() == HttpStatus.SC_OK)                
    {                   

        String strResult = EntityUtils.toString(httpResponse.getEntity());

        return strResult;

    }
    return null;

}

}