从Android发送Json到wcf

Question

我在vb.net中创建了一个wcf服务。服务代码在这里：

 <ServiceContract()>
 Public Interface IServiceUpload
  <OperationContract()> _
  <WebInvoke(Method:="POST", BodyStyle:=WebMessageBodyStyle.Wrapped, RequestFormat:=WebMessageFormat.Json, ResponseFormat:=WebMessageFormat.Json,UriTemplate:="UploadFile")>
    Function UploadFile(F As FileStructure) As String

<DataContract()> _
Class FileStructure

    <DataMember()> _
    Public Property gp1 As String
 End Class
End Interface

我通过Android从此代码发送json数据：

        JSONStringer UPJ = new JSONStringer();

        try {
            UPJ.object()
                        .key("F").object().key("gp1")
                        .value("50").endObject().endObject();
        } catch (JSONException e) {
            e.printStackTrace();
        }
   OutputStream outputStream = new  BufferedOutputStream(urlConnection.getOutputStream());
            BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(outputStream, "UTF-8"));
          String upload=jsonObject2.toString();

            writer.write(UPJ.toString());

我的问题是我在尝试阅读服务中的F.gp1时总是返回http错误400。我认为发送的json是null。帮帮我吧！