HttpURLConnection OutputStream出现java问题

Question

我有以下代码：

String urlString = API_URL + "/checkins/add";  
            String inputdata = "v=20111111"+"&venueId="+venueid+"&broadcast="+broadcast+"&oauth_token="+mAccessToken;  
            String content="";  
            try {  
                URL url = new URL( urlString );
                HttpURLConnection con = (HttpURLConnection) url.openConnection();
                con.setRequestMethod("POST");
                con.setDoOutput( true ); 
                con.connect();  

                Log.v("RESPONSE MESSAGE", con.getResponseMessage());

                OutputStreamWriter out = new OutputStreamWriter(con.getOutputStream());  
                out.write(inputdata);  
                out.flush();              

我得到了一个错误：

Exception: OutputStream unavailable because request headers have already been sent!

这是为什么