Cannot write output after reading input

Question

I'm writing a program that connects to a servlet thanks to a HttpURLConnection but I stuck while checking the url

public void connect (String method) throws Exception {

server = (HttpURLConnection) url.openConnection ();
server.setDoInput (true);
server.setDoOutput (true);
server.setUseCaches (false);
server.setRequestMethod (method);
server.setRequestProperty ("Content-Type", "application / xml");

server.connect ();

/*if (server.getResponseCode () == 200)
{
System.out.println ("Connection OK at the url:" + url);
System.out.println ("------------------------------------------- ------- ");
}
else
System.out.println ("Connection failed"); 

}*/

I got the error :

java.net.ProtocolException: Cannot write output after reading input.

if i check the url with the code in comments but it work perfectly without it unfortunately, I need to check the url so i think the problem comes from the getResponseCode method but i don t know how to resolve it

Thank you very much

Answer 1

The HTTP protocol is based on a request-response pattern: you send your request first and the server responds. Once the server responded, you can't send any more content, it wouldn't make sense. (How could the server give you a response code before it knows what is it you're trying to send?)

So when you call server.getResponseCode(), you effectively tell the server that your request has finished and it can process it. If you want to send more data, you have to start a new request.

Looking at your code you want to check whether the connection itself was successful, but there's no need for that: if the connection isn't successful, an Exception is thrown by server.connect(). But the outcome of a connection attempt isn't the same as the HTTP response code, which always comes after the server processed all your input.

Answer 2

I think the exception is not due toprinting url. There should some piece of code which is trying to write to set the request body after the response is read.

This exception will occur if you are trying to get HttpURLConnection.getOutputStream() after obtaining HttpURLConnection.getInputStream()

Here is the implentation of sun.net.www.protocol.http.HttpURLConnection.getOutputStream:

public synchronized OutputStream getOutputStream() throws IOException {

     try {
         if (!doOutput) {
             throw new ProtocolException("cannot write to a URLConnection"
                            + " if doOutput=false - call setDoOutput(true)");
         }

         if (method.equals("GET")) {
             method = "POST"; // Backward compatibility
         }
         if (!"POST".equals(method) && !"PUT".equals(method) &&
             "http".equals(url.getProtocol())) {
             throw new ProtocolException("HTTP method " + method +
                                         " doesn't support output");
         }

         // if there's already an input stream open, throw an exception
         if (inputStream != null) {
             throw new ProtocolException("Cannot write output after reading 
                input.");
         }

         if (!checkReuseConnection())
             connect();

         /* REMIND: This exists to fix the HttpsURLConnection subclass.
          * Hotjava needs to run on JDK.FCS.  Do proper fix in subclass
          * for . and remove this.
          */

         if (streaming() && strOutputStream == null) {
             writeRequests();
         }
         ps = (PrintStream)http.getOutputStream();
         if (streaming()) {
             if (strOutputStream == null) {
                 if (fixedContentLength != -) {
                     strOutputStream = 
                        new StreamingOutputStream (ps, fixedContentLength);
                 } else if (chunkLength != -) {
                     strOutputStream = new StreamingOutputStream(
                         new ChunkedOutputStream (ps, chunkLength), -);
                 }
             }
             return strOutputStream;
         } else {
             if (poster == null) {
                 poster = new PosterOutputStream();
             }
             return poster;
         }
     } catch (RuntimeException e) {
         disconnectInternal();
         throw e;
     } catch (IOException e) {
         disconnectInternal();
         throw e;
     }
 }

Answer 3

I have this problem too, what surprises me is that the error is caused by my added code System.out.println(conn.getHeaderFields());

Below is my code:

HttpURLConnection conn=(HttpURLConnection)url.openConnection();
conn.setRequestMethod("POST");
configureConnection(conn);
//System.out.println(conn.getHeaderFields()); //if i comment this code,everything is ok, if not the 'Cannot write output after reading input' error happens
conn.connect();
OutputStream os = conn.getOutputStream();
os.write(paramsContent.getBytes());
os.flush();
os.close();