title: HTTP chunked transfer
body: <p>I'm writing a client app that connects to a service using the chunked transfer encoding.  The service occasionally disconnects and I was told it was because we're sending a zero chunk in the request, so <code>Tomcat</code> closes the connection.  </p>
  
  <p>I'm using the <code>Java</code> <code>HttpUrlConnection</code> class to make the connection and I have no idea why it would be sending a zero chunk and how to prevent it from doing that.</p>
  
  <p>Here's the code.</p>
  
  <pre><code>URL m5url = new URL("https://hostedconnect.m5net.com/bobl/bobl?name=org.m5.apps.v1.cti.ClickToDial.subscribe");
  StringBuffer sb = new StringBuffer("&lt;?xml version=\"1.0\" encoding=\"UTF-8\"standalone=\"yes\"?&gt;" 
                                     + "&lt;Command&gt;" 
                                     + "&lt;Name&gt;org.m5.apps.v1.cti.ClickToDial.subscribe&lt;/Name&gt;"
                                     + "&lt;Id&gt;1&lt;/Id&gt;" 
                                     + "&lt;User&gt;" + m5username + "&lt;/User&gt;" 
                                     + "&lt;Password&gt;" + m5password + "&lt;/Password&gt;" 
                                     + "&lt;FormattedXml&gt;true&lt;/FormattedXml&gt;" 
                                     + "&lt;ShallowResponse&gt;FULL&lt;/ShallowResponse&gt;" 
                                     + "&lt;/Command&gt;");
  
  conn = (HttpURLConnection) m5url.openConnection();
  conn.setRequestMethod("POST");
  conn.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");
  conn.setReadTimeout(SESSION_TIMEOUT);
  conn.setChunkedStreamingMode(0);
  conn.setDoOutput(true);
  conn.setDoInput(true);
  conn.setUseCaches(false);
  
  out = new DataOutputStream(conn.getOutputStream());
  conn.connect();
  out.writeBytes(sb.toString());
  out.flush();
  </code></pre>
  
  <p>When I do <code>inputstream.readline</code> it's <code>null</code>, but sometimes it works and sometimes it doesn't.</p>
  
  <p><img src="https://i.stack.imgur.com/mxbzR.png" alt="enter image description here">
  Ok, so I'm thoroughly confused. I abandoned using the HttpURLConnection and started using the Socket class and writing all the headers and data manually.  Without sending the zero chunk it seems to work all the time.  With the zero chunk it seemed to be working all the time except when I ran it in the debugger, where it got the same error as above.  So I put a sleep(100) after it sends the headers and before it sends the data and ran it without the debugger and it consistently got the error.  So I assume in the HttpURLConnection class there's a delay after sending the headers which is why it works sometimes and doesn't other times.  I could just not send the zero chunk but I'd really like to know why that would cause the error.  Any ideas?  I'm thinking there's a bug in Tomcat.</p>
  
  <p>Here's the code.</p>
  
  <pre><code>public class M5Connection
  {
      public static final String urlBase = "/bobl/bobl";
      public static final String ENCODING = "ISO-8859-1";
      public static final String DELIMITER = "\r\n";
      protected URL url;
      private InputStream inputStream;
      protected OutputStream outputStream;
      protected Socket socket;
      protected BufferedReader reader;
      private boolean bProcessedHeaders;
  
      protected String resp = null;
      protected String errorMessage = null;
  
  
      /**
       * Start a new connection to the BOBL server.
       * @param server server name:port to connect to
       * @throws IOException
       */
      protected void initConnection(String server, int timeout) throws IOException
      {
          url = new URL(server + urlBase);
          int port = url.getPort();
          if (server.startsWith("https"))
          {
              if (port == -1) port = 443;
              else
                  if (port == 80 || port == -1)port = 8080;
          }
  
          if (server.startsWith("https") == false)
          {
              socket = new Socket(url.getHost(), port);
          }
          else
          {           
              SocketFactory socketFactory = SSLSocketFactory.getDefault();
              socket = socketFactory.createSocket(url.getHost(), port);
          }
  
          socket.setSoTimeout(timeout);
          socket.setKeepAlive(true);
          socket.setSoLinger(false, 0);
          inputStream = socket.getInputStream();
          outputStream = socket.getOutputStream();
          reader = new BufferedReader(new InputStreamReader(inputStream));
      }
  
      public void initHttpsConnection(String server, int timeout) throws IOException
      {
          initConnection(server,timeout);
          sendHeaders();
          bProcessedHeaders = false;
      }
  
      private void sendHeaders() throws IOException {
          String path = url.getPath();
          StringBuffer outputBuffer = new StringBuffer();
          outputBuffer.append("POST " + path + " HTTP/1.1" + DELIMITER);
          outputBuffer.append("Host: " + url.getHost() + DELIMITER);
          outputBuffer.append("User-Agent: CometTest" + DELIMITER);
          outputBuffer.append("Connection: keep-alive" + DELIMITER);
          outputBuffer.append("Content-Type: text/plain" + DELIMITER);
          outputBuffer.append("Transfer-Encoding: chunked" + DELIMITER);
          outputBuffer.append(DELIMITER);
          byte[] outputBytes = outputBuffer.toString().getBytes(ENCODING);
          outputStream.write(outputBytes);
          outputStream.flush();
      }
  
      /** Send some data to the server, HTTP/1.1 chunked style. */
      public void send(String chunkData) throws IOException {
          byte[] chunkBytes = chunkData.getBytes(ENCODING);
          String hexChunkLength = Integer.toHexString(chunkBytes.length);
          StringBuffer outputBuffer = new StringBuffer();
          outputBuffer.append(hexChunkLength);
          outputBuffer.append(DELIMITER);
          outputBuffer.append(chunkData);
          outputBuffer.append(DELIMITER);
          byte[] outputBytes = outputBuffer.toString().getBytes(ENCODING);
          outputStream.write(outputBytes);
          outputStream.flush();
  
          outputBuffer = new StringBuffer();
          outputBuffer.append("0");
          outputBuffer.append(DELIMITER);
          outputBuffer.append(DELIMITER);
          outputBytes = outputBuffer.toString().getBytes(ENCODING);
          outputStream.write(outputBytes);
          outputStream.flush();
      }
  
      /**
       * Wait for a response from the server.
       * @return the string that the server returned.
       * @throws IOException
       */
      public String getRawResponse() throws IOException
      {
          String s;
  
          // just after we connect we expect to see the HTTP headers. Read and discard
          if (!bProcessedHeaders) {
              while (true){
                  String line = reader.readLine();
                  System.out.println("HEADER: " + line);
  
                  if (line == null || line.equals("\r\n") || line.equals(""))
                      break;
              }
              bProcessedHeaders = true;
          }
  
          while (true)
          {       
              s = getChunk();     
  
              if (s == null)
                  return null;
  
              if (s.equals("")) {
                  continue;
              }
  
              // server will not emit XML if it is having real troubles
              if (s.charAt(0) != '&lt;' || s.startsWith("&lt;html&gt;")) {
                  System.out.println("Server says: " + s);
                  continue;
              }
              return s;
          }
      }   
  
      /**
       * Expect chunked excoding back from the server. Read and return a chunk.
       * @return a string containing the HTTP chunk
       * @throws IOException
       */
      private String getChunk() throws IOException
      {
          StringBuffer buf = new StringBuffer();
          while (true)
          {
          // HTTP chunked mode, expect to see a line with the length in hex of the chunk that follows
              String s = reader.readLine();           
  
              if (s == null)
                  throw new IOException();
              if (s.length() == 0)
                  continue;
  
              int toread;
              try {
                  toread = Integer.parseInt(s, 16);
              } catch (NumberFormatException e) {
                  System.out.println("Number format error: " + s);
                  return "";
              }
  
              if (toread == 0)
              {
                  return null;
              }
  
              // read the chunk
              char[] data = new char[toread];
              int read = 0;
              while (read != toread)
              {
                  read += reader.read(data, read, toread - read);
              }
              buf.append(data, 0, read);
  
              // for some reason tomcat only sends data in up to 8192 byte chunks
              if (toread != 8192)
                  break;
          }
          return buf.toString();
      }   
  
      public void close()
      {
          try { socket.close(); } catch (IOException e) {}
      }
  
      public static void main(String[] args) throws Exception
      {
          M5Connection cnx = new M5Connection();
          cnx.initHttpsConnection("https://hostedconnect.m5net.com/bobl/bobl?name=org.m5.apps.v1.cti.ClickToDial.subscribe", 0);
  
          Thread.sleep(100);
          //
          // Create and send an XML command to listen for call state changes on our TN
          //
          String format = "&lt;?xml version=\"1.0\" encoding=\"UTF-8\" standalone=\"yes\"?&gt;" +
          "&lt;Command&gt;" +
          "    &lt;Name&gt;org.m5.apps.v1.cti.ClickToDial.subscribe&lt;/Name&gt;" +
          "    &lt;Id&gt;1&lt;/Id&gt;" +
          "    &lt;User&gt;&lt;/User&gt;" +
          "    &lt;Password&gt;&lt;/Password&gt;" +
          "    &lt;FormattedXml&gt;true&lt;/FormattedXml&gt;" +
          "    &lt;ShallowResponse&gt;FULL&lt;/ShallowResponse&gt;" +
          "&lt;/Command&gt;";
          String command = format;
          System.out.println("SENDING " + command + "\n ------------ ");
          cnx.send(command);
  
          //
          // Now just wait for the responses
          //
          while (true)
          {
              String resp = cnx.getRawResponse();
              System.out.println(resp);
          }
      }
  }
  </code></pre>