download manager don't work!!

[IdolOfIdle]

here is my code. it connects to the server successfully.But connect.getContentLength() returns -1; and downloaded file contains zero bytes. any suggestion will be deeply appreciated.

try {
      // Open connection to URL.
      HttpURLConnection connection =
        (HttpURLConnection) url.openConnection();

      // Specify what portion of file to download.
      connection.setRequestProperty("Range",
        "bytes=" + downloaded + "-");

      // Connect to server.
      connection.connect();

      // Make sure response code is in the 200 range.
      if (connection.getResponseCode() / 100 != 2) {
      	System.out.println(connection.getResponseCode());
        error();
      }
      System.out.println(connection.getResponseCode());
      System.out.println(connection.getResponseMessage());
      // Check for valid content length.
      int contentLength = connection.getContentLength();
      if (contentLength < 1) {
        error();
      }

      /* Set the size for this download if it
         hasn't been already set. */
      if (size == -1) {
        size = contentLength;
        stateChanged();
      }

      // Open file and seek to the end of it.
      file = new RandomAccessFile(getFileName(url), "rw");
      file.seek(downloaded);

      stream = connection.getInputStream();
      while (status == DOWNLOADING) {
        /* Size buffer according to how much of the
           file is left to download. */
        byte buffer[];
        if (size - downloaded > MAX_BUFFER_SIZE) {
          buffer = new byte[MAX_BUFFER_SIZE];
        } else {
          buffer = new byte[size - downloaded];
        }

        // Read from server into buffer.
        int read = stream.read(buffer);
        if (read == -1)
          break;

        // Write buffer to file.
        file.write(buffer, 0, read);
        downloaded += read;
        stateChanged();
      }