java BufferedReader specific length returns NUL characters

Question

I have a TCP socket client receiving messages (data) from a server. messages are of the type length (2 bytes) + data (length bytes), delimited by STX & ETX characters.

Answer 1

Here is a sample server that I have used for testing The main rcv is structured like

while((chars_read = from_server.read(buffer)) != -1)
{
    to_user.write(buffer,0,chars_read);
    to_user.flush();
}

The actual whole server is below ...

   public static void main(String[] args) throws IOException
   {
      try
      {
         if (args.length != 2)
            throw new IllegalArgumentException("Wrong number of Args");

         String   host = args[0];
         int      port = Integer.parseInt(args[1]);

         Socket   s = new Socket(host,port);

         final Reader from_server = new InputStreamReader(s.getInputStream());
         PrintWriter to_server = new PrintWriter(new OutputStreamWriter(s.getOutputStream()));

         BufferedReader from_user = new BufferedReader(new InputStreamReader(System.in));
         final PrintWriter to_user = new PrintWriter(new OutputStreamWriter(System.out));

         to_user.println("Connected to " + s.getInetAddress() + ":" + s.getPort());

         Thread t = new Thread()
         {
            public void run()
            {
               char [] buffer = new char[1024];
               int chars_read;
               try
               {
                  while((chars_read = from_server.read(buffer)) != -1)
                  {
                     to_user.write(buffer,0,chars_read);
                     to_user.flush();
                  }
               }
               catch(IOException e)
               {
                  to_user.println(e);
               }

               to_user.println("Connection closed by server");
               to_user.flush();
               System.exit(0);
            }
         };

         t.setPriority(Thread.currentThread().getPriority() + 1);
         t.start();

         String line;
         while ((line = from_user.readLine()) != null)
         {
            to_server.println(line);
            to_server.flush();
         }

         //t.stop();
         s.close();
         to_user.println("Connection closed by client");
         to_user.flush();
      }
      catch(Throwable e)
      {
         e.printStackTrace();
         System.err.println("Usage : java TCPClient <hostname> <port>");
      }
   }

Answer 2

The InputStream read method may return short reads; you must check the return value to determine how many characters were read, and continue reading in a loop until you get the number you wanted. The method may block, but it only blocks until some data is available, not necessarily all the data you requested.

Most people end up writing a "readFully" method, like DataInputStream, which reads the amount of data expected, or throws an IOException:

static public int readFully(InputStream inp, byte[] arr, int ofs, int len) throws IOException {
    int                                 rmn,cnt;

    for(rmn=len; rmn>0; ofs+=cnt, rmn-=cnt) {
        if((cnt=inp.read(arr,ofs,rmn))==-1) { 
            throw new IOException("End of stream encountered before reading at least "+len+" bytes from input stream"); 
            }
        }
    return len;
    }