Re: Non destructive read of socket

Tom Anderson <>
Thu, 8 Oct 2009 00:40:59 +0100
On Thu, 8 Oct 2009, Gilbert wrote:

Kevin McMurtrie wrote:

It doesn't sound like that's going to work reliably. Data existing and
data flowing aren't the same thing. I think what you need is a message
dispatcher thread, or demultiplexer if you want to call it that,
handling the stream. It will route heartbeats off to their handler and
app data off to another handler. Being that it operates independently
of app data streams, it will be able to identify exactly the last time
and type of data that passed through.

That sounds like it could help with something I'm doing reading messages
from a serial port. There are about 20 message types that require
slightly different handling depending on the message identifier byte. I
don't really want the message dispatcher to have to know about all the
different handlers and the "standard" listener pattern means that all
handlers receive all messages and ignore those that they don't want
which seems wasteful. What I think I'd like to be able to do is to
register a listener for a specific message id. Is there anything
available that I could extend or would I have to roll my own

I can't think of anything. But this isn't hard:

interface MessageHandler {
  public void handle(int messageType, InputStream in) throws IOException;

class MessageDispatcher implements Runnable {
  private final InputStream in;
  private final Map<Integer, MessageHandler> handlers;

  public void run() {
  // you'll need some exception handling here
  // and some thread control somewhere
  while (true) {
  int messageType =;
  if (messageType == -1) throw new EOFException();
  MessageHandler handler = handlers.get(messageType);
  if (handler == null) throw new IOException("bad message type: " + messageType);
  handler.handle(messageType, in);

Instead of a map, you could use an array of length 256 (or less), keyed by
the byte. Or you could have an enum of message types, turn the message
type byte into an enum value, then use an EnumMap.


This is the best kind of weird. It can make a corpse laugh back to
death. -- feedmepaper

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