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After reading several webpages about networking, in attempt to trying to understand basic networking, (I've never done networking before) I've made a few classes that is designed to make somewhat of a simple chat room. The following classes will be posted below:

ChatServer: http://paste.strictfp.com/32591 ((Edited recently)Creating the actual server that waits for people to connect to it, in this case, through port 9045)

ChatSession: http://paste.strictfp.com/32583 (When a client is found, from the server code above, it creates a new session, which just basically reads the messages that are sent by the clients)

ChatClient: http://paste.strictfp.com/32584 (Allows a client to write to the server)

ServerRunner: http://paste.strictfp.com/32585 (Main method for running the server)

ClientRunner: http://paste.strictfp.com/32586 (Main method for running a client, which connects to the server)

I know the code above isn't the best, considering the fact that I didn't add any checks to see if the socket/client got disconnected, or something got interrupted. But then again, this was just a practice in order to help me try and grasp the concept of networking.

So these 5 classes together work fine, but I have a question/concern that would be very much appreciated if answered:

How do I send a message from the server to a client?

The reason why I'm asking is this because I want to make a simple multiplayer tic-tac-toe game, in which there will be a server, and 2 clients (both representing players) and basically what I had in mind was that whenever the clients click on buttons, I'd send a message to the server. And then I would send the message back to both clients in order to modify both of their games. And I'm just a little confused about how it works, due to my lack of networking knowledge. I'd appreciate if you didn't redirect me to another URL, unless very very very simple, because I prefer if people explain it to me in simplest terms. If someone can help me, it'd be very much appreciated.

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2 Answers 2

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There are two meanings for the word "server" which can be confusing:

  1. Server is what is not at the customer (i.e. it the central hub to which every customer/user connects to)
  2. Server is the one receiving requests. Servers never initiate conversations, they only always respond to incoming messages.

So what you try to do is to turn your server into a client (and the ChatClient would become a server).

This explains why this seemingly simple task is so hard: Servers aren't meant to talk to clients out of the blue. Real world example: A clerk from your bank suddenly shows up on your doorstep and tries to sell you something at your home. Odd, isn't it?

Solutions:

  1. You can have the client send a message to the server every few minutes. The message would be "Is there something new?". This is called "polling".
  2. You can have the client send a message and let it wait for the answer. There is no need for the server to reply to messages right away. There will be timeouts, though, that you need to handle. But the client could send one "Is there something new?" and then wait until it gets a reply. If you move this into a thread, the client could do something else while it waits for an answer. This is polling but it's passive.
  3. You can use a bi-directional protocol like WebSocket.
  4. You can turn your client into a real server: Open a server socket, let the central hub know the port number and let it connect there to update your client. While this works, it needs two connections and they are a bit expensive. This works well for small amounts of clients but as the number grows, it eventually becomes a problem.
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Well basically while I was trying to think of a way to do this, I modified the ChatServer class in hopes that it would actually send data to all the clients. I modified it to this: paste.strictfp.com/32588 And basically what I did was when the server found a new client, I added the socket's output stream to a list of writers and basically when I wanted to write data to all the clients, I would just call the write(String) method, as shown in the paste.
A clerk from your bank suddenly shows up on your doorstep and tries to sell you something at your home. Odd, isn't it? I wish that would be perceived as being odd... ;)
I like the passive polling idea. +1
@JoshM: You can call write() on the server side but how will the clients know that they have to read?
@Aaron Inside the ChatClient paste, I added a Scanner which checks for messages from the server, but I'm not receiving any messages for some reason using that write() method, so that's why I'm kind of confused.
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One thing you have to decide before going further is how to structure your application-level protocol on top of plain TCP sockets. This is about which side initiates the conversation, what sort of messages you want to pass back and forth, and so on.

Then the next often sticky point is deciding the layout of the messages themselves - should they be text or binary, and how the receiving side will know if it got a complete message. The usual confusion here is about not understanding that TCP connection gives you a bi-directional stream of bytes, and you have to figure out where messages start and end. Traditional options here are:

  • Delimited messages - works best for text-based protocols. The simplest form is line-is-a-message notation, where new-line is your delimiter.
  • Fixed-length message. Best for binary.
  • Length-prefixed messages - you have a fixed length header that tells how many bytes will follow.
  • Self-describing messages like s-expressions, XML, etc. Usually text.

Hope this helps a bit.

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I'm sorry I don't really understand majority of the things you just explained to me. Although I did read plenty of websites, I am looking for simple explanations.
Here's more or less an expanded version of what I am talking about: en.wikipedia.org/wiki/Communications_protocol
It's not difficult at all, just don't assume all those white-bearded CS folks were pipe-dreaming for the last 50 years, they actually had some good ideas :)
It's just very frustrating. I've tried adding all the writers to a list of writers, in this example: paste.strictfp.com/32588 , but I'm not receiving them on the client's end, even though I'm constantly checking for messages from the server. And I don't understand it at all

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