Sending a message to multiple clients with one call

[Vega Zimur]

At the moment it's something like this:

package test;

import java.net.*;
import java.io.*;
import java.util.Vector;

class Server extends Thread {
	Vector<Serializable> vser;
	Vector<SocketCollection> vsc;
	ServerSocket ss;
	static final int port=1234;

	class Emitter extends Thread {
		public void run() {
			for (SocketCollection sc : vsc)
				try { sc.oos.writeObject(vser); }
				catch (IOException e) {}
			try { sleep(2000); }
			catch (InterruptedException e) {}
		}
	}

	class SocketCollection {
		Socket sock;
		ObjectOutputStream oos;

		SocketCollection(Socket psock) throws IOException {
			sock=psock;
			oos=new ObjectOutputStream(psock.getOutputStream());
		}
	}

	Server() {
		vser=new Vector<Serializable>();
		vsc=new Vector<SocketCollection>();
		ss=null;
	}

	public void run() {
		try {
			ss=new ServerSocket(port);
			Emitter emito=new Emitter();
			emito.start();
			while (true)
				try { vsc.add(new SocketCollection(ss.accept())); }
			catch(IOException e) {}
		} catch (IOException e) { System.err.println("Couldn't create the server"); }
	}

	public static void main(String[] args) {
		Server srv=new Server();
		srv.start();
		// Do something with srv.vser e.g. adding new elements
	}
}

This code works fine.
But in line 16 you can see that the content of the stream has to be generated again and again for every Client. -> is there a way to send it to all streams at once?

[Rob Spoor]

TCP doesn't allow for multicast. UDP (using DataGramSocket) does, but it's far from reliable. Data you send may not arrive.

[Vega Zimur]

:/
I've got an idea how to do it in TCP but I haven't tested it yet:

package test;

import java.io.OutputStream;
import java.io.IOException;
import java.util.Vector;

abstract class MultiOutputStream extends OutputStream{
  Vector<OutputStream> outputs;
  
  MultiOutputStream(Vector<OutputStream> vos) {
    outputs=vos;
  }
  
  public void write(byte[] b) throws IOException {
    for (OutputStream os : outputs) os.write(b);
  }
  public void write(byte[] b, int off, int len) throws IOException {
    for (OutputStream os : outputs) os.write(b,off,len);
  }
  public abstract void write(int b) throws IOException;
}

[Rob Spoor]

You'll have to overwrite all OutputStream methods, including flush and close, and use a loop to call that same method on all contained output streams. But that basic code you have there should work.

[Vega Zimur]

yes, you're right, so I'll also overwrite the flush method.

But the good thing is that you can add new clients by just adding them to the Vector:

package test;

import java.util.Vector;
import java.io.*;
import java.net.*;

class Main {
	static class Acceptor extends Thread {
		Vector<OutputStream> vos;
		Vector<Socket> vs;
		ServerSocket ss;
		
		Acceptor(ServerSocket pss, Vector<OutputStream> pvos, Vector<Socket> pvs) {
			vos=pvos;
			vs=pvs;
			ss=pss;
		}
		public void run() {
			Socket sock;
			while(true) {
				try {
					vs.add(sock=ss.accept());
					vos.add(sock.getOutputStream());
				} catch (IOException e) {}
			}
		}
		
	}

	static public void main(String[] args) {
		MultiOutputStream mos;
		ObjectOutputStream oos;
		Vector<OutputStream> vos;
		Vector<Socket> vs;
		ServerSocket ss;
		Acceptor acpt;

		vos=new	Vector<OutputStream>();
		vs=new Vector<Socket>();
		oos=new ObjectOutputStream(mos=new MultiOutputStream(vos));
		acpt=new Acceptor(ss=new ServerSocket(1234),vos,vs);
		acpt.start();
		
		while(true) {
			try { oos.writeObject(new java.util.Date()); }
			catch (IOException e) {}
			try { Thread.sleep(1000); }
			catch (InterruptedException e) {}
		}
	}
}

[Vega Zimur]

Ok, I've tested it and it nearly works.
The only problem is that the ObjectOutputStream sends the typeName of java.util.Date only once, so when there's a client which connnects later doesn't know what to do with the incoming data.
How do I force the ObjectOutputStream to print the classDescriptors.

here's the stdout("..." is for the data sections):
Initializing
Configuring streams
Starting server
Server is running
Sending time
...java.util.Date...
Sending time
......
Sending time
......

[Vega Zimur]

yippee

When I change the server and the client a bit it works:

public void run() { // the Accept Function of the server
    ...
    try { // Add the socket+outputStream to the Vectors
        ObjectOutputStream oos=new ObjectOutputStream(sock.getOutputStream()); // Create a new ObjectOutputStream
        oos.writeObject(new java.util.Date()); // Make client familiar with the header of "java.util.Date"
        vos.add(sock.getOutputStream()); // Now the client is ready to receive the data of the global ObjectOutputStream
    } catch (IOException) {}
    ...
}

now I can also attach a File using FileOutputStream or f.e. the stream System.out