Broadcast Message in Java

Question 1

I suggest you to use java messaging service and one of it's implementations such as ApacheMQ.

A good starting point is here.

Question 2

I would suggest using a database with a trigger and store procedure. Here is an example of calling java methods from the database. A message queue will work, but that is an overly complex solution.

Here's an excerpt from an example of how to create a procedure and call it via a trigger:

First, you add the following Java method to the class DBTrigger

CREATE OR REPLACE PROCEDURE add_emp (
  emp_no NUMBER, emp_name VARCHAR2, dept_name VARCHAR2)
AS LANGUAGE JAVA 
NAME 'DBTrigger.addEmp(int, java.lang.String, java.lang.String)';

Then, you create the INSTEAD OF trigger:

CREATE OR REPLACE TRIGGER emps_trig 
INSTEAD OF INSERT ON emps
FOR EACH ROW
CALL add_emp(:new.empno, :new.ename, :new.dname);

Question 3

Since you tagged this with CORBA, you could use the Event Service to broadcast a notice to all interested clients.

Question 4

To enable the server to send packets of information to the client/clients. A datagram, by definition, is “an independent, self-contained message sent over the network whose arrival, arrival time, and content are not guaranteed”. Essentially, we are opening a DatagramSocket in order to send DatagramPacket messages to the client. We are using the datagram classes (instead of standard sockets) because they allow us to broadcast information to multiple clients, that are all connected to a MulticastSocket.

import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.net.UnknownHostException;

public class MulticastSocketServer {

    final static String INET_ADDR = "224.0.0.3";
    final static int PORT = 8888;

    public static void main(String[] args) throws UnknownHostException, InterruptedException {
        // Get the address that we are going to connect to.
        InetAddress addr = InetAddress.getByName(INET_ADDR);

        // Open a new DatagramSocket, which will be used to send the data.
        try (DatagramSocket serverSocket = new DatagramSocket()) {
            for (int i = 0; i < 100; i++) {
                String msg = "Sent message no " + i;

                // Create a packet that will contain the data
                // (in the form of bytes) and send it.
                DatagramPacket msgPacket = new DatagramPacket(msg.getBytes(),
                        msg.getBytes().length, addr, PORT);
                serverSocket.send(msgPacket);

                System.out.println("Server sent packet with msg: " + msg);
                Thread.sleep(500);
            }
        } catch (IOException ex) {
            ex.printStackTrace();
        }
    }
}

One thing that we need to take into consideration here, is that there are specific addresses that allow us to use a MulticastSocket are limited, specifically in the range of 224.0.0.0 to 239.255.255.255. Some of them are reserved, like 224.0.0.0. The address that we are using, 224.0.0.3, can be used safely.

Regarding the client, we are going to move a little bit differently. We are going to create a client class, that will accept incoming messages from the server, and then we are going to duplicate this class. The point here is that by using the same code, we can connect to the server seamlessly, while having as many clients as we like.

import java.io.IOException;
import java.net.DatagramPacket;
import java.net.InetAddress;
import java.net.MulticastSocket;
import java.net.UnknownHostException;
public class MulticastSocketClient 
{

    final static String INET_ADDR = "224.0.0.3";
    final static int PORT = 8888;

    public static void main(String[] args) throws UnknownHostException {
        // Get the address that we are going to connect to.
        InetAddress address = InetAddress.getByName(INET_ADDR);

        // Create a buffer of bytes, which will be used to store
        // the incoming bytes containing the information from the server.
        // Since the message is small here, 256 bytes should be enough.
        byte[] buf = new byte[256];

        // Create a new Multicast socket (that will allow other sockets/programs
        // to join it as well.
        try (MulticastSocket clientSocket = new MulticastSocket(PORT)){
            //Joint the Multicast group.
            clientSocket.joinGroup(address);

            while (true) {
                // Receive the information and print it.
                DatagramPacket msgPacket = new DatagramPacket(buf, buf.length);
                clientSocket.receive(msgPacket);

                String msg = new String(buf, 0, buf.length);
                System.out.println("Socket 1 received msg: " + msg);
            }
        } catch (IOException ex) {
            ex.printStackTrace();
        }
    }
}

First, we start the clients, which will keep waiting for incoming packets of information. As soon as we start the server, it will send the information packets and the clients will receive them and print the information on the screen

Question 5

ZeroMQ could be what you are searching, but I have only used it in C, C++ and Python, so I am not totally sure of how well it is usable in Java. But they have implemented the Publisher-Subscriber pattern for their sockets and at least the static library of the version 3.2.2 under ubuntu 12.04 is stable and works very well.

Question 6

As a "quick fix" write the output to file and have the second app read the file.

This is not elegant at all, but if you code to interfaces you can later replace the implementation with something better.

Question 7

A good solution would be an implementation of the OMG Data Distribution Standard (DDS). With DDS there is just a global data space with a dynamic discovery protocol. See for example RTI DDS, there is a free community edition.