How can I listen to UDP ports in Java?

Question

I have to listen 3 different ports for UDP in my Java project. I implemented my system as follows:

I have a UDPMessageListener class which implements Runnable. I want to create 3 threads running as this object.

I have an interface called " UDPPacketProcessor " which has a single onPacketReceived method.

Every UDPMessageListener has a UDPPacketProcessor instance, and directs the UDP packet to the object registered as its UDPPacketProcessor.

And I have a DatabaseProc Class which needs to serve to the messages coming from 3 different UDP ports. DatabaseProc implements UDPPacketProcessor to register itself to those 3 UDPMessageListener classes.

In theory, according my Java knowledge there cannot be any problem, 3 Runnable threads listens to 3 ports, and on packet received, calls a method of my main singleton object's method. However, when I run the program, only one of the ports is listened. I can only the HBMessageListener works correctly, the others do not respond. When I bypass the HBMessageListener (comment out the line I run it) I can now see that only AlarmMessageListener works. Where am I wrong?

UDPMessageListener.java

public class UDPMessageListener implements Runnable {

int port;
byte[] receiveData;
DatagramSocket udpListeningSocket;
UDPPacketProcessor processor;
public UDPMessageListener(UDPPacketProcessor listener,int localPort){
    port = localPort;
    this.processor = listener;
    receiveData = new byte[SRPDefinitions.BYTE_BUFFER_MAX_LENGTH];
    try {
        udpListeningSocket = new DatagramSocket(port);
    } catch (SocketException e) {
        System.out.println("Socket bind error in port: " + port);
        e.printStackTrace();
    }
}

@Override
public void run() {
    while(true){
        DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
        try {
            udpListeningSocket.receive(receivePacket);
            System.out.println("Received UDP Packet from Port:" + port);
            processor.onPacketReceived(receivePacket, port);

        } catch (IOException e) {
            System.out.println("UDP Listener end up with an exception:");
            e.printStackTrace();
        }

    }
}
}

UDPPacketProcessor.java

public interface UDPPacketProcessor {
public void onPacketReceived(DatagramPacket receivedPacket,int localPort);
}

DatabaseProc.java

public class DatabaseProc implements UDPPacketProcessor{

private static DatabaseProc instance = null; // for singleton.

byte[] receiveData;
byte[] sendData;
ByteBuffer systemMessageByteBuffer;

UDPMessageListener HBMessageListener;
UDPMessageListener AlarmMessageListener;
UDPMessageListener TrackMessageListener;

private DatabaseProc(){
    receiveData = new byte[SRPDefinitions.BYTE_BUFFER_MAX_LENGTH];
    sendData = new byte[SRPDefinitions.BYTE_BUFFER_MAX_LENGTH];
    HBMessageListener = new UDPMessageListener(this,SRPDefinitions.HB_PORT);
    AlarmMessageListener = new UDPMessageListener(this,SRPDefinitions.ALARM_PORT);
    TrackMessageListener = new UDPMessageListener(this,SRPDefinitions.TRACK_PORT);
}

public void run(){
    runListeners();
}

private void runListeners(){
    HBMessageListener.run();
    AlarmMessageListener.run();
    TrackMessageListener.run();
}

public static DatabaseProc getInstance(){
    if(instance == null){
        instance = new DatabaseProc();
    }
    return instance;
}

@Override
public void onPacketReceived(DatagramPacket receivedPacket, int localPort) {
    String strIPAddress =receivedPacket.getAddress().toString();

    ByteBuffer buffer = ByteBuffer.allocate(receivedPacket.getLength());
    System.out.println("Received Packet Length: " + receivedPacket.getLength() + "/" + receivedPacket.getData().length);
    buffer.order(ByteOrder.LITTLE_ENDIAN);

    buffer.put(receivedPacket.getData(),0,receivedPacket.getLength());
    buffer.position(0);

    if(localPort == SRPDefinitions.HB_PORT){
        System.out.println("HB Message Received from " + strIPAddress + "!");
        SRPHeartBeatMessage message = new SRPHeartBeatMessage(buffer);
        //message.print();
    }
    if(localPort == SRPDefinitions.ALARM_PORT){
        System.out.println("ALARM Message Received from " + strIPAddress + "!");
        SRPAlarmMessage message = new SRPAlarmMessage(buffer);
        message.print();
    }
    if(localPort == SRPDefinitions.TRACK_PORT){
        System.out.println("TRACK Message Received from " + strIPAddress + "!");
    }   
}

Answer 1

The issue is in DatabaseProc class. Change runListeners to as following.

private void runListeners(){
    new Thread(HBMessageListener).start();
    new Thread(AlarmMessageListener).start();
    new Thread(TrackMessageListener).start();
}

EDIT

Explanation is when UDPMessageListener.run() is directly called, It is not creating a new Thread and executing asynchronously. With your code as it is the execution will never come out of HBMessageListener.run() method as it is in a infinite loop. You need to parallelize the Listeners so that all can listen.