UDP server and multiple clients (1v1 game) - multithreading

Question

I'm trying to create an UDP server with two slots per new instance of the game and clients.
It has to be UDP cause of the game mechanics. I wrote this:
public class Server {
private static HashSet<Integer> ports = new HashSet<Integer>(); // these are player's IDs
static ArrayList<InetAddress> addresses = new ArrayList<InetAddress>();
public static boolean player1_active = false, player2_active = false;
public static PlayerCredits p1, p2, temp;
public static void main(String[] args) throws Exception {
// The default port
int serverport = 8888;
DatagramSocket udpServerSocket = new DatagramSocket(serverport);
DatagramPacket sendpacket;
System.out.println("Server started...\n");
while(true)
{
byte[] receiveData = new byte[1024];
DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
udpServerSocket.receive(receivePacket);
String clientMessage = (new String(receivePacket.getData())).trim();
System.out.println("Client Connected - Socket Address: " + receivePacket.getSocketAddress());
System.out.println("Client message: \"" + clientMessage + "\"");
InetAddress clientIP = receivePacket.getAddress();
addresses.add(clientIP);
System.out.println("Client IP Address & Hostname: " + clientIP + ", " + clientIP.getHostName() + "\n");
// Get the port number which the recieved connection came from
int clientport = receivePacket.getPort();
System.out.println("Adding "+clientport);
ports.add(clientport);
// Response message
String returnMessage = clientMessage.toUpperCase();
System.out.println(returnMessage);
// Create an empty buffer/array of bytes to send back
byte[] sendData = new byte[1024];
temp = new PlayerCredits(clientport, clientIP);
if(!player1_active){
p1 = new PlayerCredits(clientport, clientIP);
player1_active = true;
}
if(temp.port != p1.port && temp.ad != p1.ad)
if(!player2_active){
p2 = new PlayerCredits(clientport, clientIP);
player2_active = true;
}
DatagramPacket sendPacket;
if(p2 == null){
String awaiting = "WAITING FOR SECOND PLAYER";
sendData = awaiting.getBytes();
sendPacket= new DatagramPacket(sendData, sendData.length, p1.ad, p1.port);
udpServerSocket.send(sendPacket);
}
else{
String awaiting = "SECOND PLAYER INCOMING";
sendData = awaiting.getBytes();
if(p2!=null){
sendPacket = new DatagramPacket(sendData, sendData.length, p2.ad, p2.port);
udpServerSocket.send(sendPacket);
}
sendPacket = new DatagramPacket(sendData, sendData.length, p1.ad, p1.port);
udpServerSocket.send(sendPacket);
}
}
}
public static class PlayerCredits{
int port;
InetAddress ad;
public PlayerCredits(int p, InetAddress a){
this.port = p;
this.ad = a;
}
}
For now I have two people connected - but as you will probably say - it's WRONG. I need to somehow implement every player on the Multisocket or something (or connect them via TCP and then listen on a new thread with a new datagram).
As I know, the best way is to create a listener server (TCP) on a main thread - accept connections and pass them to 'Player's constructors' and in Player's class make the whole fun.
If there is someone that could help and tell me, how it should be done like in 3 steps like (I don't need code):
Connection with TCP
Create new threads with players and listen on UDP
Start the game when 2 players are connected
Also, how to stop the 'third connection' and redirect it to a new game instance? (cause it's 1v1).