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NullPointerException for PrintWriter thats initialized in the run method of a thread

im making a networked game that has a server which creates a clientHandler thread every time a client joins. I want to ask the first client that joined if it wants to start the game every time a new client joins, giving it the current number of players connected. Writting through the clientHandlers printwritter gives a nullPointerException, even though ive started the thread before doing this. what could be the problem?
Here is the server code:
`public class Server implements Runnable{
private ArrayList<ClientHandler> handlers = new ArrayList<>();
private ArrayList<Player> players = new ArrayList<>();
private Game game;
private boolean start;
public static void main(String[] args) {
Server server = new Server();
Thread s = new Thread(server);
s.start();
}
public void login(String name){
//todo
for (ClientHandler c : handlers){
if (c.getName().equals(name)){
alreadyTaken(name);//todo
}
else{
players.add(new HumanPlayer(name,c));//todo
}
}
}
public void setStart(){
start = true;
}
private void alreadyTaken(String name) {
}
public void setTurn(ServerHandler sh){
//todo
}
public void updateView(){
}
public String hello() {
return "Hello"; //?
}
public void place(String s){
}
#Override
public void run() {
ServerSocket serverSocket;
try {
serverSocket = new ServerSocket(1800);
} catch (IOException e) {
throw new RuntimeException(e);
}
System.out.println("----Server----");
while (!serverSocket.isClosed()) {
try {
Socket socket = serverSocket.accept();
ClientHandler handler = new ClientHandler(socket,handlers,this);
handlers.add(handler);
Thread h = new Thread(handler);
h.start();
System.out.println("A new client has connected");
System.out.println(handlers.get(0));
handlers.get(0).out.println("START? "+ handlers.size());
if (start){
System.out.println("start request works");
}
} catch (IOException e) {
throw new RuntimeException(e);
}
}
}
}
`
And here's the client handler code:
`public class ClientHandler implements Runnable{
private Socket socket;
private ArrayList<ClientHandler> handlers;
private Server server;
public PrintWriter out;
private BufferedReader in;
private String name;
public ClientHandler(Socket socket, ArrayList<ClientHandler> handlers, Server server){
this.socket = socket;
this.handlers = handlers;
this.server = server;
}
public void broadcastMessage(String msg){
System.out.println("Broadcasting");
for (ClientHandler s : this.handlers){
s.out.println("Player: " + msg);
}
}
public static String removePrefix(String s, String prefix)
{
if (s != null && s.startsWith(prefix)) {
return s.split(prefix, 2)[1];
}
return s;
}
public String getName(){
return name;
}
#Override
public void run() {
try {
out = new PrintWriter(new OutputStreamWriter(socket.getOutputStream()),true);
in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
new Thread(() -> {
while(socket.isConnected()){
String msg;
try {
msg = in.readLine();
while(msg!=null){
switch (msg.split(" ")[0]){
case "LOGIN":
name = removePrefix(msg,"LOGIN ");
server.login(name);//todo
break;
case "HELLO":
server.hello();//todo
break;
case "PLACE":
server.place(removePrefix(msg,"PLACE "));
break;
case "QUIT":
//todo
break;
case "STOP":
//todo
break;
case "START":
server.setStart();
default:
broadcastMessage(msg);
break;
}
msg = in.readLine();
}
} catch (IOException e) {
throw new RuntimeException(e);
}
}
}).start();
} catch (IOException e) {
throw new RuntimeException(e);
}
}
}`
I tried making a method in the client handler class which does the same thing. The server would just call that instead of writting directing through the PrintWriter, but i got the same error.

Starting a thread does not mean it is guaranteed to actually finish executing the first statement in its run() method before start() returns. In fact,
Usually it won't - starting a thread takes some time, and start() returns as soon as it can.
A JVM that runs a few statements in the thread you just started before start() returns is 'correct' - that is fine. A JVM that doesn't is also fine. Generally you don't want threads, because nothing is predictable anymore. At the very least you want to keep 'inter-thread comms' down to a minimum. Anytime a single field is used from more than one thread, things get very tricky.
What you need is synchronized or other tools to insert predictability in this code.
First, fix a bug
Your ClientHandler's run() code starts another thread for no reason. Take all that out, your run() method in ClientHandler should set up out and in and then immediately do while (socket.isConnected())
Synchronizing
At the very basic level, make a locker object and use notify/wait:
private final Object lock = new Object();
#Override public void run() {
try {
synchronized (lock) {
out = ...;
in = ...;
lock.notifyAll();
}
while (socket.isConnected()) { ... }
out definitely cannot be public here, you can't refer to a stream from multiple threads and expect things to work out!
Just 'fixing' your code involves then using something like:
public OutputStream getOutputStream() {
synchronized (lock) {
while (out == null) {
lock.wait();
}
}
return out;
}
Which will ensure that any thread that wants the out will wait for the other thread to get far enough, but, really, this is just setting you up for another 20 threading problems down the line. Instead, you want one object responsibile for all communication (both outgoing and incoming), and a concurrency-capable queue (there are various collections in the java.util.concurrent package good for this). Then:
Any other threads that want to just send data dump their message in the queue.
You have either 1 thread doing all comms, or 2 (one doing incoming, and one doing outgoing), both dedicated. The outgoing one just loops forever, grabbing objects from the queue and sending them.
If a thread wants to send a message and wait for the response, you need to use .wait() or nicer API from e.g. java.util.concurrent, or, use callback hell - you pass a closure with the code to run once the result is received.

Java Concurrent Sockets: not being able to share a variable between threads

I happen to have a problem with some attempts at reading the same variable in socket multi-threading, not being able to share it among threads.
It works as an app where an employer assigns work to an employee. Through his interface the employer can add and assignment to an ArrayList inside a class named ListadoPedidos.
When the employer's ServerSocket accepts an employee Socket, it starts a TCP connection and launches the following thread:
public class HiloServer implements Runnable{
private ListadoPedidos peds=new ListadoPedidos();
private ListadoOperarios operarios=new ListadoOperarios();
private ListadoSockets sockets=new ListadoSockets();
private SocketServer s;
public HiloServer(SocketServer sock, JFrame frame, ListadoPedidos pedidos) {
s=sock;
peds=pedidos;
}
/* (non-Javadoc)
* #see java.lang.Runnable#run()
*/
#Override
public void run() {
boolean agregar;
Socket nuevo;
try {
while(true) {
// ACEPTA OPERARIOS QUE DESEEN CONECTARSE
s.aceptar();
nuevo=s.getSocket();
sockets.addSocket(nuevo);
new NuevoCliente();
HiloDatos hd=new HiloDatos(s, nuevo,operarios,peds,sockets);
Thread t=new Thread(hd);
t.start();
}
}
catch (IOException e) {
e.printStackTrace();
}
}
}
*note that I send the object where the assigments added are stored.
Then it starts another thread that will work as a sort of "validation" for a number the employee's have to insert and send through his Swing interface to truly enter the system. This thread is generated everytime a new socket employee makes a TCP connection to the ServerSocket employer. It goes like:
public class HiloDatos implements Runnable {
private int n;
private Socket cliente;
private SocketServer server;
private int opRecibido;
private ListadoOperarios ops;
private ListadoPedidos peds;
private ListadoSockets socks;
public HiloDatos(SocketServer ss, Socket nuevo, ListadoOperarios operarios, ListadoPedidos pedidos, ListadoSockets sockets) {
cliente=nuevo;
server=ss;
ops=operarios;
peds=pedidos;
socks=sockets;
}
#Override
public void run() {
server.setSocket(cliente);
boolean agregar, aceptado=false;
try {
do {
// RECIBE EL NRO OPERARIO Y VERIFICA SU EXISTENCIA
agregar=true;
opRecibido=Integer.parseInt(server.recibir());
for(int c=0;c<ops.getOperarios().size();c++) {
if (opRecibido==ops.getOperarios().get(c)) {
new ErrorRepetido();
agregar=false;break;
}
}
if (agregar==true) {
ops.addOperarios(opRecibido);
server.enviar("Si");
aceptado=true;
}
}while(aceptado==false);
HiloPedidos hp=new HiloPedidos(server,opRecibido,ops,peds,socks);
Thread t=new Thread(hp);
t.start();
}catch (NumberFormatException e) {
new ErrorDatos();
} catch (ConnectException e) {
new ErrorConexion();
} catch (SocketException e) {
try {
socks.getSockets().remove(socks.getSockets().indexOf(cliente));
cliente.close();
} catch (IOException e1) {
new ErrorFlujo();
}
new WarnSocket();
} catch (IOException e) {
try {
socks.getSockets().remove(socks.getSockets().indexOf(cliente));
cliente.close();
} catch (IOException e1) {
new ErrorFlujo();
}
new WarnFlujo();
}
}
}
And lastly it launches yet another Thread that looks for that same validation number from the thread above in the ArrayList of assignments ("pedidos" of class ListadoPedidos) i kept passing from thread to thread, and if it finds a "new" one, it should send it to the connected socket:
public class HiloPedidos implements Runnable {
private Pedido ped;
private SocketServer server;
private int op;
private ListadoOperarios ops;
private ListadoPedidos peds;
private ListadoSockets socks;
public HiloPedidos(SocketServer ss, int opRecibido, ListadoOperarios operarios, ListadoPedidos pedidos, ListadoSockets sockets) {
server=ss;
opRecibido=op;
ops=operarios;
peds=pedidos;
socks=sockets;
}
#Override
public void run() {
int cambio=0, nuevo;
Pedido pedRecibido;
try {
while(true) {
// ENVÍA PEDIDOS
nuevo=peds.Contar(op);
if(nuevo==cambio) {
cambio=peds.Contar(op);
pedRecibido=peds.TraerNuevo(op, cambio);
server.enviarObjeto(pedRecibido);
}
}}
catch (NumberFormatException e) {
new ErrorDatos();
} catch (ConnectException e) {
new ErrorConexion();
} catch (SocketException e) {
try {
socks.getSockets().remove(socks.getSockets().indexOf(server.getSocket()));
server.getSocket().close();
} catch (IOException e1) {
new ErrorFlujo();
}
new WarnSocket();
} catch (IOException e) {
try {
socks.getSockets().remove(socks.getSockets().indexOf(server.getSocket()));
server.getSocket().close();
} catch (IOException e1) {
new ErrorFlujo();
}
new WarnFlujo();
}
}
}
Problem is that the last thread can't really notice a change in the list, as i debugged it and never reached the breakpoint inside the condition of sending the assignment. The class ListadoPedidos goes like this:
public class ListadoPedidos {
private static volatile ArrayList<Pedido> pedidos=new ArrayList<>();
public ListadoPedidos() {
}
public ArrayList<Pedido> getPedidos() {
return pedidos;
}
public synchronized void addPedidos(Pedido pedido) {
pedidos.add(pedido);
}
public int Contar(int o) {
int n=0;
for (Pedido p: pedidos) {
if (p.getNro_operario()==o) {
n++;
}
}
return n;
}
public Pedido TraerNuevo(int o, int c) {
int n=0;
Pedido nuevo = new Pedido();
for (Pedido p: pedidos) {
if (p.getNro_operario()==o) {
n++;
}
if (n==c) {
nuevo=p;break;
}
}
return nuevo;
}
}
Contar is the one that counts for an assignment with the value nrooperario same as the value it brings from the thread, and TraerNuevo brings the assignment to be sended (never reached this method).
I tried declaring the ArrayList as volatile and all but nothing works. Mind that even if i use socket connections, the problem has more to do with shared varaible not being able to update between threads. Any help will be appreciated.

Try this, basically, synchronize access.
public class ListadoPedidos {
private static volatile ArrayList<Pedido> pedidos=new ArrayList<>();
public ListadoPedidos() {
}
/**
* Here DO NOT return the arrayList. The underlying implementation is not threadsafe
*/
// public ArrayList<Pedido> getPedidos() {
// return pedidos;
// }
public synchronized void addPedidos(Pedido pedido) {
pedidos.add(pedido);
}
public synchronized int Contar(int o) {
int n=0;
for (Pedido p: pedidos) {
if (p.getNro_operario()==o) {
n++;
}
}
return n;
}
public synchronized Pedido TraerNuevo(int o, int c) {
int n=0;
Pedido nuevo = new Pedido();
for (Pedido p: pedidos) {
if (p.getNro_operario()==o) {
n++;
}
if (n==c) {
nuevo=p;break;
}
}
return nuevo;
}
}

The amount of code you have given us makes it difficult to answer your question. To be honest, the Spanish does not help either. But I can give you some general advice.
Let us start with the question. What exactly is the question? From what I can understand, it comes down to: "how can two threads read the same variable?"
Even if that is not the question, try to make the question as clear as possible for yourself.
Then start with a new test-project separate from the project you are working on. Write the minimal amount of code that you think should work. If it does not work, write even less code that does work (e.g. use static variables to make things even more simple). Go back and forth until you have code that can answer your question. If you cannot get it to work, step back and think about assumptions that you made that might not be true.
If you still cannot figure it out, come back here with the minimal amount of code that you think should work and a clear question.
This method of "trying it with minimal code in a test-project" is something I still use after years of programming to solve problems. When I solve a problem this way, I usually learn something new and often I discover that I made an assumption that did not hold true.

Why is my boolean not being changed?

So I'm trying to create a client/server program. I want to know when my client disconnects of his own accord, so I've setup a heartbeat system. Every 6 seconds my client sends a ping to my server, if the client doesn't send a ping for a total of 30 seconds the client is considered disconnected and removed from the current connections list (for which I plan to implement a GUI). Or at least, that's the plan.
ConnectionManager.java
public class ConnectionManager implements Runnable{
static Socket connection;
private ArrayList<Thread> allConnections;
private ArrayList<Connection> allConnectionList;
private ServerSocket server;
private int id = 0;
public ConnectionManager() {
allConnections = new ArrayList<Thread>();
allConnectionList = new ArrayList<Connection>();
}
#Override
public void run() {
try {
server = new ServerSocket(5555);
System.out.println("Server is running!");
while(true) {
connection = server.accept();
Connection a = new Connection(connection, id);
Runnable runnable = a;
allConnectionList.add(a);
allConnections.add(new Thread(runnable));
allConnections.get(allConnections.size() - 1).start();
id++;
}
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
public void removeConnection(int id) {
allConnections.remove(id);
allConnectionList.remove(id);
}
Connection.java
public class Connection implements Runnable {
private Socket a;
public boolean amIActive;
private int id;
public Connection(Socket a, int id) {
amIActive = true;
this.a = a;
this.id = id;
}
public void onConnect() {
try {
String TimeStamp = new java.util.Date().toString();
String formattedAddress = a.getInetAddress().toString().replace("/", "");
System.out.println("Received connection from: " + formattedAddress + " at " + TimeStamp);
Runnable runnable = new ConnectionListener(this);
Thread connectionThread = new Thread(runnable);
connectionThread.start();
String returnCode = "Server repsonded to " + a.getInetAddress().toString().replace("/", "") + " at "+ TimeStamp + (char) 13;
BufferedOutputStream os = new BufferedOutputStream(a.getOutputStream());
OutputStreamWriter osw = new OutputStreamWriter(os, "US-ASCII");
osw.write(returnCode);
osw.flush();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
#Override
public void run() {
onConnect();
System.out.println("We got this far!");
while(amIActive) {
whileTrue();
}
System.out.println("This code never gets run because we get stuck in the while loop above");
Main.b.removeConnection(id);
System.out.println("Connection was closed from " + a.getInetAddress());
}
public void setOffline(boolean state) {
this.amIActive = state;
}
public void whileTrue() {
}
public Socket getSocket() {
return a;
}
ConnectionListener.java
public class ConnectionListener implements Runnable{
public Connection myConnection;
public boolean receivedHeartbeat;
public int missedHeartbeats = 0;
public ConnectionListener(Connection a) {
this.myConnection = a;
}
#Override
public void run() {
Runnable runnable = new Heartbeat(this);
Thread thread = new Thread(runnable);
thread.start();
while(myConnection.amIActive) {
try {
BufferedInputStream is;
is = new BufferedInputStream(myConnection.getSocket().getInputStream());
InputStreamReader isr = new InputStreamReader(is);
StringBuffer process = new StringBuffer();
int character;
while((character = isr.read()) != 13) { //GETTING STUCK HERE BECAUSE STUPID.
if(character == -1) {
myConnection.setOffline(true);
} else {
process.append((char)character);
}
}
handleInput(process);
} catch (Exception e) {
e.printStackTrace();
}
}
}
public void handleInput(StringBuffer process) {
String messageSent = process.toString();
if(messageSent.equals("Ping!")) {
receivedHeartbeat = true;
}
}
Heartbeat.java
public class Heartbeat implements Runnable{
private ConnectionListener b;
public Heartbeat(ConnectionListener a) {
b = a;
}
#Override
public void run() {
while(true) {
try {
Thread.sleep(1000);
if(b.missedHeartbeats > 5) {
b.myConnection.amIActive = false;
System.out.println("Setting amIActiveToFalse!");
}
if(b.receivedHeartbeat) {
b.receivedHeartbeat = false;
} else {
b.missedHeartbeats++;
}
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
My console is spammed with System.out.println("Setting amIActiveToFalse!"); from Heartbeat.java. But the while loop in Connection.java keeps running. I believe this might be something to do with my threading, but I can't figure it out.

When you have a non-volatile variable, there is no guarentee of visability of a change in one thread to another. In particular, if the JVM detects that a thread doesn't alter a boolean it can inline it, meaning you will never see the value change.
The simple solution is to make the boolean volatile and it will not be inlined and one thread will see when another changes it.
For more details http://vanillajava.blogspot.com/2012/01/demonstrating-when-volatile-is-required.html

The trivial answer to this is: make the variable volatile.
Without this, it is allowed for the thread changing the value to basically keep its updates in cache, committing them to main memory some time later.
This allows threaded code to run much faster, since it can keep its variables in cache rather than having to fetch from main memory. However, the consequence of this is that other threads don't see the update.
Making the variable volatile prevents this from happening: a thread always reads the value from main memory, and writes are immediately committed.
I say that this is the trivial answer because it doesn't necessarily fix all of your problems. There may also be an atomicity issue: in between one thread reading the variable and writing it again, another thread might sneak in and change its value, which may or may not put the first thread into an undefined state from the perspective of its invariants.
Specifically:
if(b.receivedHeartbeat) { b.receivedHeartbeat = false;
It is possible that some other thread can change b.receivedHeartbeat to false after this thread evaluates it to true, so this iteration is erroneously counted as a "non-missed" heartbeat.
This can be fixed by making the variable a (non-volatile) AtomicBoolean, on which there is an atomic compare-and-set method, which avoids such race conditions.
Java Concurrency In Practice is a great reference on these issues, I wholeheartedly recommend it. Look for the topics "visibility" and "atomicity".
Also read the advanced chapter on the Java Memory Model. That made me doubt myself at first, but made me a much stronger programmer after I digested it.

There are a couple issues I saw while debugging the code you posted, but I was able to successfully get the heartbeat functionality working.
In the Connection Listener class I don't think the if statement with .equals("Ping!") will match, because of the newline character at the end of each line.
In the Connection Listener class I would probably put the socket's Input Stream at the top of the loop not inside the loop. (I don't think this will break it but it's probably nicer this way)
ConnectionListener Updates:
public void run() {
Runnable runnable = new Heartbeat(this);
Thread thread = new Thread(runnable);
thread.start();
BufferedReader br = null;
try {
//is = new BufferedInputStream(myConnection.getSocket().getInputStream());
br = new BufferedReader(new InputStreamReader(myConnection.getSocket().getInputStream()));
} catch (IOException e1) {
// TODO Auto-generated catch block
e1.printStackTrace();
}
while(myConnection.amIActive) {
try {
String processLine = br.readLine();
System.out.println("handleInput:" + processLine);
handleInput(processLine);
} catch (Exception e) {
System.out.println("Exception!");
e.printStackTrace();
}
}
}
public void handleInput(String messageSent) {
if(messageSent.startsWith("Ping!")) { //Need to use startsWith, or add newline character
receivedHeartbeat = true;
System.out.println("receivedHeartbeat!");
}
}
Also, in your Heartbeat class make sure you reset the missedHeartbeats counter to 0 on true:
if(b.receivedHeartbeat) {
b.receivedHeartbeat = false;
b.missedHeartbeats = 0;
} else {
b.missedHeartbeats++;
}

Java Socket - Updating instance variable List from Thread

I have a simple XO game which have multiple clients play XO Chess with each other. I built a sever which communicate with every clients connected. ( When a client connect to server, i create a new class (implement Runnable) handle that socket then start the Thread ... I have a List store all the client in the outer class ) When a client want to start a game , he make himself a new server and send the info of the Room(his name, IP address, port...) to the Server. And in the server I have a List which add a that room to the list and send it to others Clients.
public class ServerController {
private ServerView view;
private ServerSocket myServer;
private int serverPort = 8881;
private List<ExchangeData> list;
private volatile List<Room> rooms;
public ServerController(ServerView view) {
this.view = view;
openServer(serverPort);
view.showMessage("TCP server is running...");
list = new ArrayList<ExchangeData>();
rooms = new ArrayList<Room>();
while (true) {
listening();
}
}
private void openServer(int portNumber) {
....
}
private void listening() {
try {
Socket clientSocket = myServer.accept();
ExchangeData ex = new ExchangeData(clientSocket);
Thread thread = new Thread(ex);
thread.start();
list.add(ex);
} catch (IOException e) {
System.out.print(e.toString());
}
}
class ExchangeData implements Runnable {
private Socket clientSocket;
private ObjectOutputStream oos;
private ObjectInputStream ois;
private boolean run = true;
public ExchangeData(Socket socket) {
try {
clientSocket = socket;
oos = new ObjectOutputStream(clientSocket.getOutputStream());
ois = new ObjectInputStream(clientSocket.getInputStream());
} catch (Exception ex) {
System.out.println(ex.toString());
}
}
public void closeConnection() {
.....
}
public void sendData(Object o) {
try {
oos.writeObject(o);
} catch (Exception ex) {
view.showMessage(ex.toString());
}
}
public void run() {
try {
while (run) {
Object data = ois.readObject();
if (data instanceof Object[]) {
Object[] o = (Object[]) data;
String s = o[0].toString();
if (s.equalsIgnoreCase("Open Room")) {
Room r = (Room) o[1];
String username = r.getUsername();
InetSocketAddress isa = r.getIsa();
**rooms.add(r);**
List<Room> l = new ArrayList<Room>(rooms);
for (ExchangeData ex : list) {
ex.sendData(username + " Open A room at : "
+ isa);
**ex.sendData(rooms);**
}
} else if (s.equalsIgnoreCase("Close Room")) {
Room room = (Room) o[1];
rooms.remove(room);
List<Room> l = new ArrayList<Room>(rooms);
for (ExchangeData ex : list) {
ex.sendData(l);
}
}
} else if (data instanceof String) {
String s = data.toString();
if (s.equalsIgnoreCase("Quit")) {
stopThread();
list.remove(this);
closeConnection();
}
}
}
} catch (Exception ex) {
try {
stopThread();
list.remove(this);
} catch (ExceptionInInitializerError e) {
e.printStackTrace();
}
}
}
}
}
The problem is when i execute rooms.add(r) , others Thread doesn't seem to see that "update". For example:
1) First player opens a Room - the rooms size is 1;
2) Second player opens a new Room - the rooms size is now 2 ( I tried to add some lines of code in side the method sendData(Object o ) which println the size of rooms and all is 2. But when the line ObjectoutputStream.write(o) is executed, it actually write the value of rooms that Thread(first player) hold before which is 1.
If i create new list like List l = new ArrayList(rooms) like i did with the "Closing Room" then send this , it work. I don't understand why ? Someone please explain this to me. Sorry for my bad English :(.

When you send the same object several time through a given ObjectOutputStream, the stream writes the full object state the first time, but only sends some reference to this object afterwards.
This allows sending complex graphs of objects with cyclic references without consuming too much bandwidth and without going into infinit loops (send A which references B, so send B which references A, so send A which references B, etc.).
So, if you want to send a fresh copy of your list, you'll have to use the reset() method of ObjectOutputStream first.

Facing issue in java MultiThreading

Im facing one problem in streaming data capture for reading the broadcast data during multithreading, pls help or suggest,
Actually there is one class which is reading data from one of the udp socket. Another class accepts the tcp connection from every client request, creates a thread for every client and request the same udp class for data. The thing is working with 1st thread which gets created. But when i request with another client from another pc/ip the packets get losted to the 2nd client/thread
I have made a workaround by creating a list where im storing the Threads outputstream object
and looping it to send the data to all the client. But this is just temporary as it ll delay the packets if clients/connections gets increased.
code for reading UDP Data
public class EventNotifier
{
private InterestingEvent ie;
public DatagramSocket clientSocket;
public String[] split_str;
byte[] receiveData;
HashMap<String, String> secMap = new HashMap<String, String>();
public EventNotifier(InterestingEvent event)
{
ie = event;
clientSocket = new DatagramSocket(9050);
receiveData = new byte[500];
}
public String getDataFeed(String client_id)
{
try
{
DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
clientSocket.receive(receivePacket);
String s = new String(receivePacket.getData());
String split_str = s.split(",");
if(secMap.containsValue(split_str[0]))
return s;
else
return "";
} catch(Exception e3) {}
}
}// end of eventNotifier class
code for multithreading handling client requests
public class multiServer
{
static protected List<PrintWriter> writers = new ArrayList<PrintWriter>();
static String client_id = "";
public static void main(String[] args)
{
try
{
ServerSocket servsock = new ServerSocket(8858);
Socket incoming;
while(true)
{
incoming = servsock.accept();
multiServerThread connection = new multiServerThread(incoming);
Thread t1 = new Thread(connection);
t1.start();
}
}
catch(IOException e)
{
System.out.println("couldnt make socket");
}
}
}
class multiServerThread extends Thread implements InterestingEvent
{
Socket incoming;
PrintWriter out=null;
PrintWriter broad=null;
BufferedReader in = null;
String cliString=null;
private EventNotifier en;
int id;
public static String udp_data;
public void interestingEvent(String str1)
{
this.udp_data = str1;
}
public String getUdpData()
{
String _udp_data = this.udp_data;
return _udp_data;
}
multiServerThread(Socket incoming)
{
this.incoming=incoming;
en = new EventNotifier(this);
}
public void run()
{
try
{
out = new PrintWriter(incoming.getOutputStream(), true);
in = new BufferedReader(new InputStreamReader(incoming.getInputStream()));
cliString = in.readLine();
multiServer.writers.add(out);
while(true)
{
try
{
udp_data = en.getDataFeed(cliString);
if(udp_data!=null && udp_data.length()>0)
{
//workaround for serving the data to all cleints who are connected
for (int i=0; i<multiServer.writers.size();i++)
{
broad=multiServer.writers.get(i);
broad.println(udp_data.trim());
}
//else will directly write to the outputstream object for every thread which is connected
// out.println(udp_data.trim());
}
}
catch (Exception e)
{
System.out.println("exception "+e);
}
Thread.sleep(1);
}
} catch(IOException e)
{
System.out.print("IO Exception :: "+ e);
}
catch(InterruptedException e)
{
System.out.print("exception "+ e);
}
}
}

You need mutual exclusion (or a different design).
For example, what will happen if two threads call multiServer.writers.add(out); concurrently?
From the ArrayList Javadocs
Note that this implementation is not synchronized. If multiple threads access an ArrayList instance concurrently, and at least one of the threads modifies the list structurally, it must be synchronized externally. (A structural modification is any operation that adds or deletes one or more elements, or [...])
Another problem is two calling udp_data = en.getDataFeed(cliString); concurrently. The second thread might overwrite the result of the first. You'll loose data!
What happens if one thread calls for (int i=0; i<multiServer.writers.size();i++) while another thread is busy doing multiServer.writers.add(out);? The size may have increased, before out has actually been added to the list!
public class multiServer
{
private List<PrintWriter> writers = new ArrayList<PrintWriter>();
public synchronized void addWriter(PrintWrite out) {
writers.add(out);
}
public synchronized void serveAllWriters(String data) {
for (int i=0; i<multiServer.writers.size();i++)
{
broad=multiServer.writers.get(i);
broad.println(data);
}
}
}
Now when a thread tries to add a writer, the synchronizeds will make sure no other thread is adding or printing. So multiServerThread should be fixed to use the new methods:
class multiServerThread extends Thread implements InterestingEvent
{
//...
private String udp_data;
//...
myMultiServer.addWriter(out);
//...
udp_data = en.getDataFeed(cliString);
if(udp_data!=null && udp_data.length()>0)
myMultiServer.serveAllWriters(udp_data.trim());
//...
}
There might be more problems, not sure I don't fully understand your code. The question you must ask yourself is, can another thread read and/or write the same data or object? Yes? Then you'll need proper synchronization.