Develop Reference

How to handle multiple streams from a lobby server to clients

So, I'm having an issue with a project of mine. I'm writing a multiplayer lobby system which will enable multiple users to join a lobby, readying themselves by pressing a key. The issue that I'm facing is when two players is readying themselves, the lobby is only printing out a message for the last player who readied themselves. The system is built up in the following way.
Main Server
package master;
import java.net.*;
import java.io.*;
import java.util.*;
import java.io.*;
import java.net.InetAddress;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.Date;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.ArrayList;
import main.Lobby;
public class MainServer {
public static final int PORT = 4444;
public static final String HOST = "localhost";
public ArrayList<Lobby> serverList = new ArrayList<>();
public static void main(String[] args) throws IOException, ClassNotFoundException {
new MainServer().runServer();
}
public void runServer() throws IOException, ClassNotFoundException {
// Creating the server
ServerSocket serverSocket = new ServerSocket(PORT);
System.out.println("Main Server initiated.");
while (true) {
Socket socket = serverSocket.accept();
try {
// Establishing the connection to the Lobby server and then adding it to its list
ObjectInputStream objectInputStream = new ObjectInputStream(socket.getInputStream());
ObjectOutputStream objectOutputStream = new ObjectOutputStream(socket.getOutputStream());
objectOutputStream.writeObject("Server created successfully.");
Lobby s = (Lobby) objectInputStream.readObject();
this.serverList.add(s);
System.out.println("Server \"" + s.name + "\" added to game list.");
} catch (IOException e) {
e.printStackTrace();
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
}
}
}
The lobby
package main;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.net.ServerSocket;
import java.net.Socket;
import java.net.UnknownHostException;
import java.util.concurrent.Semaphore;
import master.MainServer;
/**
* The Class Server.
*/
public class Lobby implements Serializable {
private static final long serialVersionUID = -21654L;
public static final int PORT = 4445;
public static final int MAX_USERS = 5000;
public static final String HOST = "localhost";
public String name = "Lobby Server";
public int clientNumber;
public int playerNumberReady = 0;
public boolean allPlayersReady = false;
public boolean OddurIsNice = false;
public static void main(String[] args) throws IOException, ClassNotFoundException {
Lobby s = new Lobby();
s.runServer();
}
public void runServer() throws IOException, ClassNotFoundException {
registerServer();
new Thread( () -> {
try {
ServerSocket serverSocket = new ServerSocket(PORT);
System.out.println("Server waiting for connections...");
while (true) {
Socket socket = serverSocket.accept();
System.out.println("User 1 is now connected");
clientNumber++;
new ObjectOutputStream(socket.getOutputStream()).writeObject("You are connected man");
Socket socket2 = serverSocket.accept();
System.out.println("User 2 is now connected");
clientNumber++;
// ObjectOutputStream objectOutputStream2 = new ObjectOutputStream(socket2.getOutputStream());
// objectOutputStream2.writeObject("You are player number " + clientNumber + ". Waiting for other players to join");
new ServerThread(socket, socket2).start();
}
} catch (IOException ex) {
ex.printStackTrace();
}
}).start();
}
private void registerServer() throws UnknownHostException, IOException, ClassNotFoundException {
// Method for establishing a connection to the MainServer
Socket socket = new Socket(MainServer.HOST, MainServer.PORT);
ObjectOutputStream objectOutputStream = new ObjectOutputStream(socket.getOutputStream());
ObjectInputStream objectInputStream = new ObjectInputStream(socket.getInputStream());
objectOutputStream.writeObject(this);
System.out.println((String) objectInputStream.readObject());
}
public class ServerThread extends Thread {
public Socket socket = null;
public Socket socket2 = null;
ServerThread(Socket socket, Socket socket2) {
this.socket = socket;
this.socket2 = socket2;
}
public void run() {
try {
// This method is for when the client want's to connect to the lobby
ObjectInputStream objectInputStream = new ObjectInputStream(socket.getInputStream());
ObjectOutputStream objectOutputStream = new ObjectOutputStream(socket.getOutputStream());
System.out.println("User 1 is now connected");
ObjectInputStream objectInputStream2 = new ObjectInputStream(socket2.getInputStream());
ObjectOutputStream objectOutputStream2 = new ObjectOutputStream(socket2.getOutputStream());
System.out.println("User 2 is now connected");
BoardGameClient joined = (BoardGameClient) objectInputStream.readObject();
System.out.println(joined.name + " is now connected.");
while(true) {
objectOutputStream.writeObject("You joined the server.");
objectOutputStream.writeObject("You are player Number " + 1);
objectOutputStream.writeObject("Press '1' if you are ready");
objectOutputStream2.writeObject("You joined the server.");
objectOutputStream2.writeObject("You are player Number " + 2);
objectOutputStream2.writeObject("Press '1' if you are ready");
if(objectInputStream.readObject().equals(1)) {
playerNumberReady++;
}
if(objectInputStream2.readObject().equals(1)) {
playerNumberReady++;
}
if(playerNumberReady != 2) {
allPlayersReady = false;
} else {
allPlayersReady = true;
}
if (allPlayersReady == false) {
objectOutputStream.writeObject("Waiting...");
objectOutputStream2.writeObject("Waiting...");
}
if (allPlayersReady == true) {
objectOutputStream.writeObject("Lets GO");
objectOutputStream2.writeObject("Lets GO");
}
while (true) {
System.out.println(objectInputStream.readObject());
}
}
} catch (ClassNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
}
And the client
package main;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.net.Socket;
import java.net.UnknownHostException;
import java.util.Scanner;
import java.util.concurrent.Semaphore;
import master.MainServer;
public class BoardGameClient implements Serializable {
private int playerName;
private static final long serialVersionUID = -6224L;
public String name = "User";
private transient Socket socket;
public transient Scanner input = new Scanner(System.in);
public static void main(String[] args) {
BoardGameClient c = new BoardGameClient();
if (args.length > 0) {
c.name = args[0];
}
try {
c.joinServer();
} catch (ClassNotFoundException | IOException e) {
System.out.println("Failed to join server.");
e.printStackTrace();
}
}
public void joinServer() throws UnknownHostException, IOException, ClassNotFoundException {
socket = new Socket(Lobby.HOST, Lobby.PORT);
ObjectOutputStream objectOutputStream = new ObjectOutputStream(socket.getOutputStream());
ObjectInputStream objectInputStream = new ObjectInputStream(socket.getInputStream());
while(true) {
objectOutputStream.writeObject(this);
BufferedReader inputReader = new BufferedReader(new InputStreamReader(System.in));
System.out.println(objectInputStream.readObject());
System.out.println(objectInputStream.readObject());
System.out.println(objectInputStream.readObject());
int ready = input.nextInt();
objectOutputStream.writeObject(ready);
System.out.println(objectInputStream.readObject());
objectOutputStream.writeObject(name + ": " + inputReader.readLine());
}
}
}
I sincerely hope, that someone will be able to help me out <3

Firstly, there's a few things that bug me about this code. Not to sound condescending but you need to avoid rewriting code as much as possible. What happens if you want 3 or more players in the future? Currently you'd have to manually create a whole socket eg socket3, and then rewrite all the code you've already written. This is bad. You've manually spent the time creating 2 sockets and then created 2 streams for both of these sockets etc etc.
This can be automated don't you think?
Secondly, you have a lot of public variables. Unless they are static and final, for the most part you should keep variables as private.
I've tinkered with your lobby class as seen below, which is more scalable. It's not perfect by any means, but I feel illustrates the direction of improvement you should be heading for. Look up SOLID OOP principles, they'll help you guaranteed.
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.net.ServerSocket;
import java.net.Socket;
import java.net.UnknownHostException;
import java.util.LinkedHashSet;
import java.util.Set;
/**
* The Class Server.
*/
public class Lobby implements Serializable {
private static final long serialVersionUID = -21654L;
public static final int PORT = 4445;
public static final int MAX_USERS = 5000;
public static final String HOST = "localhost";
private static final int MIN_USERS = 2;
private String name = "Lobby Server";
private int clientNumber;
private boolean gameRunning = false;
// set of client connections
private final Set<ServerThread> clientConnectionThreads = new LinkedHashSet<>();
public static void main(String[] args) throws IOException, ClassNotFoundException {
Lobby s = new Lobby();
s.createLobby();
}
public void createLobby() throws IOException, ClassNotFoundException {
// waits for all players to ready up in a different thread
new Thread(this::waitReady).start();
registerServer();
// Listens for clients
runServer();
}
public void runServer() {
// closes serverSocket automatically in this way
try (ServerSocket serverSocket = new ServerSocket(PORT)) {
System.out.println("Server waiting for connections...");
long ids = 0;
while (!gameRunning) {
// accepts a new client connection
Socket socket = serverSocket.accept();
if (clientConnectionThreads.size() >= MAX_USERS) {
// tell user server is full and dont add the connection
} else {
// calculates the new id of the incoming player and adds them to the lobby
ids++;
this.clientConnectionThreads.add(new ServerThread(ids, socket));
System.out.println("User " + ids + " is now connected");
}
}
} catch (IOException ex) {
ex.printStackTrace();
}
}
/*
* loops until every player is ready and there is enough players and then starts
* the game.
*/
public void waitReady() {
while (true) {
try {
if (areAllReady() && this.clientConnectionThreads.size() >= MIN_USERS) {
startGame();
return;
}
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
// returns true if all users are ready
public boolean areAllReady() {
return clientConnectionThreads.stream().allMatch(ServerThread::isReady);
}
public void startGame() {
System.out.println("Starting game...");
this.gameRunning = true;
clientConnectionThreads.forEach(ServerThread::startGame);
// do game stuff
}
// i havent touched this function
private void registerServer() throws UnknownHostException, IOException, ClassNotFoundException {
// Method for establishing a connection to the MainServer
Socket socket = new Socket(MainServer.HOST, MainServer.PORT);
ObjectOutputStream objectOutputStream = new ObjectOutputStream(socket.getOutputStream());
ObjectInputStream objectInputStream = new ObjectInputStream(socket.getInputStream());
objectOutputStream.writeObject(this);
System.out.println((String) objectInputStream.readObject());
}
public class ServerThread extends Thread {
private final Socket socket;
private final ObjectInputStream in;
private final ObjectOutputStream out;
private final long id;
boolean ready = false;
private ServerThread(long id, Socket socket) throws IOException {
// does some basic initialization
this.socket = socket;
this.id = id;
in = new ObjectInputStream(socket.getInputStream());
out = new ObjectOutputStream(socket.getOutputStream());
// starts this connection thread
this.start();
}
public boolean isReady() {
return ready;
}
public void run() {
try {
// sets up the client and waits for their input
BoardGameClient joined = (BoardGameClient) in.readObject();
System.out.println(joined.name + " is now connected.");
out.writeObject("You joined the server.");
out.writeObject("You are player Number " + id);
out.writeObject("Press '1' if you are ready");
out.flush();
// waits for user to return ready
while (!ready) {
try {
int input = in.readInt();
System.out.println("input: " + input);
ready = input == 1;
} catch (ClassCastException e) {
e.printStackTrace();
}
}
out.writeObject("Waiting for players...");
} catch (ClassNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
public void startGame() {
// send client message etc etc
}
}
public String getName() {
return name;
}
}
I basically didn't change any of the other classes, except a few lines within the client class to make this work. (I've changed the ready input type from writeObject() to writeInt())
I haven't tested this for problems, but I know it works at least on a basic level.
I also suggest using writeUTS()/readUTS() instead of writeObject()/readObject() for sending and receiving Strings across streams as this will add extra complexity to the code.

Sending messages to clients TCP connection fails with an nullpointexception [duplicate]

This question already has answers here:
What is a NullPointerException, and how do I fix it?
(12 answers)
Closed 6 years ago.
Hi all :) Sorry for this really long question but this needs some explaination.
I was given an assignment where i have to turn a very simple game into a 2 player multiplayer game. The reason why we have to make this game is to learn more about threads and concurrency. I have never worked with concurrency nor with multiple threads.
My idea is to create a TCP server like i have done in GameServer.java where i create a new ServiceObject for each player. I create a thread for each ServiceObject where i will recieve, handle and send commands from a client.
Gameserver.java
package server;
import java.io.IOException;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.ArrayList;
public class GameServer {
public static void main(String[] args) throws IOException {
ServerSocket server = new ServerSocket(6789);
System.out.println("Waiting for clients to connect . . . ");
Socket s1 = server.accept();
System.out.println("Clients connected.");
PlayerService servicep1 = new PlayerService(s1);
Thread t1 = new Thread(servicep1);
Socket s2 = server.accept();
System.out.println("Clients connected.");
PlayerService servicep2 = new PlayerService(s2);
Thread t2 = new Thread(servicep2);
t1.start();
t2.start();
servicep1.sendDataToClient("ready");
servicep2.sendDataToClient("ready");
}
}
PlayerService.java
package server;
import java.io.IOException;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.net.Socket;
import java.util.ArrayList;
import java.util.Scanner;
import java.util.concurrent.LinkedBlockingQueue;
import game2016.Player;
public class PlayerService extends Thread {
private Socket s;
private PlayerService opponent;
private Scanner in;
private PrintWriter out;
public PlayerService(Socket aSocket) {
this.s = aSocket;
}
public void setOpponent(PlayerService opponent) {
this.opponent = opponent;
}
public void run() {
try {
in = new Scanner(s.getInputStream());
out = new PrintWriter(s.getOutputStream());
try {
doService();
} finally {
// s.close();
}
} catch (IOException exception) {
exception.printStackTrace();
}
}
public void doService() throws IOException {
while (true) {
if (!in.hasNext()) {
return;
}
String command = in.next();
if (command.equals("QUIT")) {
return;
} else
recieveFromClient(command);
}
}
public void recieveFromClient(String command) throws IOException {
System.out.println(command);
if(command.equals("player")) {
String newPlayerName = in.next();
int xPos = in.nextInt();
int yPos = in.nextInt();
String direction = in.next();
// sendDataToOpponent("addOpponent " + newPlayerName + " " + xPos + " " + yPos + " " + direction);
}
}
public void sendDataToClient(String response) {
out.write(response + "\n");
out.flush();
}
public void sendDataToOpponent(String response) {
opponent.sendDataToClient(response);
}
}
To send data from one client to another client i have a reference to the opponents servicelayer as i can invoke the sendDataToOpponent() method to send data to him and if the server have to communicate i can just invoke sendDataToClient() from the server.
My problem is that i want to postpone opening my clients GUI to both clients have connected.
Main.java(Client) - GUI code have been left out
private static Socket s;
private static InputStream instream;
private static OutputStream outstream;
private static Scanner in;
private static PrintWriter out;
private static boolean isOpponentConnected;
public static void main(String[] args) throws Exception {
openConnection();
reciever();
waitOpponentConected();
launch(args);
}
public static void waitOpponentConected() throws Exception {
while(!isOpponentConnected) {
System.out.println("Waiting for opponent");
Thread.sleep(2000);
}
System.out.println("Opponent is ready now");
}
public static void openConnection() throws IOException {
s = new Socket("localhost", 6789);
System.out.println("Connection established");
instream = s.getInputStream();
outstream = s.getOutputStream();
in = new Scanner(instream);
out = new PrintWriter(outstream);
}
public static void responseFromServer() throws IOException {
try {
while(in.hasNext()) {
String response = in.next();
if(response.equals("ready")) {
isOpponentConnected = true;
System.out.println("Ready");
}
}
} catch (Exception e) {
}
}
public static void reciever() {
Task<Void> task = new Task<Void>() {
#Override
protected Void call() throws Exception {
while(true) {
responseFromServer();
}
}
};
new Thread(task).start();
}
public static void sendCommandToServer(String command) throws IOException {
out.print(command + "\n");
out.flush();
}
I've created a Thread to recieve commands from the server, and when both clients have connected to the server it sends a string 'ready' to each of the clients. My thought was that The Main-thread sleeps till isOpponentConnected is true.
But my gameserver fails and prints out a nullpointer exception when the second clients connects to the server. I've spend to days reading and trying to fix this bug. When i run the code in debug mode, both clients recieves the ready signal and the GUI starts for both clients.
Exception in thread "main" java.lang.NullPointerException
at server.PlayerService.sendDataToClient(PlayerService.java:67)
at server.GameServer.main(GameServer.java:23)
Can you guys see anything i'm obviously doing wrong?
I think this queston is interesseting because it's not just the nullpointerexception, it's about structering TCP server-client relationships and the chain when things are initialized and ready when threads and connections are made.

It should be fixable from inside the PlayerService.java class you have posted.
I suggest moving:
in = new Scanner(s.getInputStream());
out = new PrintWriter(s.getOutputStream());
from public void run() to your PlayerService constructor:public PlayerService(Socket aSocket)
It looks like the function sendDataToClient is trying to use the out variable before it gets initialised.

Server Client Exception

I have a problem in running server-client program. When i run my server program , it keeps on running and never ends up. On other side, when i run my client program it throws an exception as shown below (my firewall is off).
The replies will be more than appreciated. Thanks
//Client Code
import java.io.*;
import java.net.*;
public class DailyAdviceClient
{
public void go()
{
try {
Socket s = new Socket("127.0.0.1", 4242);
InputStreamReader read = new InputStreamReader(s.getInputStream());
BufferedReader z = new BufferedReader(read);
String advice = z.readLine();
System.out.println("today you should" + advice);
z.close();
}
catch (IOException ex)
{
ex.printStackTrace();
}
}
public static void main(String[] args)
{
DailyAdviceClient x = new DailyAdviceClient();
x.go();
}
}
//Server Code
import java.io.*;
import java.net.*;
public class DailyAdvisor
{
String[] advicelist = { "take your time", "be patient",
"don't be diplomatic", " life is really short", "try to fix things" };
public void go()
{
try
{
ServerSocket s = new ServerSocket(4242);
while (true)
{
Socket m = s.accept();
PrintWriter writer = new PrintWriter(m.getOutputStream());
String advice = getAdvice();
writer.println(advice);
writer.close();
writer.flush();
System.out.println(advice);
}
} catch (IOException ex)
{
ex.printStackTrace();
}
}
private String getAdvice()
{
int random = (int) (Math.random() * advicelist.length);
return advicelist[random];
}
public static void main(String[] args)
{
DailyAdvisor x = new DailyAdvisor();
x.go();
}
}

The Server never ends up because you used a while(true) loop. It is necessary for your server to keep listening to new client connections through the accept() method.
About the exception, your code runs fine both locally and using a remote machine. Thus a network configuration error could be the cause and you must check if both server/client could see each other using the ping command. If this is the case, then check if the server is listening to the client using netstat.

Stuck with Multi-Client Chat Application

I'm trying to make a MultiClient Chat Application in which the chat is implemented in the client window. I've tried server and client code for the same. I've got two problems:
A. I believe the code should work but, server to client connections are just fine but information isn't transferred between clients.
B. I need a way to implement private one-to-one chat in case of more than two clients, I've used a class to store the information of the Socket object returned for each connection being established, but I can't figure out how to implement it.
The server code is:
import java.io.*;
import java.net.*;
class ClientInfo {
Socket socket;
String name;
public ClientInfo(Socket socket, String name) {
this.socket = socket;
this.name = name;
}
}
public class server {
private ObjectInputStream input[] = null;
private ObjectOutputStream output[] = null;
private String value = null;
private static ServerSocket server;
private Socket connection = null;
private static int i = -1;
public static void main(String args[]) {
try {
server = new ServerSocket(1500, 100);
while (true) {
System.out.println("Waiting for connection from client");
Socket connection = server.accept();
i++;
System.out.println("Connection received from " + (i + 1) + " source(s)");
//System.out.println(i);
new ClientInfo(connection, "Client no:" + (i + 1));
innerChat inc = new server().new innerChat(connection);
}
} catch (Exception e) {
System.out.println("Error in public static void main! >>>" + e);
}
}// end of main!!!
class innerChat implements Runnable {
private Socket connection = null;
public innerChat(Socket connection) {
this.connection = connection;
Thread t;
t = new Thread(this);
t.start();
}
public void run() {
try {
output[i] = new ObjectOutputStream(connection.getOutputStream());
output[i].flush();
input[i] = new ObjectInputStream(connection.getInputStream());
} catch (Exception e) {
}
}
}
}
And the client code is
import java.net.*;
import java.io.*;
import javax.swing.JFrame;
import javax.swing.JTextArea;
import javax.swing.JTextField;
import javax.swing.JButton;
import javax.swing.*;
import java.awt.event.*;
public class ChatappClient {
private static int port = 1500;
JFrame window = new JFrame("Chat");
JButton sendBox = new JButton("Send");
JTextField inputMsg = new JTextField(35);
JTextArea outputMsg = new JTextArea(10, 35);
private ObjectInputStream input;
private ObjectOutputStream output;
public static void main(String[] args) throws Exception {
ChatappClient c = new ChatappClient();
c.window.setVisible(true);
c.window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
c.run();
}
public ChatappClient() {
inputMsg.setSize(40, 20);
sendBox.setSize(5, 10);
outputMsg.setSize(35, 50);
inputMsg.setEditable(true);
outputMsg.setEditable(false);
window.getContentPane().add(inputMsg, "South");
window.getContentPane().add(outputMsg, "East");
window.getContentPane().add(sendBox, "West");
window.pack();
sendBox.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
try {
output.writeObject(inputMsg.getText());
outputMsg.append("\n" + "Client>>>" + inputMsg.getText());
output.flush();
} catch (IOException ie) {
outputMsg.append("Error encountered! " + ie);
}
inputMsg.setText("");
}
});
inputMsg.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
try {
output.writeObject(inputMsg.getText());
outputMsg.append("\n" + "Client>>>" + inputMsg.getText());
output.flush();
} catch (IOException ie) {
outputMsg.append("Error encountered! " + ie);
}
inputMsg.setText("");
}
});
}
private void run() throws IOException {
Socket clientSocket = new Socket("127.0.0.1", port);
output = new ObjectOutputStream(clientSocket.getOutputStream());
output.flush();
input = new ObjectInputStream(clientSocket.getInputStream());
outputMsg.append("I/O Success");
String value = null;
while (true) {
try {
value = (String) input.readObject();
} catch (Exception e) {
}
outputMsg.append(value + "\n");
}
}
}

Your code looks like it could be improved quite a bit. Your main method for instance should have none of that code in it. It should start your Server class, and that's it (and note that class names should begin with an upper case letter as per Java standards which I strongly advise you to follow).
I'm trying to make a MultiClient Chat Application in which a Server does nothing but listen and create connections.
The Server is going to have to do more than that. It will need to create Clients, it will need to maintain a collection such as an ArrayList of Clients such as an ArrayList<ChatappClient> Otherwise how do you expect the Server to connect two Clients together?
I think that in all you're going to need to think the structure and connections of this program out in more depth before starting to write code.

java : spawning new thread is causing original thread to halt

I have the following code in which I spawn a thread listen which is supposed to constantly listen to any incoming TCP messages, after this thread is run I want the main thread to be used for sending messages but as soon as I initiate listen.run() it seems that main thread does not run any further. I want it to continue to run the while loop but it never reaches it.
package tcpclient;
import java.io.*;
import java.net.*;
import java.net.UnknownHostException;
import java.util.Scanner;
import java.util.logging.Level;
import java.util.logging.Logger;
public class client {
//instance vars
static Socket cSocket =null;
static PrintWriter out = null;
static BufferedReader in = null;
//server info
static String serverName = null;
static int serverPort = 0;
static String userName=null;
//listening vars
static Thread listen;
static String incoming=null;
/**
* #param args the command line arguments
*/
public static void main(String[] args) throws IOException {
try {
System.out.println("\n\n\nTCP Chat Client\n\nEnter server name:");
Scanner scan = new Scanner(System.in);
//get server info from user
serverName = scan.nextLine();
System.out.println("\nEnter port number:");
serverPort = Integer.parseInt(scan.nextLine());
System.out.println("\nEnter your username:");
userName = scan.nextLine();
//make connection to server
cSocket = new Socket(serverName, serverPort);
out = new PrintWriter(cSocket.getOutputStream(), true);
in = new BufferedReader(new InputStreamReader(cSocket.getInputStream()));
//send username to server
out.println(userName);
//start listening
listen = new Thread(){
#Override
public void run(){
try {
incoming = in.readLine();
while (!(incoming.equals(null))) {
System.out.print(incoming);
incoming = in.readLine();
}
} catch (IOException ex) {
Logger.getLogger(client.class.getName()).log(Level.SEVERE, null, ex);
}
}
};
listen.run();
String rcvrname="wefwef";
String message=null;
//start messaging
while(!(rcvrname.equals("exit"))){
System.out.println("Enter reciever name");
out.println(scan.nextLine());
System.out.println("Enter message");
out.println(scan.nextLine());
}
out.close();
in.close();
cSocket.close();
}
catch (UnknownHostException ex) {
System.err.println("\ncan't find that host\n");
}
catch (IOException ex) {
Logger.getLogger(client.class.getName()).log(Level.SEVERE, null, ex);
}
finally{
in.close();
out.close();
cSocket.close();
}
}
}

You want:
listen.start();
not
listen.run();
Also the preferred way of doing this since Java 5 is to use an ExecutorService:
Runnable listen = new Runnable() {
public void run() { ... }
}
ExecutorService exec = Executors.newSingleThreadExecutor();
exec.submit(listen);
and when you want to stop it:
exec.shutdown();
exec.awaitTermination(Long.MAX_VALUE, TimeUnit.NANOSECONDS);
or simply:
exec.shutdownNow();

Here is the problem:
listen.run();
You mustn't call run() itself! You should call start(), this spawns a thread, calling run() just call a function.