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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.

Need help on a simple server/multiple client chat application

this is the scenario .. i have 2 clients connected to a server.. i want them to be able to chat with eachother. After a couple of messages i get this error.
Exception in thread "Thread-0" java.lang.ClassCastException:
java.io.ObjectStreamClass cannot be cast to message.Mesaj
at server.ServerThread.readAndWrite(ServerThread.java:43)
at server.ServerThread.run(ServerThread.java:61)
java.io.StreamCorruptedException: invalid type code: 00
at java.io.ObjectInputStream.readObject0(Unknown Source)
at java.io.ObjectInputStream.readObject(Unknown Source)
at server.ServerThread.readAndWrite(ServerThread.java:43)
at server.ServerThread.run(ServerThread.java:61)
This is the client:
package client;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.net.Socket;
import java.util.Scanner;
import message.Mesaj;
public class Client {
public static int port=4321;
public static Socket socket;
public static ObjectOutputStream oo;
public static ObjectInputStream oi;
public static Scanner sc;
public Client() throws IOException{
socket = new Socket ("localhost",4321);
oi = new ObjectInputStream(socket.getInputStream());
oo = new ObjectOutputStream(socket.getOutputStream());
}
public static void listen() throws ClassNotFoundException, IOException{
while(true){
Mesaj m = (Mesaj) oi.readObject();
if(m!=null){
System.out.println("mesajul este: " + m.getMesaj());
}
}
}
public static void write() throws IOException{
sc= new Scanner(System.in);
while(true){
String trimite= sc.nextLine();
Mesaj m = new Mesaj();
m.setMesaj(trimite);
oo.writeObject(m);
oo.flush();
}
}
public static Thread t = new Thread(){
public void run(){
try {
listen();
} catch (ClassNotFoundException | IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
};
public static Thread t2 = new Thread(){
public void run(){
try {
write();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
};
public static void main(String[] args) throws IOException{
new Client();
t.start();
t2.start();
}
This is the Server:
package server;
import java.io.IOException;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.ArrayList;
public class Server {
public int port;
public static Socket socket;
public static ServerSocket serverSocket;
public Server() throws IOException{
this.port=4321;
serverSocket = new ServerSocket(port);
}
public static void main (String [] args){
try {
new Server();
} catch (IOException e1) {
// TODO Auto-generated catch block
e1.printStackTrace();
}
System.out.println("Server functionabil..asteptam conexiune client");
while(true){
try {
socket= serverSocket.accept();
ServerThread st= new ServerThread(socket);
st.start();
System.out.println("Conexiune realizata -client conectat");
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (ClassNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
}
..and this is the Server Thread:
package server;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.net.Socket;
import java.util.ArrayList;
import message.Mesaj;
public class ServerThread extends Thread {
boolean running;
public static ObjectOutputStream oo;
public static ObjectInputStream oi;
public static Mesaj m;
public static Socket socket;
public ServerThread(Socket socket) throws ClassNotFoundException{
try {
running=true;
this.socket=socket;
oo = new ObjectOutputStream(socket.getOutputStream());
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
public static void readAndWrite() throws ClassNotFoundException, IOException{
oi = new ObjectInputStream(socket.getInputStream());
while(true){
m= (Mesaj) oi.readObject();
if(m!=null){
oo.writeObject(m);
oo.flush();
System.out.println(m.getMesaj());
}
}
}
public void run(){
System.out.println("Server Thread contectat");
try {
readAndWrite();
} catch (IOException | ClassNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
EDIT:
This is the message class:
package message;
import java.io.Serializable;
public class Mesaj implements Serializable {
private String mesaj;
public String getMesaj() {
return mesaj;
}
public void setMesaj(String mesaj) {
this.mesaj = mesaj;
}
}

You having concurrency issues.
Both serverthreads access the same static outputstream, meaning that there is a change for corruption as Object streams aren't designed for this.
In ServerThread, remove static from all the variables and the method "readAndWrite".
public ObjectOutputStream oo;
public ObjectInputStream oi;
public Mesaj m;
public Socket socket;
...
public void readAndWrite() throws ClassNotFoundException, IOException{
If you want to access the output streams from multiple threads, you should synchronize on them.

Java write to .csv file

I am trying to write to a .csv file, but I keep getting the error:
Exception in thread "main" java.lang.Error: Unresolved compilation problems:
void is an invalid type for the variable writeToFile
Syntax error on token "(", ; expected
Syntax error on token ")", ; expected
The error is associated with the line:
void writeToFile(String Filename){
Here is my code:
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.FileWriter;
public class writeFileExample {
public static void main(String[] args) {
void writeToFile(String Filename){
double steps=0;
File file=new File(Filename);
file.createNewFile();
FileWriter writer=new FileWriter(file);
try {
//Integrate integrate=new Integrate();
//for (steps=10;steps<1000000;steps=steps*10){
//double area_value=integrate.integrate_function(steps);
writer.write("Steps"+","+"Area");
//}
//System.out.println(area_value);
writer.flush();
writer.close();
System.out.println("Done");
} catch (IOException e) {
e.printStackTrace();
} finally {
try {
if (writer != null) {
writer.close();
}
} catch (IOException e) {
e.printStackTrace();
}
}
}
}
}
I can't see any syntax errors.
Taking into account Reimeus' comment below I edited it a bit. I now have:
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.FileWriter;
void writeToFile(String Filename){
public class writeFileExample {
public static void main(String[] args) {
double steps=0;
etc.
I am getting the error:
Exception in thread "main" java.lang.Error: Unresolved compilation problem:
Filename cannot be resolved to a variable
Any help appreciated.

Java doesnt support nested methods. Move writeToFile out of the main method

public class Test {
public static void main(String[] args) {
try {
writeToFile("c:\\abc.csv");
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
static public void writeToFile(String Filename) throws IOException
{
.
.
.
.
}
}

Running xjc from java code

I am using xjc to generate classes from xsd. The generation has to happen inside the java code. Right now I have done it like this:
Process child = Runtime.getRuntime().exec(command);
try {
System.out.println("waiting...");
child.waitFor();
System.out.println("waiting ended..");
} catch (InterruptedException e) {
e.printStackTrace();
return false;
}
The output for the above program is:
waiting...
I have to use the classes after they are generated. The problem here is that the subprocess never exits and the control is never back to the java program!
Is there a way to do this without getRuntime().exec() ?

You can actually use the driver class (com.sun.tools.xjc.Driver) behind the command line tool. This worked for me:
import com.sun.tools.xjc.BadCommandLineException;
import com.sun.tools.xjc.Driver;
import com.sun.tools.xjc.XJCListener;
import org.xml.sax.SAXParseException;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
public class Generator {
public static void main(String[] args) throws BadCommandLineException, IOException {
final String targetDir = "jaxb-files";
Path path = Paths.get(targetDir);
if(!Files.exists(path)) {
Files.createDirectories(path);
}
Driver.run(new String[]{"-d", targetDir,
"D:\\dev\\onepoint\\tui\\java\\xsdjsonschema\\src\\main\\xsd\\test.xsd"}, new XJCListener() {
#Override
public void error(SAXParseException e) {
printError(e, "ERROR");
}
#Override
public void fatalError(SAXParseException e) {
printError(e, "FATAL");
}
#Override
public void warning(SAXParseException e) {
printError(e, "WARN");
}
#Override
public void info(SAXParseException e) {
printError(e, "INFO");
}
private void printError(SAXParseException e, String level) {
System.err.printf("%s: SAX Parse exception", level);
e.printStackTrace();
}
});
}
}

try this
Process child = Runtime.getRuntime().exec(command);
BufferedReader in = new BufferedReader(
new InputStreamReader(child.getInputStream()));
String line = null;
while ((line = in.readLine()) != null) {
System.out.println(line);
}

NullPointerException in Thread's run method

I would really appreciate help with my program. It is some sort of chat server with multiple clients.
Here's the server code:
package com.server;
import java.io.IOException;
import java.net.ServerSocket;
import java.net.Socket;
public class Server {
public static int PORT;
private ServerSocket server;
private Socket socket;
public Server(int port) throws IOException {
PORT = port;
server = new ServerSocket(PORT);
System.out.println("server started");
try {
while (true) {
socket = server.accept();
try {
new ServeClient(socket);
} catch (IOException e) {
socket.close();
}
}
} finally {
server.close();
}
}
public static void main(String[] args) throws IOException {
int port = Integer.parseInt(args[0]);
Server server = new Server(port);
}
}
I start the server and then create a Client. The server receives connection socket from socket
and creates a ServeClient Thread.
Here's ServeClient code:
package com.server;
import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.net.Socket;
import java.util.Enumeration;
import java.util.Vector;
import com.gui.WindowManager;
public class ServeClient extends Thread {
private final Socket socket;
private BufferedReader in;
private PrintWriter out;
private String msg;
public static final String ENDSTRING = "END";
public static Vector clients = new Vector();
public ServeClient(final Socket socket) throws IOException {
this.socket = socket;
System.out.println("socket " + socket);
in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(
socket.getOutputStream())), true);
start();
}
public void run() {
try {
clients.add(this);
while (true) {
msg = in.readLine();
if (msg == ENDSTRING)
break;
broadcast(msg);
}
System.out.println("closing...");
} catch (IOException e) {
System.err.println("IO EXCEPTION");
} finally {
try {
socket.close();
} catch (IOException e) {
System.err.println("SOCKET NOT CLOSED");
}
}
}
#SuppressWarnings("deprecation")
public void broadcast(String msg) {
synchronized (clients) {
Enumeration<ServeClient> e = clients.elements();
while (e.hasMoreElements()) {
ServeClient serveClient = e.nextElement();
try {
synchronized (serveClient.out) {
serveClient.out.println(msg);
}
} catch (Exception eee) {
serveClient.stop();
}
}
}
}
}
What i get is a NullPointerException when ServeClient invokes run() method
server started
socket Socket[addr=/127.0.0.1,port=51438,localport=8888]
Exception in thread "Thread-0" java.lang.NullPointerException
at com.server.ServeClient.run(ServeClient.java:33)
line 33 is the line with first "try" statement in ServeClient run() method

com.server.ServeClient.run(ServeClient.java:33)
I don't believe that it's happening at the try.
Open up an IDE, turn on debugging, and step through until you can see what's happening. That's the fastest way to figure out what you've missed.
There's an object that you're assuming is fine that is not. Find it.
Here's an example of how to do this properly:
http://www.kodejava.org/examples/216.html

Your problem is with the order in which static instance variables are initialised. Try doing something like:
...
private static Vector clients = null;
...
if (clients==null) {
clients = new Vector(); // consider putting this in a synchronized block
}
before you add the client to the vector.

Sorry for necroing such an old issue but it seemed like this problem wasn't resolved, so I'll give a bit of input from my end.
I've had a similar problem and the compiler also kept telling me that the problem was at the start() method. However, when I commented out the thread part and just ran the code on the same thread as the UI, the compiler directed me to the real source of the problem: the code inside the thread.
After making sure the code didn't give an error, I enclosed the code with the original thread code, and it stopped giving me the NullPointerException error.
Hope this helps someone along the way.

Remove the duplicate class declaration in JPanel.
I was trying to run a timer thread that updated a clock in the main application window.
I had created the JFrame with Eclipse/WindowBuilder and had followed a tutorial on how to make a timer. I had copied the declaration of the textfield into the class declaration to make it available for the entire class, but forgot to remove the Class Id in front of the widget definition. So it still initialized the local instance and not the global one. Thus when I accessed the global one it was still null.