Multiple client threading "NullPointerException"
Hello Java Forum World :)
Coming up to a problem that should be simple...I think. But I can't get through it. I have a simple chat program, and some GUI that goes along with it. The program is an application, so the applet examples I've tried havent been working for me. ...well for that matter...the application examples arent working either. I think the problem is somehow related to the threading? Its either not receiving the data, or not sending it. Could anyone please look at my code? :)
3 Files
-Client
-MultiServer
-MultiServerThread
***Client***
import java.io.*;
import java.net.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class Client extends javax.swing.JFrame implements Runnable
private String message = "";
private String chatServer;
private Socket client;
private int port = 4444;
boolean DEBUG = true;
Socket socket;
String host = "127.0.0.1";
boolean trysted;
DataOutputStream o;
DataInputStream i;
protected Thread listener;
protected TextArea output;
protected TextField input;
/** Creates new form Client */
public Client( String host, InputStream i, OutputStream o ) {
super("Client");
i = new DataInputStream (new BufferedInputStream (i));
o = new DataOutputStream (new BufferedOutputStream (o));
initComponents();
chatServer = host; // set server to which this client connects
setSize( 300, 650 );
setVisible( true );
listener = new Thread (this);
listener.start();
public void start()
} // end method runClient
/////// send message to server
private void sendData( String message )
{ System.out.println("sendData()"+message);
try {
o.writeUTF ( message );
o.flush ();
} catch (IOException ex) {
ex.printStackTrace();
listener.stop ();
//input.setText ("");
private void initComponents() {
jInternalFrameChat = new javax.swing.JInternalFrame();
jTextFieldCompose = new javax.swing.JTextField();
jPanel1 = new javax.swing.JPanel();
jButton1 = new javax.swing.JButton();
jButton2 = new javax.swing.JButton();
jScrollPaneConversation = new javax.swing.JScrollPane();
jTextAreaConversation = new javax.swing.JTextArea();
jInternalFrameDraw = new javax.swing.JInternalFrame();
jInternalFrameTextSymbols = new javax.swing.JInternalFrame();
jButtonTextSymbolPlus = new javax.swing.JButton();
jMenuBar1 = new javax.swing.JMenuBar();
jMenuFile = new javax.swing.JMenu();
jMenuItemQuit = new javax.swing.JMenuItem();
jMenuConnecton = new javax.swing.JMenu();
jMenuItemDisconect = new javax.swing.JMenuItem();
getContentPane().setLayout(new java.awt.FlowLayout());
setTitle("Whiteboard");
addWindowListener(new java.awt.event.WindowAdapter() {
public void windowClosing(java.awt.event.WindowEvent evt) {
exitForm(evt);
jInternalFrameChat.setTitle("Chat");
jInternalFrameChat.setName("intFrameChat");
jInternalFrameChat.setPreferredSize(new java.awt.Dimension(200, 200));
jInternalFrameChat.setVisible(true);
jTextFieldCompose.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
sendData( jTextFieldCompose.getText() );
jTextFieldCompose.setText( "" );
jInternalFrameChat.getContentPane().add(jTextFieldCompose, java.awt.BorderLayout.NORTH);
jButton1.setText("Send");
jButton1.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
sendData(jTextFieldCompose.getText() );
jTextFieldCompose.setText( "" );
jPanel1.add(jButton1);
jButton2.setText("Insert Symbol");
jButton2.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
jButton2ActionPerformed(evt);
jPanel1.add(jButton2);
jInternalFrameChat.getContentPane().add(jPanel1, java.awt.BorderLayout.SOUTH);
jTextAreaConversation.setColumns(2);
jTextAreaConversation.setLineWrap(true);
jTextAreaConversation.setWrapStyleWord(true);
jScrollPaneConversation.setViewportView(jTextAreaConversation);
jInternalFrameChat.getContentPane().add(jScrollPaneConversation, java.awt.BorderLayout.CENTER);
getContentPane().add(jInternalFrameChat);
jInternalFrameDraw.setTitle("Draw");
jInternalFrameDraw.setPreferredSize(new java.awt.Dimension(230, 233));
jInternalFrameDraw.setVisible(true);
getContentPane().add(jInternalFrameDraw);
jInternalFrameTextSymbols.getContentPane().setLayout(new java.awt.FlowLayout());
jInternalFrameTextSymbols.setPreferredSize(new java.awt.Dimension(130, 133));
jButtonTextSymbolPlus.setText("+");
jButtonTextSymbolPlus.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
jButtonTextSymbolPlusActionPerformed(evt);
jInternalFrameTextSymbols.getContentPane().add(jButtonTextSymbolPlus);
getContentPane().add(jInternalFrameTextSymbols);
jMenuFile.setText("File");
jMenuItemQuit.setText("Quit");
jMenuItemQuit.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
jMenuItemQuitActionPerformed(evt);
jMenuFile.add(jMenuItemQuit);
jMenuBar1.add(jMenuFile);
jMenuConnecton.setText("Connection");
jMenuItemDisconect.setText("Item");
jMenuConnecton.add(jMenuItemDisconect);
jMenuBar1.add(jMenuConnecton);
setJMenuBar(jMenuBar1);
pack();
private void jButtonTextSymbolPlusActionPerformed(java.awt.event.ActionEvent evt) {
jTextFieldCompose.setText(jTextFieldCompose.getText()+"+");
private void jButton2ActionPerformed(java.awt.event.ActionEvent evt) {
jInternalFrameTextSymbols.setVisible(true);
private void jTextFieldComposeActionPerformed(java.awt.event.ActionEvent evt) {
jTextAreaConversation.setText(jTextAreaConversation.getText()+'\n'+"Student: "+jTextFieldCompose.getText());
jTextFieldCompose.setText("");
private void jMenuItemQuitActionPerformed(java.awt.event.ActionEvent evt) {
System.exit(0);
/** Exit the Application */
private void exitForm(java.awt.event.WindowEvent evt) {//GEN-FIRST:event_exitForm
System.exit(0);
public static void main( String args[] ) throws IOException
{System.out.println("main");
if (args.length != 2)
throw new RuntimeException ("Syntax: ChatClient <host> <port>");
Socket s = new Socket (args[0], Integer.parseInt (args[1]));
new Client ("Chat " + args[0] + ":" + args[1],
s.getInputStream (), s.getOutputStream ());
public void run() {
try {
while (true) {
String line = i.readUTF ();
output.appendText (line + "\n");
} catch (IOException ex) {
ex.printStackTrace ();
} finally {
listener = null;
//input.hide ();
validate ();
try {
o.close ();
} catch (IOException ex) {
ex.printStackTrace ();
}// end run
private javax.swing.JButton jButton1;
private javax.swing.JButton jButton2;
private javax.swing.JButton jButtonTextSymbolPlus;
private javax.swing.JInternalFrame jInternalFrameChat;
private javax.swing.JInternalFrame jInternalFrameDraw;
private javax.swing.JInternalFrame jInternalFrameTextSymbols;
private javax.swing.JMenuBar jMenuBar1;
private javax.swing.JMenu jMenuConnecton;
private javax.swing.JMenu jMenuFile;
private javax.swing.JMenuItem jMenuItemDisconect;
private javax.swing.JMenuItem jMenuItemQuit;
private javax.swing.JPanel jPanel1;
private javax.swing.JScrollPane jScrollPaneConversation;
private javax.swing.JTextArea jTextAreaConversation;
private javax.swing.JTextField jTextFieldCompose;
}***MultiServer***
import java.net.*;
import java.io.*;
class MultiServer
public MultiServer (int port) throws IOException {
ServerSocket server = new ServerSocket (port);
while (true) {
Socket client = server.accept ();
System.out.println ("Accepted from " + client.getInetAddress ());
ChatHandler c = new ChatHandler (client);
c.start ();
public static void main (String args[]) throws IOException {
if (args.length != 1)
throw new RuntimeException ("Syntax: MultiServer <port>");
new MultiServer (Integer.parseInt (args[0]));
}*** MultiServerThread ***
import java.io.*;
import java.net.*;
import java.util.*;
class MultiServerThread extends Thread
protected Socket s;
protected DataInputStream i;
protected DataOutputStream o;
public MultiServerThread(Socket s) throws IOException {
this.s = s;
i = new DataInputStream (new BufferedInputStream (s.getInputStream ()));
o = new DataOutputStream (new BufferedOutputStream (s.getOutputStream ()));
protected static Vector handlers = new Vector ();
public void run () {
try {
handlers.addElement (this);
while (true) {
String msg = i.readUTF ();
broadcast (msg);
} catch (IOException ex) {
ex.printStackTrace ();
} finally {
handlers.removeElement (this);
try {
s.close ();
} catch (IOException ex) {
ex.printStackTrace();
protected static void broadcast (String message) {
synchronized (handlers) {
Enumeration e = handlers.elements ();
while (e.hasMoreElements ()) {
ChatHandler c = (ChatHandler) e.nextElement ();
try {
synchronized (c.o) {
c.o.writeUTF (message);
c.o.flush ();
} catch (IOException ex) {
c.stop ();
} // end class
changed Client.constructor()-- public Client( String host, InputStream is, OutputStream os ) {
super("Client");
this.i = new DataInputStream (new BufferedInputStream (is));
this.o = new DataOutputStream (new BufferedOutputStream (os));----
changed MultiServer.constructor()--MultiServerThread c = new MultiServerThread (client);----
changed MultiServerThread.broadcast()--MultiServerThread c = (MultiServerThread) e.nextElement ();----
Error at Client.class:
java Client 127.0.0.1 4444
main
sendData()a
java.lang.NullPointerException
at Client.run(Client.java:215)
at java.lang.Thread.run(Unknown Source)
Error at MultiServer.class:
Accepted from /127.0.0.1
Accepted from /127.0.0.1
java.io.EOFException
at java.io.DataInputStream.readUnsignedShort(Unknown Source)
at java.io.DataInputStream.readUTF(Unknown Source)
at java.io.DataInputStream.readUTF(Unknown Source)
at MultiServerThread.run(MultiServerThread.java:23)
java.io.EOFException
at java.io.DataInputStream.readUnsignedShort(Unknown Source)
at java.io.DataInputStream.readUTF(Unknown Source)
at java.io.DataInputStream.readUTF(Unknown Source)
at MultiServerThread.run(MultiServerThread.java:23)
Error at other Client.class instance:
main
java.lang.NullPointerException
at Client.run(Client.java:215)
at java.lang.Thread.run(Unknown Source)
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import java.io.EOFException;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.net.ServerSocket;
import java.net.Socket;
import java.awt.BorderLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JFrame;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;
import java.util.concurrent.Executors;
import java.util.concurrent.ExecutorService;
import javax.swing.JFrame;
public class ServerTest
public static void main( String args[] )
Server application = new Server();
application.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
application.runServer();
class Server extends JFrame
private JTextField enterField;
private JTextArea displayArea;
private ObjectOutputStream output;
private ObjectInputStream input;
private ServerSocket server;
private Socket connection;
private int counter = 1;
private ExecutorService serverExecutor;
private MultiServer clients[];
public Server()
super( "Server" );
enterField = new JTextField();
enterField.setEditable( false );
enterField.addActionListener(
new ActionListener()
public void actionPerformed( ActionEvent event )
sendData( event.getActionCommand() );
enterField.setText( "" );
add( enterField, BorderLayout.NORTH );
displayArea = new JTextArea();
add( new JScrollPane( displayArea ), BorderLayout.CENTER );
setSize( 300, 150 );
setVisible( true );
public void runServer()
serverExecutor = Executors.newCachedThreadPool();
try
server = new ServerSocket( 12345, 100 );
while ( true )
try
waitForConnection();
getStreams();
processConnection();
catch ( EOFException eofException )
displayMessage( "\nServer terminated connection" );
finally
closeConnection();
counter++;
catch ( IOException ioException )
ioException.printStackTrace();
private void waitForConnection() throws IOException
displayMessage( "Waiting for connection\n" );
connection = server.accept();
serverExecutor.execute( new MultiServer(server, connection));
displayMessage( "Connection " + counter + " received from: " + connection.getInetAddress().getHostName() );
private void getStreams() throws IOException
output = new ObjectOutputStream( connection.getOutputStream() );
output.flush();
input = new ObjectInputStream( connection.getInputStream() );
displayMessage( "\nGot I/O streams\n" );
private void processConnection() throws IOException
String message = "Connection successful";
sendData( message );
setTextFieldEditable( true );
serverExecutor.execute(new MultiServer(server, connection));
do
try
message = ( String ) input.readObject();
displayMessage( "\n" + message );
catch ( ClassNotFoundException classNotFoundException )
displayMessage( "\nUnknown object type received" );
} while ( !message.equals( "CLIENT>>> TERMINATE" ) );
private void closeConnection()
displayMessage( "\nTerminating connection\n" );
setTextFieldEditable( false );
try
output.close();
input.close();
connection.close();
catch ( IOException ioException )
ioException.printStackTrace();
private void sendData( String message )
try
output.writeObject( "SERVER>>> " + message );
output.flush();
displayMessage( "\nSERVER>>> " + message );
catch ( IOException ioException )
displayArea.append( "\nError writing object" );
private void displayMessage( final String messageToDisplay )
SwingUtilities.invokeLater(
new Runnable()
public void run()
displayArea.append( messageToDisplay );
private void setTextFieldEditable( final boolean editable )
SwingUtilities.invokeLater(
new Runnable()
public void run()
enterField.setEditable( editable );
class MultiServer extends Thread
public MultiServer(ServerSocket servers, Socket connection)
servers = server;
public void run(){System.out.print("Yes");}
}Check out the "Supporting Multiple Clients" bit here: http://java.sun.com/docs/books/tutorial/networking/sockets/clientServer.html
Start a Thread for each client. I'd recommend you create a class:
class Client
Socket socket;
ObjectOutputStream out;
ObjectInputStream in;
...any other client-specific data you need...
public void sendMessage(String s) { ...send to this client...; }
...any other client-specific methods you need...;
}Create one of those when accept() gets a new connection, then start the thread for that client.
You may want to keep a LinkedList that contains all the Client objects. E.g. if you want to send a message to all clients you'd loop over the LinkedList and send to each in turn. Synchronize the list appropriately. Removing clients from the list when they close down can be interesting :-) so be careful. -
Thanks to the help I got from the good people of this forum today I modified the chat server program which I am making and now it is working a bit better. Right now it can accept several (up to 8) clients and store their open sockets in a SocketCollection list which can later be accessed so that messages get sent to all of them. Problem is that when I send a message, it is displayed instantaneously on the client that sent that message (my machine), but is displayed only after the other clients press enter on the other clients' machines (for testing purposes it was the same machine, different port opened for communication with server which is also on the same machine). To clarify that - all clients and server are the same machine (that shouldn't be a problem, right?). Code for printing incoming data for client program is below:
while ((fromServer = in.readLine()) != null) {
if (fromServer!=null){
System.out.println(fromServer);
System.out.println("print on client screen msg received from server");
fromUser = stdIn.readLine();
if (fromUser != null) {
System.out.println("Client: " + fromUser);
out.println(fromUser);
}Code to deliver the message to all clients from the server application is below as well:
while ((inputLine = in.readLine()) != null) {
PrintWriter multiOut;
for (int x=0; x<8; x++){
System.out.println("INSIDE MULTI-TELL FOR LOOP");
if (socketCollection[x]!=null){
Socket c=socketCollection[x];
try{
System.out.println("tried to send to all\nSocket c = " + c);
out=new PrintWriter(c.getOutputStream(), true);
out.println(c.getInetAddress() + inputLine);
catch(IOException ioe){}
}In the server's DOS window I can clearly see that it enters to display the message to multiple clients and it should send it. The sockets are all different and they point to different ports on different clients' machines. Maybe the problem is just in the client program's code. If you could help me out, it will be greatly appreciated. Thank you very much.The sockets get created one by one when each client connects. Afterwards when the thread is made, that socket gets copied into the SocketCollection array which can afterwards be accessed in order for the server to distribute the message to all its connected clients.
The for loop in the second code example where it takes SocketCollection[x] and gets its outputStream and all that is where it prints the message and sends it to each client (sends it but it is not displayed on the client side until the client sends a message to the server again). My question is how to make it so the client does not have to send a message in order to see the server message (message to all clients) that was sent before. -
Send data to multiple clients from a server
My problem statement is this:
A server is created, say X. Multiple clients are created, say A, B & C. If X sends a message to A it should reach only A and should not go to B or C. Similarly if X sends message to B it should not reach A or C. I made a one to one communication with the following code:
//Server
import java.io.*;
import java.net.*;
class X
public static void main(String args[])throws Exception
ServerSocket ss=new ServerSocket(4321);
try
Socket s=ss.accept();
BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
DataOutputStream out = new DataOutputStream(s.getOutputStream());
String str=in.readLine();
out.writeBytes(str+"\n");
s.close();
ss.close();
catch (IOException e){}
//Client A
import java.io.*;
import java.net.*;
class A
public static void main(String args[])throws Exception
try
Socket s=new Socket("localhost",4321);
BufferedReader in=new BufferedReader(new InputStreamReader(s.getInputStream()));
System.out.println(in.readLine());
s.close();
catch(Exception e){}
}But i dont know how to keep track of each client. Because all the clients sockets are created at the same port, ie. 4321. I think thread will help. But even then i dont know how to identify each client. Somebody please help. Thanks in advance.
Edited by: sowndar on Dec 5, 2009 1:21 AMYoungWinston wrote:
sowndar wrote:
Ok. I think i have to attach an unique ID to each client message. So that, with the help of that ID, the server can identify each client. Have i caught ur point?If you don't mind using a port per client, you could do something like a receptionist taking incoming calls (on 4321 only).
- Hi I'm Client xyz.
- Hi Client xyz, please call me back on port abcd and I'll put you straight through to our server.
Since 4321 is an "open line" you might have to have some sort of ID so that Clients know which return messages are meant for them, but messages on the other ports would all be direct Client to Server. Also, the Server is then is charge of port allocation and communication on the "open" port is kept to a minimum.4321 is the socket that the server is listening to. It's not what the actual communication will be carried out over. Run this, then telnet to port 12345 a few times
public class TestServerSocket {
public static void main(String[] args) throws Exception {
ServerSocket server = new ServerSocket(12345);
while (true) {
Socket socket = server.accept();
System.err.println(socket.getPort());
}Notice how each inbound connection is allocated a unique outbound socket. -
Calling pl/sql api through multiple java threads
Hi All,
I need to call a pl/sql api from multiple java threads simultaneously and all thread will use same db connection.
I want to know if all the threads will simultaneously call the pl/sql api then will the local variable inside pl/sql procedure be shared between them or they will get separate instances of variables.
TIAYou cannot make multiple parallel client calls over the same Oracle session handle. There is a single non-threaded/non-fibre serialised server process servicing client requests for that session. (physical process on Linux/Unix, thread in the oracle.exe process on Windows).
Each thread on the client side, needs its very own Oracle session. Thus each thread will have a server process footprint on the server. Which could be problematic if 10 clients each starts 10 threads - as it means a 100 processes on the server are needed to service these client threads.
Have a look at Overview of OCI Multithreaded Development in the Oracle® Call Interface Programmer's Guide for how to use the threading call interface of the OCI - as oppose to rolling your own where each thread manually needs to deal with is OCI session context. -
Networking: problems servering multiple clients
hi all
i'm writing a simple client server system, with a multithread server, in order to serve multiple clients.
the client's requests to connect to the server arrive to a port (ie 1025), and then the server, through a method returns to the client another port number, and then the comunication between them starts through the new port.
all work very fine, but i tried, with 2 computers, to start two clients at the "same time" (with a gap of few milliseconds), and my system "crashes".
i think that is a problem due to the second request that arrives while the comunication of the port from the server to the client happens.
is there a way to "queue" the requests arriving to the 1025 port of my server?
if i wasn't clear i can post some code
thanx in advance
sandroYes, teh code I posted does nothing more then listen for incoming conections and create a new Thread wich gets the Socket created by the accept to play with. This will happen for any incoming connection on the right port and will always be handeled the same.
As you'll see in the code i posted, there is some time between ServerSocket.accept returning a Socket and ServerSocket.accept being started again. This time shouldn't be to long to be sure the serversocket is listening for incoming connections when they arrive, so don't do to much inside the loop. If your system should handle a lot of connections simultaneously wou might have to optimise this be doing thing like having a few ClientThreads created allready to save the time of creating a new Thread. This becomes more important if you ClientThread is complex and slow to create. But when handelin less the say 25 clients you should be fine with this.
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