Develop Reference

Java multi-client chat server private message options

I am new at stackoverflow and I am sorry if this kind of a question is asked before but did a quick search and I could not find any title like mine. I am working on a multi-client chat application on Java. I was following the tutorials and I can send messages that every user in the application can see. But I wonder how to create and send a private message to a spesific user into the chat.
import java.io.*;
import java.net.*;
import java.util.HashSet;
import java.util.Set;
public class ChatServer {
private int port;
private Set<String> userNames = new HashSet<>();
private Set<UserThread> userThreads = new HashSet<>();
public ChatServer(int port) {
this.port = port;
}
public static void main(String[] args) {
new ChatServer(9999).execute();
}
private void execute() {
try {
ServerSocket serverSocket = new ServerSocket(9999);
System.out.println("Server is running");
while (true) {
Socket socket = serverSocket.accept();
System.out.println("New user connected");
UserThread newUser = new UserThread(socket, this);
userThreads.add(newUser);
newUser.start();
}
} catch (IOException e) {
e.printStackTrace();
}
}
public void addUserName(String s) {
this.userNames.add(s);
}
public void broadcast(String serverMessage, UserThread excludeUser) {
for (UserThread aUser : userThreads) {
if (aUser != excludeUser)
aUser.sendMessage(serverMessage);
}
}
}
The code above is my server code.
public void run() {
Console console = System.console();
String userName = console.readLine("Enter your username : ");
writer.println(userName);
String text;
do {
text = console.readLine("[" + userName + "]: ");
if (text.startsWith("[")) {
isTargeted = true;
this.aimUserName = text.substring(text.indexOf("[") + 1, text.indexOf("]"));
//System.out.println("Private Message to: " + aimUserName);
} else {
isTargeted = false;
}
writer.println(text);
} while (!text.equals("bye"));
try {
socket.close();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
and this code above is a part of my write thread class. As you can see, if a message starts with '[name]' part, the "name" means the user that we want to send a private message. By doing this, I can get the name of the user that I want to send a private message but I could not figure out how to broadcast this message just to that spesific user. I believe I need to configure my broadcast function in ChatServer class but I don't really know how to do. What steps should I follow?
--Edit--
I've been working on my question and I did some additions to solve my problem. First of all, I think I should share everything I have to you. I shared my ChatServer class previously. Other classes I have are:
import java.io.IOException;
import java.net.Socket;
public class ChatClient {
public static void main(String[] args) {
new ChatClient().execute();
}
private void execute() {
try {
Socket socket = new Socket("localhost", 3);
System.out.println("Connected to chat server");
new ReadThread(socket, this).start();
new WriteThread(socket, this).start();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
import java.net.*;
import java.io.*;
public class ReadThread extends Thread{
private BufferedReader reader;
private Socket socket;
private ChatClient client;
public ReadThread(Socket socket, ChatClient client) {
this.socket = socket;
this.client = client;
InputStream input;
try {
input = this.socket.getInputStream();
this.reader = new BufferedReader(new InputStreamReader(input));
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
public void run() {
while(true) {
try {
String response = this.reader.readLine();
System.out.println("\n" + response);
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
break;
}
}
}
}
import java.net.Socket;
import java.io.*;
public class UserThread extends Thread {
private Socket socket;
private ChatServer server;
PrintWriter writer = null;
public String userName;
public UserThread(Socket socket, ChatServer chatServer) {
this.socket = socket;
this.server = chatServer;
}
public void run() {
try {
InputStream input = socket.getInputStream();
BufferedReader reader = new BufferedReader(new InputStreamReader(input));
OutputStream output = socket.getOutputStream();
writer = new PrintWriter(output,true);
String userName = reader.readLine();
this.userName = userName;
server.addUserName(userName);
String serverMessage = "New user connected: " + userName;
server.broadcast(serverMessage,this);
String clientMessage;
do {
clientMessage = reader.readLine();
serverMessage = "[" + userName + "] : " + clientMessage;
server.broadcast(serverMessage, this);
}while(!clientMessage.equals("bye"));
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
public void sendMessage(String serverMessage) {
writer.println(serverMessage);
}
}
import java.net.*;
import java.io.*;
public class WriteThread extends Thread {
private Socket socket;
private ChatClient client;
private PrintWriter writer;
public WriteThread(Socket socket, ChatClient client) {
this.socket = socket;
this.client = client;
OutputStream output;
try {
output = socket.getOutputStream();
this.writer = new PrintWriter(output, true);
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
public void run() {
Console console = System.console();
String userName = console.readLine("Enter your username : ");
writer.println(userName);
String text;
do {
text = console.readLine("[" + userName + "]: ");
if(text.startsWith("[")){
String aimUserName = text.substring(text.indexOf("[")+1,text.indexOf("]"));
System.out.println("Private Message to: " + aimUserName);}
writer.println(text);
}while(!text.equals("bye"));
/*do {
text = console.readLine("[" + userName + "]: ");
writer.println(text);
}while(!text.equals("bye"));*/
try {
socket.close();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
These codes work properly and I can multi-chat very clean. But while working on private chat stuff, I added to the ChatServer the line of:
public void privatebr(String serverMessage, String targetUserName){
for(UserThread aUser: userThreads){
if(aUser.userName == targetUserName)
aUser.sendMessage(serverMessage);
}
to the UserThread, I edited the part as:
String clientMessage;
do {
clientMessage = reader.readLine();
serverMessage = "[" + userName + "] : " + clientMessage;
if(clientMessage.startsWith("[")){
String targetUserName = clientMessage.substring(clientMessage.indexOf("[")+1,clientMessage.indexOf("]"));
serverMessage = "[" + userName + "] : " + clientMessage;
server.privatebr(serverMessage, targetUserName);
}else{
server.broadcast(serverMessage, this);
}
}while(!clientMessage.equals("bye"));
But when I did all these edits, the normal multi-chat progress became broken where is my fault? Why everything has broken?

Good question! To answer the question you asked is that you should maintain a Map of Users to their Socket connections, so that way with DMs you can just select the user(s) that you want to message. You will also need a messaging protocol for that (see below)
...But I have to tell you that using Sockets and SocketServer classes in today's day and age is like re-inventing the wheel. The place to start in doing a chat server is using the web sockets protocol. Even under this, you will probably want to define a message protocol (like I did - I created a messaging protocol using JSON and message types, where the string message in the websocket event onMessage first gets parsed into an object)
There are implementations for supporting WS on all platforms: java, .net, python, php etc. This should be your starting point.
--- Update ---
I understand where you are coming from. To help you along in understanding Sockets / ServerSockets, here are a couple of pointers & resources
DatagramSockets (aka UDP): This is a different transmission protocol than the regular TCP, used by Shockwave and then Flash, and is the fundamental reason that Flash is problematic. I strongly recommend against this
Data & Object Input/OutputStreams: "Data" streams are Java only (can't connect to technologgy built on other platforms). Object streams are similar, except you are transporting actual whole objects through the stream (also Java only) No one* (almost no one) uses these anymore.
SocketException: Using java.net.[Server]Socket(s), you are likely to encounter this exception. It happens when you are waiting for more data (through a read / readLine call) on a socket, and the socket closes. It took me a long time to figure this out, but THIS EXCEPTION IS YOUR FRIEND! You get it when the connection has closed (either on the client or server side). It allows the thread that was waiting on the socket to wake up, and allows you to do whatever clean-up you need to do. SocketException is a subclass of IOException, so you may not even realize what this is. But now at least I have warned you
Streams vs. Writers and Readers: Writers and Readers are for interpreting raw bytes as Java characters and Strings. This is necessary, as there are multiple text formats (i.e. ascii, windows-xx, utf-8, utf-16). Readers and Writers help you read and write text in different text formats (and also interpreting Images from image formats).
Buffered Writers and Streams: These are for INEFFICIENT reading and writing. For writing, this means enabling you to write part of a message and not send it until you are ready. For reading, this means reading streams line by line for example rather than reading everything at one go.
TUS: tjacobs/io - https://sourceforge.net/p/tus/code/HEAD/tree/tjacobs/io/ this is an old collection of Java libraries I put on SourceForge years ago, but a lot of the classes here pertain to dealing with Sockets. In particular, see SocketServerEx, DataFetcher, Main/App, Timeout, and maybe IOUtils. And of everything, really look at DataFetcher which is a lightweight threadableframework for Callback I/O listening.
Good luck and have fun!

why this method is not giving me the subnet mask in android

I have a method that gets the subnet mask using android shell command. I tested on adb and works; however, I just put it in a method and I can get the output displayed on the android monitor console. If there's an easier way please advise. Thanks. By the way I'm running this in main activity thread (no Asynctask)
/*
* method to return private subnet mask
*/
public String getPrivateSubnet() {
String output = "";
final String SUBNET_CMD = "/system/bin/ifconfig wlan0 | find \"Mask\"";
try {
Process p = Runtime.getRuntime().exec(SUBNET_CMD);
// p.wait();
Log.v("SUBNET OUTPUT", p.toString());
BufferedReader in = new BufferedReader(new InputStreamReader(p.getInputStream()));
String inputLine;
while ((inputLine = in.readLine()) != null) {
Log.v("SUBNET", inputLine);
}
in.close();
} catch (IOException ex) {
ex.printStackTrace();
}
Log.v("SUBNET", output);
return output;
}

If you wish to use ifconfig use:
ifconfig wlan0 | awk '/netmask/{print $4}'
EDIT: I did some really quick coding of myself to find this one. Yes the Java API lets you use the NetworkInterface class to get the ipv4 subnetmask of an interface. I made a piece of code that may be helpful for you. This code gives you the CIDR value of each interface (for example: 24 would be 255.255.255.0). See more on this here: https://en.wikipedia.org/wiki/IPv4_subnetting_reference
import java.net.*;
import java.util.Enumeration;
public class Main {
public static void main(String[] args) throws UnknownHostException, SocketException {
Enumeration<NetworkInterface> networkInterfaces = NetworkInterface.getNetworkInterfaces();
while (networkInterfaces.hasMoreElements()) {
NetworkInterface networkInterface = networkInterfaces.nextElement();
try {
for (InterfaceAddress address : networkInterface.getInterfaceAddresses()) {
System.out.println(address.getNetworkPrefixLength());
}
} catch (NullPointerException e) {
e.printStackTrace();
}
}
}
}

Continuously read objects from an ObjectInputStream in Java

I have a problem using an ObjectInputStream and I have been struggling with it for 2 days now. I tried to search for a solution but unfortunately found no fitting answer.
I am trying to write a client/server application in which the client sends objects (in this case a configuration class) to the server. The idea is that connection keeps alive after sending the object so it is possible to send a new object if necessary.
Here are the important parts of my client code:
mSocket = new Socket("192.168.43.56", 1234);
mObjectIn = new ObjectInputStream(mSocket.getInputStream());
mObjectOut = new ObjectOutputStream(mSocket.getOutputStream());
mObjectOut.writeObject(stubConfig);
mObjectOut.flush();
In the above code, I left out some try/catch blocks to keep the code readable for you.
The server side looks as follows:
mHostServer = new ServerSocket(port);
mSocket = mHostServer.accept();
// create streams in reverse oreder
mObjectOut = new ObjectOutputStream(mConnection.getOutputStream());
mObjectOut.flush();
mObjectIn = new ObjectInputStream(mConnection.getInputStream());
while (mIsSocketConnected)
{
StubConfig = (StubConfiguration)mObjectIn.readObject();
}
What I want to achieve is that as long at the socketconnection is alive, the server is listening for incoming config objects.
When I run my program however, I got an EOFException in the while loop at server side. I receive the first config object without any problems in the first iteration of the while loop but after that I get an EOFException every time readObject() is called.
I am looking for a way to solve this. Can anyone put me in the good direction?
EDIT: What I read about the EOFException is that it is thrown when you want to read from a stream when the end of it is reached. That means that for some reason the stream ended after the object has been send. Is there a way to reinitialize the streams or so??

EOFException is thrown by readObject() when the peer has closed the connection. There can never be more data afterwards. Ergo you can't have written multiple objects at all: you closed the connection instead.

try using this
Server side
1.Server running on a separate thread
public class ServeurPresence implements Runnable {
public final static int PORT = 20000 ;
public final static String HOSTNAME = "localhost" ;
public static enum Action {CONNEXION, MSG, DECONNEXION,USER, FINCLASSEMENT};
ServerSocket serveur ;
static List<String> names ;
*/
public ServeurPresence()
{
System.out.println("Start Server...");
try
{
serveur = new ServerSocket(PORT) ;
new Thread(this).start();
//javax.swing.SwingUtilities.invokeLater(new Runnable() { public void run() { createAndShowGUI();} } );
}
catch (IOException e)
{
e.printStackTrace();
}
}
/**
* #param args
*/
public static void main(String[] args)
{
new ServeurPresence();
}
#Override
public void run()
{
System.out.println("server runs");
while(true)
{
try {
Socket sock = serveur.accept();
ServiceClientsThread thread= new ServiceClientsThread(sock);
thread.start();
}
catch (IOException e)
{
System.out.println("Error with socket");
e.printStackTrace();
}
}
}
}
2. A Thread to handle each Client:ServiceClientThread
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.net.Socket;
import java.util.ArrayList;
import java.util.List;
public class ServiceClientsThread extends Thread{
private Socket sock ;
ServiceClientsThread(Socket sock)
{
//super();
this.sock=sock;
}
#Override
public void run()
{
DataInputStream is ;
DataOutputStream os ;
String name =null ;
try {
is = new DataInputStream(sock.getInputStream()) ;
os = new DataOutputStream(sock.getOutputStream()) ;
ServeurPresence.Action act ;
do {
// read Action
act = ServeurPresence.Action.valueOf(is.readUTF()) ; // read string -> enum
System.out.println("action :"+act);
switch (act) {
case CONNEXION :
name = is.readUTF(); //read client name
System.out.println("Name :"+name);
os.writeUTF("Hi");//send welcome msg
break ;
case MSG :
String msg = is.readUTF();
os.writeUTF("OK");//response
break ;
case DECONNEXION :
System.out.println(name+" is logged out");
break ;
}
} while (act!=ServeurPresence.Action.DECONNEXION) ;
// the end
is.close();
os.close();
sock.close();
} catch (IOException e)
{
System.out.println("Error with "+name+" socket");
e.printStackTrace();
}
}
}
3. Client side
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.net.InetAddress;
import java.net.Socket;
import java.net.UnknownHostException;
public class Client {
/**
*
*/
Client(String name)
{
System.out.println("Start Client...");
try {
Socket sock = new Socket(ServeurPresence.HOSTNAME,ServeurPresence.PORT) ;
DataOutputStream os = new DataOutputStream(sock.getOutputStream()) ;
DataInputStream is = new DataInputStream(sock.getInputStream()) ;
System.out.println("Send "+name+" to server");
// CONNECTION : Action then value
os.writeUTF(ServeurPresence.Action.CONNEXION.name()) ; // send action : write enum -> String
os.writeUTF(name) ; // send the name
//read server welcome msg
String msg = is.readUTF();
System.out.println("Welcome msg: "+msg);
/* Your actions here : see example below */
try
{
Thread.currentThread().sleep(4000);
os.writeUTF(ServeurPresence.Action.MSG.name()) ; // send action : write enum -> String
os.writeUTF("My message here") ; // send msg
Thread.currentThread().sleep(4000);
msg = is.readUTF();//server response message
}
catch (InterruptedException e)
{
e.printStackTrace();
}
/************************************************/
//CLOSE
os.writeUTF(ServeurPresence.Action.DECONNEXION.name()) ; // send action
System.out.println("Log out");
os.close();
sock.close();
}
catch (UnknownHostException e)
{
System.out.println(ServeurPresence.HOSTNAME+ " unknown");
e.printStackTrace();
}
catch (IOException e)
{
System.out.println("Impossible to connect to "+ServeurPresence.HOSTNAME+ ":"+ServeurPresence.PORT);
e.printStackTrace();
}
}
}
4. In your case use readObject()/writeObject() instead of readUTF()/writeUTF() to write your config objects

Try this and let me know how it goes:
while (1==1)
{
StubConfig = (StubConfiguration)mObjectIn.readObject();
Thread.sleep(100); //Saves CPU usage
}

Very late answer, but just for future reference. I have been having problems sending Objects via sockets because the method flush() is not working properly.
I solved this problem just by switching flush() to reset().

Java process out of memory when creating thread on CentOS 6

I get this error basically less then an hour after i fire this nohup java -cp server.jar:mysql-connector.jar com.server.test.EchoMulti &. The error:
Exception in thread "main" java.lang.OutOfMemoryError: unable to create new native thread
at java.lang.Thread.start0(Native Method)
at java.lang.Thread.start(Unknown Source)
at com.server.test.EchoMulti.main(EchoMulti.java:14)
I'm new to servers and Java. This is the first thing I did on a server and in Java and it's a multiplayer for Android game. The game is doing quite good so there are many people trying to play the multiplayer. I already fixed the mySQL max_connections but now i got whis problem. I would also like to know if there is a way to restart process automatically when it gets an error that causes a crach. And a way to automatically start it on reboot.
The EchoMulti class that's causing it:
package com.server.test;
import java.net.*;
import java.io.*;
public class EchoMulti {
public static void main(String[] args) throws IOException {
int portNumber = 25003;
boolean listening = true;
try (ServerSocket serverSock = new ServerSocket(portNumber)) {
while (listening) {
new EchoServer(serverSock.accept()).start();
}
} catch (IOException e) {
System.err.println("Could not listen on port " + portNumber);
System.exit(-1);
}
}
}
The EchoServer class:
package com.server.test;
import java.net.*;
import java.io.*;
import java.sql.*;
import java.lang.Thread;
public class EchoServer extends Thread {
private Socket sock = null;
public EchoServer(Socket sock) {
super("EchoServer");
this.sock = sock;
}
public void run(){
String url = "jdbc:mysql://IP:3306/DB";
String username = "USER";
String password = "PASSWORD";
Connection con = null;
Statement sta = null;
ResultSet resultSet = null;
try {
//System.out.println("Connecting database...");
con = DriverManager.getConnection(url, username, password);
//System.out.println("Database connected!");
} catch (SQLException e) {
throw new RuntimeException("Cannot connect the database!", e);
}
try (
PrintWriter out = new PrintWriter(sock.getOutputStream(), true);
BufferedReader in = new BufferedReader(new InputStreamReader(sock.getInputStream()));
) {
String inputLine;
while ((inputLine = in.readLine()) != null) {
if(inputLine.equalsIgnoreCase("get")){
SEND SOMETHING
} else {
String catched[] = inputLine.split("\\;");
if(catched[0].equalsIgnoreCase("sub")){
SEND SOMETHING
} else if(catched[0].equalsIgnoreCase("res")){
SEND SOMETHING
}
}
}
} catch (IOException e) {
System.out.println("Exception caught when trying to listen on port 25003 or listening for a connection");
System.out.println(e.getMessage());
} finally {
//System.out.println("Closing the connection.");
if (con != null) try { con.close(); } catch (SQLException ignore) {}
try { sock.close(); } catch (IOException ignore) {} finally {}
}
}
public void close() throws IOException {
if (sock != null) sock.close();
}
}

You can also have a memory leak somewhere, or something preventing the correct garbage collection of used memory.
I would do the following three things.
Modify the EchoServer class
You can add a static member to understand if, when the crash occurs, the number of existing thread in memory is the one you expect (they should be as many as the connected players).
public class EchoServer extends Thread {
private Socket sock = null;
private static Integer CONNECTED = 0;
public EchoServer(Socket sock) {
try{
super("EchoServer");
this.sock = sock;
CONNECTED++;
} catch (Exception ex)
{
Logger.getLogger(EchoServer.class.getName()).log(Level.SEVERE, "Currently CONNECTED: "+CONNECTED);
}
}
.... your stuff here
#Override
public void finalize(){
super.finalize();
CONNECTED--;
}
}
If have more threads than connected players something is wrong.
Use a profiler Use a profiler to observe the memory usage and identify bottlenecks. Also be sure there aren't "strange" objects surviving generations or that the memory is not increasing over time without an increment of the players. NetBeans has a nice profiler that can be connected via TCP to live apps too.
Use -Xmx Use java -Xmx1g to launch the server.

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.