I’m learning how to implement a multi-user chat server using TCP in Java. I came across an example of on this on the web, but the something seems to be wrong in the ChatClient.java file. Ideally once the client connects to the server, the client is supposed to provide a nick name and if this nick name is valid… the server returns an “OK” message and the chat session can commence.
However, this doesn’t seem to work. Once I enter a nick name I don’t get an “OK” message back from the server and consequently can’t chat using the client.
The link for the original tutorial website is here (it’s the last example): http://pguides.net/java/tcp-client-server-chat
I tried searching the forum thread for this article… but it seems dead. I’d really appreciate it if someone could explain to me why it won’t work.
ChatServer.java:
/* ChatServer.java */
import java.net.ServerSocket;
import java.net.Socket;
import java.io.IOException;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.Hashtable;
public class ChatServer {
private static int port = 1001; /* port to listen on */
public static void main (String[] args) throws IOException {
ServerSocket server = null;
try {
server = new ServerSocket(port); /* start listening on the port */
} catch (IOException e) {
System.err.println("Could not listen on port: " + port);
System.err.println(e);
System.exit(1);
}
Socket client = null;
while(true) {
try {
client = server.accept();
} catch (IOException e) {
System.err.println("Accept failed.");
System.err.println(e);
System.exit(1);
}
/* start a new thread to handle this client */
Thread t = new Thread(new ClientConn(client));
t.start();
}
}
}
class ChatServerProtocol {
private String nick;
private ClientConn conn;
/* a hash table from user nicks to the corresponding connections */
private static Hashtable<String, ClientConn> nicks =
new Hashtable<String, ClientConn>();
private static final String msg_OK = "OK";
private static final String msg_NICK_IN_USE = "NICK IN USE";
private static final String msg_SPECIFY_NICK = "SPECIFY NICK";
private static final String msg_INVALID = "INVALID COMMAND";
private static final String msg_SEND_FAILED = "FAILED TO SEND";
/**
* Adds a nick to the hash table
* returns false if the nick is already in the table, true otherwise
*/
private static boolean add_nick(String nick, ClientConn c) {
if (nicks.containsKey(nick)) {
return false;
} else {
nicks.put(nick, c);
return true;
}
}
public ChatServerProtocol(ClientConn c) {
nick = null;
conn = c;
}
private void log(String msg) {
System.err.println(msg);
}
public boolean isAuthenticated() {
return ! (nick == null);
}
/**
* Implements the authentication protocol.
* This consists of checking that the message starts with the NICK command
* and that the nick following it is not already in use.
* returns:
* msg_OK if authenticated
* msg_NICK_IN_USE if the specified nick is already in use
* msg_SPECIFY_NICK if the message does not start with the NICK command
*/
private String authenticate(String msg) {
if(msg.startsWith("NICK")) {
String tryNick = msg.substring(5);
if(add_nick(tryNick, this.conn)) {
log("Nick " + tryNick + " joined.");
this.nick = tryNick;
return msg_OK;
} else {
return msg_NICK_IN_USE;
}
} else {
return msg_SPECIFY_NICK;
}
}
/**
* Send a message to another user.
* @recepient contains the recepient's nick
* @msg contains the message to send
* return true if the nick is registered in the hash, false otherwise
*/
private boolean sendMsg(String recipient, String msg) {
if (nicks.containsKey(recipient)) {
ClientConn c = nicks.get(recipient);
c.sendMsg(nick + ": " + msg);
return true;
} else {
return false;
}
}
/**
* Process a message coming from the client
*/
public String process(String msg) {
if (!isAuthenticated())
return authenticate(msg);
String[] msg_parts = msg.split(" ", 3);
String msg_type = msg_parts[0];
if(msg_type.equals("MSG")) {
if(msg_parts.length < 3) return msg_INVALID;
if(sendMsg(msg_parts[1], msg_parts[2])) return msg_OK;
else return msg_SEND_FAILED;
} else {
return msg_INVALID;
}
}
}
class ClientConn implements Runnable {
private Socket client;
private BufferedReader in = null;
private PrintWriter out = null;
ClientConn(Socket client) {
this.client = client;
try {
/* obtain an input stream to this client ... */
in = new BufferedReader(new InputStreamReader(
client.getInputStream()));
/* ... and an output stream to the same client */
out = new PrintWriter(client.getOutputStream(), true);
} catch (IOException e) {
System.err.println(e);
return;
}
}
public void run() {
String msg, response;
ChatServerProtocol protocol = new ChatServerProtocol(this);
try {
/* loop reading lines from the client which are processed
* according to our protocol and the resulting response is
* sent back to the client */
while ((msg = in.readLine()) != null) {
response = protocol.process(msg);
out.println("SERVER: " + response);
}
} catch (IOException e) {
System.err.println(e);
}
}
public void sendMsg(String msg) {
out.println(msg);
}
}
ChatClient.java:
/* ChatClient.java */
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;
import java.io.PrintWriter;
import java.net.Socket;
import java.net.UnknownHostException;
public class ChatClient {
private static int port = 1001; /* port to connect to */
private static String host = "localhost"; /* host to connect to */
private static BufferedReader stdIn;
private static String nick;
/**
* Read in a nickname from stdin and attempt to authenticate with the
* server by sending a NICK command to @out. If the response from @in
* is not equal to "OK" go bacl and read a nickname again
*/
private static String getNick(BufferedReader in,
PrintWriter out) throws IOException {
System.out.print("Enter your nick: ");
String msg = stdIn.readLine();
out.println("NICK " + msg);
String serverResponse = in.readLine();
if ("SERVER: OK".equals(serverResponse)) return msg;
System.out.println(serverResponse);
return getNick(in, out);
}
public static void main (String[] args) throws IOException {
Socket server = null;
try {
server = new Socket(host, port);
} catch (UnknownHostException e) {
System.err.println(e);
System.exit(1);
}
stdIn = new BufferedReader(new InputStreamReader(System.in));
/* obtain an output stream to the server... */
PrintWriter out = new PrintWriter(server.getOutputStream(), true);
/* ... and an input stream */
BufferedReader in = new BufferedReader(new InputStreamReader(
server.getInputStream()));
nick = getNick(in, out);
/* create a thread to asyncronously read messages from the server */
ServerConn sc = new ServerConn(server);
Thread t = new Thread(sc);
t.start();
String msg;
/* loop reading messages from stdin and sending them to the server */
while ((msg = stdIn.readLine()) != null) {
out.println(msg);
}
}
}
class ServerConn implements Runnable {
private BufferedReader in = null;
public ServerConn(Socket server) throws IOException {
/* obtain an input stream from the server */
in = new BufferedReader(new InputStreamReader(
server.getInputStream()));
}
public void run() {
String msg;
try {
/* loop reading messages from the server and show them
* on stdout */
while ((msg = in.readLine()) != null) {
System.out.println(msg);
}
} catch (IOException e) {
System.err.println(e);
}
}
}
Edit
From comment
This is what happens when I run the code:
client side:
Enter your nick: learnHK
hello
SERVER: INVALID COMMAND
not working?
SERVER: INVALID COMMAND
and on the server side:
Nick learnHK joined.
And that’s about it. The client isn’t getting the “OK” response message from the server and so can’t commence the chat. Thanks.
Your problem is that this is not a simple chat server where you just write whatever you want and it’s sent back. The server wants specific commands.
Here is an example session I just had:
As you can see, when I sent just the string “hello” I get an error back. But when I use “MSG arrow hello” it is parsed as a proper command by the server.