Develop Reference

Sending a Message to Multiple Clients from One Server Simultaneously?

I am currently working on a TCP Sockets program in Java in which two or more separate clients connect to a single server, and that server will send a message to both of these connected clients simultaneously.
I've tried to work through the code multiple times, but I can't quite seem to be able to have one message be sent to both clients.
Below is a reduced version of my entire code, condensed down to just the issue I'm having.
I've also included a video, just to save you the effort of having to copy and run my code!
[STREAMABLE LINK]
When one client is connected, I just have to write one message in the server, press send, and it shows up in the client.
When two clients are connected, I have to write two messages in the server and press send twice, and one message goes to one client and the other to the next client.
How can I make it so I only send one message from the server that goes to all clients?
I greatly appreciate any and all help.
SERVER CODE:
import java.io.*;
import java.net.*;
import java.util.*;
// Server class
class Server {
public class countLogic {
public static int client_count = 0;
}
public static void main(String[] args) {
System.out.println("[SERVER]");
ServerSocket server = null;
try {
server = new ServerSocket(1234);
server.setReuseAddress(true);
while (true) {
Socket client = server.accept();
countLogic.client_count++;
System.out.println("Client ("+countLogic.client_count+") connected: " + client.getInetAddress().getHostAddress());
ClientHandler clientSock = new ClientHandler(client);
new Thread(clientSock).start();
}
}
catch (IOException e) {
e.printStackTrace();
}
finally {
if (server != null) {
try {
server.close();
}
catch (IOException e) {
e.printStackTrace();
}
}
}
}
// ClientHandler class
private static class ClientHandler implements Runnable {
private final Socket clientSocket;
public ClientHandler(Socket socket)
{
this.clientSocket = socket;
}
public void run()
{
PrintWriter out = null;
BufferedReader in = null;
try {
Scanner sc = new Scanner(System.in);
out = new PrintWriter(clientSocket.getOutputStream(), true);
in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
String line = null;
while (true) {
line = sc.nextLine();
out.println(line);
}
}
catch (IOException e) {
e.printStackTrace();
}
finally {
try {
if (out != null) {
out.close();
}
if (in != null) {
in.close();
clientSocket.close();
}
}
catch (IOException e) {
e.printStackTrace();
}
}
}
}
}
CLIENT CODE:
import java.io.*;
import java.net.*;
import java.util.*;
// Client class
class Client {
// driver code
public static void main(String[] args)
{
System.out.println("[CLIENT 1]");
try (Socket socket = new Socket("localhost", 1234)) {
PrintWriter out = new PrintWriter(
socket.getOutputStream(), true);
BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
Scanner sc = new Scanner(System.in);
String line = null;
while (!"exit".equalsIgnoreCase(line)) {
System.out.println("Server: "+ in.readLine());
}
// closing the scanner object
sc.close();
}
catch (IOException e) {
e.printStackTrace();
}
}
}

Read/write in simple client-server app in Java

I'm new with Java and I'm trying to learn threads and socket. So decide to make simple client-server application following official java tutorial. My idea is simple - server wait for connection, if appears, it makes new thread with new socket, input and output. Client side -> make connection; new thread with socket, input, output and stdIn (to read line and after that send it to the server). But something is wrong (don't have any idea why) with my code. The connection is established, there's no exceptions. Could someone explain why doesn't work and how to fix it? Also could you have any suggestions about the code (probably it's not with best practices and things like that):
Client side:
public class Client {
private BufferedReader reader;
private Socket sock;
private PrintWriter writer;
public static void main(String[] args) {
Client client = new Client();
client.go();
}
public void go() {
setUpNetworking();
}
private void setUpNetworking() {
try{
sock = new Socket("127.0.0.1", 5000);
System.out.println("Network established");
ServerThread serverThread= new ServerThread(sock);
serverThread.start();
System.out.println("Type your message: ");
} catch (IOException e) {
System.out.println("Problem with establishing the network: " + e);
}
}
class ServerThread extends Thread {
Socket socket;
PrintWriter out;
BufferedReader in;
BufferedReader stdIn;
ServerThread(Socket socket) {
this.socket = socket;
try{
out = new PrintWriter(socket.getOutputStream());
in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
stdIn = new BufferedReader(new InputStreamReader(System.in));
}catch (IOException e) {
System.out.println("Problem with trying to read/write to server: " + e);
}
}
#Override
public void run() {
String fromServer;
String fromClient;
while(true){
try{
if((fromServer = in.readLine()) != null) System.out.println(" " + fromServer);
else if((fromClient = stdIn.readLine()) != null) out.println(fromClient);
}catch(Exception e) {
System.out.println("msg exception: " + e);
}
}
}
}
}
Server side:
public class Server {
//Run server until keepGoing = false
private boolean keepGoing = true;
public static void main(String[] args) {
Server server = new Server();
server.go();
}
public void go() {
try {
ServerSocket serverSocket = new ServerSocket(5000);
while(keepGoing) {
Socket clientSocket = serverSocket.accept();
ClientThread t = new ClientThread(clientSocket);
t.start();
}
} catch (IOException e) {
System.out.println("Problem with socket/network: " + e);
}
}
class ClientThread extends Thread {
Socket clientSocket;
PrintWriter out;
BufferedReader in;
ClientThread(Socket clientSocket) {
this.clientSocket = clientSocket;
try{
out = new PrintWriter(clientSocket.getOutputStream());
in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
} catch (IOException e) {
System.out.println("Problem with creating in/out: " + e);
}
}
#Override
public void run() {
String message;
while(keepGoing) {
try{
message = in.readLine();
out.println(message);
System.out.println(message);
} catch (IOException e){
System.out.println("Exception while try to read line: " + e);
}
}
}
}
}
PS I've changed a bit the code - instead of made ClientThread Class, I made new runnable class and pass that variable to thread class. Inspired by this question: "implements Runnable" vs. "extends Thread".

I think the problem is that both server and client are waiting for any input. Server:
message = in.readLine();
Client:
if((fromServer = in.readLine()) != null)
System.out.println(" " + fromServer);
else if((fromClient = stdIn.readLine()) != null)
out.println(fromClient);
But the client code already blocks on the fromServer = in.readLine() part, so it never gets to read from standard in, and thus nothing will be sent out to the server.
You could move your attempt to read from standard in to the setUpNetworking method, right after the System.out.println("Type your message: ");. Build a loop there which you exit if the user types "exit" or "quit" or something like that:
BufferedReader stdIn = new BufferedReader(new InputStreamReader(System.in));
String read = "";
do {
read = stdIn.readLine();
System.out.println("Read from stdin: " + read);
serverThread.send(read);
}
while (!read.equals("exit"));
The ServerThread.send() method is simple:
void send(String string) {
System.out.println("Sending to server: " + string);
out.println(string);
}
However, to make it work, you either have to flush the stream manually after writing to out, or use the following constructor:
out = new PrintWriter(socket.getOutputStream(), true);
See the PrintWriter's JavaDoc: True means auto-flush on newline.
I tested this setup and it worked for me. I was able to send something from the client to the server.
However, this is only the first step. I would implement both reading and writing as separate threads, for both client and server. And there is no graceful shutdown of sockets implemenented yet. A more complete yet simple example can be found on Oracle.

the ReadLine doesnt wait for input

I have a socket client sending text to a socket server but the ReadLine doesnt seem to wait to receive a line before proceeding. Here is the of the server receiving the text:
public void run() {
try {
serveurSocket = new ServerSocket(PORT_ID);
connexionSocket = serveurSocket.accept();
BufferedReader reader = new BufferedReader(new InputStreamReader(connexionSocket.getInputStream()));
PrintWriter writer = new PrintWriter(connexionSocket.getOutputStream(), true);
messageRecu = "";
while (true) {
messageRecu = reader.readLine();
messageRecu = messageRecu.toUpperCase();
writer.println(messageRecu);
}
//reader.close();
//writer.close();
} catch (IOException e) {
System.out.println(e.getMessage());
e.printStackTrace();
}
}
After establishing the socket between client and server, the execution halts at reader.readLine until I send manually a string thru the socket. Which is normal and wanted. Codes resumes and its fine until its loops back to reader.ReadLine() where it will read a "null" line instead of waiting for input from the socket like it did the first time... this will obviously mess up the next command to uppercase. So how can I fix this?
EDIT: I'll add the client side if that can help understand.
public class ClientSocket {
private Socket clientSocket;
public boolean isClosed() { return clientSocket.isClosed(); }
public boolean connectToSocket (String ip, int port) {
try {
clientSocket = new Socket(ip, port);
return true;
}
catch (IOException e) {
System.out.println(e);
return false;
}
}
public String sendToServer(String messageClient) {
String messageRecu = "";
try {
BufferedReader reader = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
PrintWriter writer = new PrintWriter(clientSocket.getOutputStream(), true);
writer.println(messageClient);
messageRecu = reader.readLine();
reader.close();
writer.close();
return messageRecu;
}
catch (IOException e) {
System.out.println(e.getMessage());
e.printStackTrace();
return messageRecu;
}
}
}
A button press will call "connectTosocket" to initiate the socket. A second button when pressed will send the content of a textfield using "sendToServer".
Server does receive the message and return it capitalized but I wish for the socket to remain open with the server and if I send an other string for the same sequence to happen. Not even sure it can be done :(

According to the documentation of BufferedReader#readLine, a null is returned if the end of stream has been reached.
Change your reading loop to :
while ((messageRecu = reader.readLine()) != null) {
messageRecu = messageRecu.toUpperCase();
writer.println(messageRecu);
}
//Get out of the loop when the end of stream is reached.
As per Reading from and Writing to a Socket chapter of the Java tutorial.
As a side note, while(true) loops are not really appreciated.

The "null" signals for end of connection from the client side - which is why the connection disconnects. If you want to support multiple requests, you should run a new ServerSocket.accept() each time and wait for a new client to connect.
KKMultiServer class:
import java.net.*;
import java.io.*;
public class KKMultiServer {
public static void main(String[] args) throws IOException {
if (args.length != 1) {
System.err.println("Usage: java KKMultiServer <port number>");
System.exit(1);
}
int portNumber = Integer.parseInt(args[0]);
boolean listening = true;
try (ServerSocket serverSocket = new ServerSocket(portNumber)) {
while (listening) {
new KKMultiServerThread(serverSocket.accept()).start();
}
} catch (IOException e) {
System.err.println("Could not listen on port " + portNumber);
System.exit(-1);
}
}
}
KKMultiServerThread class:
import java.net.*;
import java.io.*;
public class KKMultiServerThread extends Thread {
private Socket socket = null;
public KKMultiServerThread(Socket socket) {
super("KKMultiServerThread");
this.socket = socket;
}
public void run() {
try (
PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
BufferedReader in = new BufferedReader(
new InputStreamReader(
socket.getInputStream()));
) {
String inputLine, outputLine;
KnockKnockProtocol kkp = new KnockKnockProtocol();
outputLine = kkp.processInput(null);
out.println(outputLine);
while ((inputLine = in.readLine()) != null) {
outputLine = kkp.processInput(inputLine);
out.println(outputLine);
if (outputLine.equals("Bye"))
break;
}
socket.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
You can read more about sockets in Oracle tutorials

Socket Issue - Only first message read

I am very new to sockets and was hoping someone could help me. I had something working but it was not sending information very quickly so i have refactored and now cannot get back to anything which works. The issue seems to be that only the first message that is published is read and then the receiver sits on client = listener.accept(); even though im pretty sure the sender is still sending messages
Can anyone see what i might be doing wrong here please?
Thanks
public class Sender {
Socket server = null;
DataInputStream inp = null;
PrintStream outp = null;
public Sender(){
server = new Socket("127.0.0.1" , 3456);
outp = new PrintStream(server.getOutputStream());
}
private void connectAndSendToServer(String message) {
outp = new PrintStream(server.getOutputStream());
outp.print(message + "\n");
outp.flush();
}
}
Receiver class
public class Receive{
public String receiveMessage(int port) {
String message= null;
ServerSocket listener = null;
Socket client = null;
try{
listener = new ServerSocket(port);
client = listener.accept();
BufferedReader br = new BufferedReader(new InputStreamReader(client.getInputStream()));
return br.readLine();
}
...
finally{
try {
if(client!=null && listener!=null){
client.close();
listener.close();
}
}
catch (IOException e) {
}
}
return message;
}
}

This because a ServerSocket is used as an entry point for a normal Socket. accept() is a blocking operation that is usually done on a different thread compared to the one that receives/sends data to normal Socket. It sits there and waits for a new connection to spawn a new Socket which is then used for data.
This means that while receiving messages you should call just readLine() to read from the specific Socket. Having an accept inside the receiveMessage is wrong just because it's a different operation and it's even blocking.
Socket socket = serverSocket.accept();
ClientThread thread = new ClientThread(socket);
class ClientThread extends Thread {
Socket socket;
public void run() {
while (!closed) {
String line = reader.readLine();
...
}
}
You don't need to have a thread for every client though, but you need at least two for sure if you want to make your server accept a number of connections greater than 1.

You are not using ServerSocket correctly. You shouldn't create a new instance for every message but use it as a data member maybe and run an infinite loop to get a new client socket connection. Because you create it locally, the socket is closed since the object is no longer used and referenced (and so GC'ed), when you return from the method.
Something like (< condition met > is pseudo-code defines your condition to accept new connections):
while(< condition met >) {
try {
client = listener.accept();
BufferedReader br = new BufferedReader(new InputStreamReader(client.getInputStream()));
String str = br.readLine();
//do something with str
} finally {
//close client socket
}
}
Better approach will be to handle client socket in a different thread so the main thread is back to accept while you can do anything with the client socket in parallel.

Try this basic Chatting Server written by me. This server simply keeps running in loop and broadcast the message send by the clients to all the other clients associated with this server.
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.ArrayList;
public class Server {
// ///----------------------------------------Instance Variable Fields
ServerSocket ss = null;
Socket incoming = null;
// ///----------------------------------------Instance Variable Fields
// ///---------------------------------------- static Variable Fields
public static ArrayList<Socket> socList = new ArrayList<Socket>();
// ///---------------------------------------- static Variable Fields
public void go() {
try {
ss = new ServerSocket(25005);
while (true) {
incoming = ss.accept();
socList.add(incoming);
System.out.println("Incoming: " + incoming);
new Thread(new ClientHandleKaro(incoming)).start();
}
} catch (IOException e) {
e.printStackTrace();
} finally {
try {
ss.close();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
class ClientHandleKaro implements Runnable {
InputStream is = null;
OutputStream os = null;
InputStreamReader isr = null;
BufferedReader br = null;
PrintWriter pw = null;
boolean isDone = false;
Socket sInThread = null;
public ClientHandleKaro(Socket sxxx) {
this.sInThread = sxxx;
}
#Override
public void run() {
if (sInThread.isConnected()) {
System.out.println("Welcamu Clienta");
System.out.println(socList);
}
try {
is = sInThread.getInputStream();
System.out.println("IS: " + is);
isr = new InputStreamReader(is);
br = new BufferedReader(isr);
os = sInThread.getOutputStream();
pw = new PrintWriter(os, true);
String s = new String();
while ((!isDone) && (s = br.readLine()) != null) {
String[] asx = s.split("-");
System.out.println("On Console: " + s);
// pw.println(s);
Thread tx = new Thread(new ReplyKaroToClient(s,
this.sInThread));
tx.start();
if (asx[1].trim().equalsIgnoreCase("BYE")) {
System.out.println("I am inside Bye");
isDone = true;
}
}
} catch (IOException e) {
System.out.println("Thanks for Chatting.....");
} finally {
try {
Thread tiku = new Thread(new ByeByeKarDo(sInThread));
tiku.start();
try {
tiku.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("Accha to hum Chalte hain !!!");
System.out.println(socList);
br.close();
pw.close();
sInThread.close();
} catch (IOException e) {
}
}
}
}
class ReplyKaroToClient implements Runnable {
public String mString;
public Socket mSocket;
public ReplyKaroToClient(String s, Socket sIn) {
this.mString = s;
this.mSocket = sIn;
}
#Override
public void run() {
for (Socket sRaW : socList) {
if (mSocket.equals(sRaW)) {
System.out.println("Mai same hun");
continue;
} else {
try {
new PrintWriter(sRaW.getOutputStream(), true)
.println(mString);
} catch (IOException e) {
System.out.println("Its in Catch");
}
}
}
}
}
class ByeByeKarDo implements Runnable {
Socket inCom;
public ByeByeKarDo(Socket si) {
this.inCom = si;
}
#Override
public void run() {
try {
new PrintWriter(inCom.getOutputStream(), true)
.println("You have Logged Out of Server... Thanks for your Visit");
} catch (IOException e) {
e.printStackTrace();
}
}
}
public static void main(String[] args) {
new Server().go();
}
}

How can I get this code (from Java tutorial on sockets) to run?

I've just started with this section of the tutorial. I only have a basic understanding of what ports are, etc.
I tried to run this code:
import java.io.*;
import java.net.*;
public class EchoClient {
public static void main(String[] args) throws IOException {
Socket echoSocket = null;
PrintWriter out = null;
BufferedReader in = null;
try {
echoSocket = new Socket("taranis", 7);
out = new PrintWriter(echoSocket.getOutputStream(), true);
in = new BufferedReader(new InputStreamReader(
echoSocket.getInputStream()));
} catch (UnknownHostException e) {
System.err.println("Don't know about host: taranis.");
System.exit(1);
} catch (IOException e) {
System.err.println("Couldn't get I/O for "
+ "the connection to: taranis.");
System.exit(1);
}
BufferedReader stdIn = new BufferedReader(
new InputStreamReader(System.in));
String userInput;
while ((userInput = stdIn.readLine()) != null) {
out.println(userInput);
System.out.println("echo: " + in.readLine());
}
out.close();
in.close();
stdIn.close();
echoSocket.close();
}
}
"Don't know about host: taranis.
Java Result: 1"
Is the error catch I get. From my limited understanding; is the echo-server something which exists on my machine? If that's the case, what do I need to do to get this running? Or am I way off?
Also why have they chosen "taranis" as a parameter?
Ive also replaced "taranis" with "localhost" to see what happened.
It ended up catching an IOException this time.
EDIT: So I've found that the echo server is disabled by default in win7 and have activated it. However I cant even connect to it on telnet. I think I may just be in over my head. I've also tried the sockets you have recommended with no success.

From the same tutorial:
... The Socket constructor used here requires the name of the machine and the port number to which you want to connect. The example program uses the host name taranis. This is the name of a hypothetical machine on our local network. When you type in and run this program on your machine, change the host name to the name of a machine on your network. Make sure that the name you use is the fully qualified IP name of the machine to which you want to connect. The second argument is the port number. Port number 7 is the port on which the Echo server listens.`
In any case, you will probably want to change taranis to "localhost" and make sure an echo service is running on your machine. If it's not, you could use something like the following code to simulate an echo server.
import java.net.Socket;
import java.util.Formatter;
import java.util.Scanner;
import java.io.IOException;
import java.net.ServerSocket;
import java.util.ArrayList;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class EchoServer {
public static void main(String[] args) {
try {
new EchoServer(INSERTPORT).execute();
} catch (IOException e) {
e.printStackTrace();
System.exit(1);
}
}
private ServerSocket serverSocket;
private int port;
private ArrayList<Client> clientList;
private ExecutorService clientRunner;
public EchoServer(int port) throws IOException {
this.port = port;
serverSocket = new ServerSocket(port);
clientRunner = Executors.newCachedThreadPool();
clientList = new ArrayList<>();
}
public void sendMessageToAll(String message) {
for (Client c : clientList) {
c.displayMessage(message);
}
}
public void execute() throws IOException {
while (true) {
clientList.add(new Client(serverSocket.accept(), this));
clientRunner.execute(clientList.get(clientList.size()-1));
}
}
private class Client implements Runnable {
private Socket clientSocket;
private Scanner input;
private Formatter output;
public Client(Socket s) throws IOException {
clientSocket = s;
input = new Scanner(clientSocket.getInputStream());
output = new Formatter(clientSocket.getOutputStream());
}
public void displayMessage(String s) {
output.format(s + "\n");
output.flush();
}
#Override
public void run() {
while(clientSocket.isConnected()) {
if(input.hasNextLine()) {
sendMessageToAll(input.nextLine());
}
}
}
}
}
Edit: Just for completeness, as you mentioned some problems running the code, you run the server (this code) and leave it running in the background, then run the client (the code you posted). I tested it, works fine.

Try this,
Use the loopback address of 127.0.0.1 instead of taranis.
Use port higher than 1024, something like 4444, 8333 etc....
I am also adding my code that i used to learn Client Server Commnu
Client Side Code:
public class ClientWala {
public static void main(String[] args) throws Exception{
Boolean b = true;
Socket s = new Socket("127.0.0.1", 4444);
System.out.println("connected: "+s.isConnected());
OutputStream output = s.getOutputStream();
PrintWriter pw = new PrintWriter(output,true);
// to write data to server
while(b){
if (!b){
System.exit(0);
}
else {
pw.write(new Scanner(System.in).nextLine());
}
}
// to read data from server
InputStream input = s.getInputStream();
InputStreamReader isr = new InputStreamReader(input);
BufferedReader br = new BufferedReader(isr);
String data = null;
while ((data = br.readLine())!=null){
// Print it using sysout, or do whatever you want with the incoming data from server
}
}
}
Server Side Code:
public class ServerTest {
ServerSocket s;
public void go() {
try {
s = new ServerSocket(44457);
while (true) {
Socket incoming = s.accept();
Thread t = new Thread(new MyCon(incoming));
t.start();
}
} catch (IOException e) {
e.printStackTrace();
}
}
class MyCon implements Runnable {
Socket incoming;
public MyCon(Socket incoming) {
this.incoming = incoming;
}
#Override
public void run() {
try {
PrintWriter pw = new PrintWriter(incoming.getOutputStream(),
true);
InputStreamReader isr = new InputStreamReader(
incoming.getInputStream());
BufferedReader br = new BufferedReader(isr);
String inp = null;
boolean isDone = true;
System.out.println("TYPE : BYE");
System.out.println();
while (isDone && ((inp = br.readLine()) != null)) {
System.out.println(inp);
if (inp.trim().equals("BYE")) {
System.out
.println("THANKS FOR CONNECTING...Bye for now");
isDone = false;
s.close();
}
}
} catch (IOException e) {
// TODO Auto-generated catch block
try {
s.close();
} catch (IOException e1) {
// TODO Auto-generated catch block
e1.printStackTrace();
}
e.printStackTrace();
}
}
}
public static void main(String[] args) {
new ServerTest().go();
}
}