Develop Reference

Detecting disconnects in sockets

I'm creating an "echo" server that upon receiving a message simply sends it back. I have managed to get multi-client working, but I want to make some kind of disconnect detection. I tried to get it working through sending a single character from the server, then replying with another character from the client. I couldn't get this to work, though.
How would you suggest I go about disconnect detection?
MessageServer.java
import java.net.*;
import java.io.*;
public class MessageServer {
static int clientCount = 0;
public static void main(String[] args) throws IOException {
try(ServerSocket servSocket = new ServerSocket(16384)){
while(true){
Socket socket = servSocket.accept();
addClient();
new ServerThread(socket, clientCount).start();
}
} catch(IOException e) {
System.out.println("Exception caught when trying to listen on port 16384 or listening for a connection");
System.out.println(e.getMessage());
}
}
public static void addClient(){
clientCount++;
}
}
ServerThread.java
import java.net.*;
import java.io.*;
public class ServerThread extends Thread {
private Socket cltSocket;
private BufferedReader in;
private PrintWriter out;
private int num;
public ServerThread(Socket clientSocket, int count) {
cltSocket = clientSocket;
num = count;
}
public void run() {
String input;
try {
out = new PrintWriter(cltSocket.getOutputStream(), true);
in = new BufferedReader(new InputStreamReader(cltSocket.getInputStream()));
System.out.println("Client " + num + " connected!");
while(cltSocket.isConnected() && !cltSocket.isClosed()){
if(in.ready()){
input = in.readLine();
if(input != null && !(input.equalsIgnoreCase("exit"))){
System.out.print("New input: ");
System.out.println(input);
out.println(input);
out.flush();
} else if(input.equalsIgnoreCase("exit")){
disconnect();
}
}
}
} catch(SocketException e) {
disconnect();
} catch (IOException e) {
e.printStackTrace();
return;
}
}
public void disconnect(){
System.out.println("Client " + num + " disconnected!");
out.close();
try {
in.close();
cltSocket.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
MessageClient.java
import java.net.*;
import java.io.*;
public class MessageClient {
public static void main(String[] args) {
if(args.length != 2) {
System.out.println("Invalid parameters! Format as: (hostname) (port)");
System.exit(1);
}
String hostname = args[0];
int port = Integer.parseInt(args[1]);
try {
Socket socket = new Socket(hostname, port);
PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
BufferedReader con = new BufferedReader(new InputStreamReader(System.in));
System.out.println("Connected!");
while(socket.isConnected() && !socket.isClosed()){
String output;
if(con.ready()) {
output = con.readLine();
out.println(output);
if(output.equalsIgnoreCase("exit")) {
socket.close();
}
}
if(in.ready()){
String li = in.readLine();
if(li != null) {
System.out.println(li);
}
}
}
System.out.println("Disconnected!");
con.close();
out.close();
in.close();
System.exit(0);
} catch(SocketException e) {
System.err.println("Socket error:" + e);
} catch(UnknownHostException e) {
System.err.println("Invalid host");
} catch(IOException e) {
System.err.println("IO Error: " + e);
}
}
}

There is a way to do that:
if you read the BufferedReader by calling BufferedReader.getLine() and the other side socket is gone, then you get an SocketException... that is a way to check a lost connection

thread sleep makes main server to sleep

I want to write a java multithreaded server. I used this code:
MultiThreadedServer.java :
package servers;
import java.net.ServerSocket;
import java.net.Socket;
import java.io.IOException;
public class MultiThreadedServer implements Runnable{
protected int serverPort = 8080;
protected ServerSocket serverSocket = null;
protected boolean isStopped = false;
protected Thread runningThread= null;
public MultiThreadedServer(int port){
this.serverPort = port;
}
public void run(){
synchronized(this){
this.runningThread = Thread.currentThread();
}
openServerSocket();
while(! isStopped()){
Socket clientSocket = null;
try {
clientSocket = this.serverSocket.accept();
} catch (IOException e) {
if(isStopped()) {
System.out.println("Server Stopped.") ;
return;
}
throw new RuntimeException(
"Error accepting client connection", e);
}
System.out.println("Request Entered: " + System.currentTimeMillis());
new Thread(
new WorkerRunnable(
clientSocket, "Multithreaded Server")
).start();
}
System.out.println("Server Stopped.") ;
}
private synchronized boolean isStopped() {
return this.isStopped;
}
public synchronized void stop(){
this.isStopped = true;
try {
this.serverSocket.close();
} catch (IOException e) {
throw new RuntimeException("Error closing server", e);
}
}
private void openServerSocket() {
try {
this.serverSocket = new ServerSocket(this.serverPort);
} catch (IOException e) {
throw new RuntimeException("Cannot open port 8080", e);
}
}
}
WorkerRunnable.java:
package servers;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.IOException;
import java.net.Socket;
public class WorkerRunnable implements Runnable{
protected Socket clientSocket = null;
protected String serverText = null;
public WorkerRunnable(Socket clientSocket, String serverText) {
this.clientSocket = clientSocket;
this.serverText = serverText;
}
public void run() {
try {
InputStream input = clientSocket.getInputStream();
OutputStream output = clientSocket.getOutputStream();
long time = System.currentTimeMillis();
output.write(("HTTP/1.1 200 OK\n\nWorkerRunnable: " + this.serverText + " - " +time +"").getBytes());
Thread.sleep(2000);
output.close();
input.close();
System.out.println("Request processed: " + time);
} catch (IOException | InterruptedException e) {
//report exception somewhere.
e.printStackTrace();
}
}
}
I used this code to run main server:
MultiThreadedServer server = new MultiThreadedServer(9000);
new Thread(server).start();
then I opened 3 tabs in the web browser and entered http://localhost:9000/ in all of them.
but tabs doesn't run concurrently and responses are received after 2 sec of each other.
I found out that when I run "Thread.sleep" in each WorkerRunnable thread, MultiThreadedServer doesn't accept any request anymore.
Any solution to run all threads concurrently?

Chat server client sometimes cannot see message from other clients on the same server

I am making 2 classes for Chat server client for a project I am working on. The problem is server can see the message that sent to it(from client) and it can send those messages out to every clients BUT each client has to type in some input first if he wants to see the message from other users. I have no clue what I did wrong. Please help me out. Thanks in advance :)
Server Class
import java.net.*;
import java.util.ArrayList;
import java.io.*;
public class TestServer extends Thread
{
protected Socket clientSocket;
public static ArrayList<Socket> ConnectionArray = new ArrayList<Socket>();
public static ArrayList<String> CurrentUsers = new ArrayList<String>();
final static int PORT = 22;
public static void main(String[] args) throws IOException
{
ServerSocket serverSocket = null;
try
{
serverSocket = new ServerSocket(PORT);
System.out.println ("Connection Socket Created");
try
{
while (true)
{
System.out.println ("Waiting for Connection");
Socket sock = serverSocket.accept();
ConnectionArray.add(sock);
System.out.println("Client connected from: " + sock.getLocalAddress().getHostName());
new TestServer (sock);
}
}
catch (IOException e)
{
System.err.println("Accept failed.");
System.exit(1);
}
}
catch (IOException e)
{
System.err.println("Could not listen on port: " + PORT);
System.exit(1);
}
finally
{
try {
serverSocket.close();
}
catch (IOException e)
{
System.err.println("Could not close port: 10008.");
System.exit(1);
}
}
}
private TestServer (Socket inSock)
{
clientSocket = inSock;
start();
}
public void run()
{
System.out.println ("New Communication Thread Started");
//System.out.println ("Client connected from: " + sock.getLocalAddress().getHostName());
try {
PrintWriter out = new PrintWriter(clientSocket.getOutputStream(),true);
BufferedReader in = new BufferedReader(new InputStreamReader( clientSocket.getInputStream()));
String inputLine;
while (true)
{
inputLine = in.readLine();
System.out.println ("Server: " + inputLine);
for(int i = 1; i <= TestServer.ConnectionArray.size(); i++)
{
System.out.println("Total Connection: " + ConnectionArray.size());
Socket TEMP_SOCK = (Socket) TestServer.ConnectionArray.get(i-1);
if (clientSocket != TEMP_SOCK)
{
PrintWriter TEMP_OUT = new PrintWriter(TEMP_SOCK.getOutputStream(), true);
TEMP_OUT.println(inputLine);
TEMP_OUT.flush();
System.out.println("Sending to: " + TEMP_SOCK.getLocalAddress().getHostName());
}
}
if (inputLine.equals("Bye."))
break;
}
out.close();
in.close();
clientSocket.close();
}
catch (IOException e)
{
System.err.println("Problem with Communication Server");
System.exit(1);
}
}
}
Client Class
import java.io.*;
import java.net.*;
public class TestClient {
public static void main(String[] args) throws IOException {
String serverHostname = new String ("127.0.0.1");
if (args.length > 0)
serverHostname = args[0];
System.out.println ("Attemping to connect to host " +
serverHostname);
Socket echoSocket = null;
PrintWriter out = null;
BufferedReader in = null;
try {
echoSocket = new Socket(serverHostname, 22);
out = new PrintWriter(echoSocket.getOutputStream(), true);
in = new BufferedReader(new InputStreamReader(echoSocket.getInputStream()));
} catch (UnknownHostException e) {
System.err.println("Don't know about host: " + serverHostname);
System.exit(1);
} catch (IOException e) {
System.err.println("Couldn't get I/O for "
+ "the connection to: " + serverHostname);
System.exit(1);
}
BufferedReader stdIn = new BufferedReader(new InputStreamReader(System.in));
String userInput;
System.out.println ("Type Message (\"Bye.\" to quit)");
while ((userInput = stdIn.readLine()) != null)
{
out.println(userInput);
if (userInput.equals("Bye."))
break;
System.out.println("Other user: " + in.readLine());
}
out.close();
in.close();
stdIn.close();
echoSocket.close();
}
}

You need a separate thread in the client for reading from the server without blocking on System.in;
Thread t = new Thread(() -> {
try {
while (!Thread.currentThread().isInterrupted()) {
System.out.println("Other user: " + in.readLine());
}
} catch (Exception e) {
e.printStackTrace();
}
});
t.start();
while ((userInput = stdIn.readLine()) != null)
{
out.println(userInput);
if (userInput.equals("Bye."))
break;
}
t.interrupt();

Why isn't my socket revieving the message?

I'm trying to learn how to use java sockets before next semester. I wrote this program that is supposed to accept accept a message from a sender and send the message to a reciever. You specify the reciever by writing the name of the client that is connected, if the client isn't connected then the server will respond by sending the message "DISCONNECTED". But for some reason the message isn't sent to the reciever. I can still send the message to the sender though. Is this the right forum to ask for this kind of help?
This is my client class
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.Socket;
public class MyTestClient {
static int port = 0;
static String ip = "";
static String name = "";
public static void main(String args[] ) {
if(args.length == 3) {
try{
port = Integer.parseInt(args[1]);
} catch(NumberFormatException e) {
System.out.println("Usage: Java MyTestClient <ip> <port> <name>");
System.exit(0);
}
ip = args[0];
name = args[2];
} else {
System.out.println("Usage: Java MyTestClient <ip> <port> <name>");
System.exit(0);
}
try {
Socket socket = new Socket(ip, port);
new Thread(new MessageHandler(socket)).start();
new Thread(new InputHandler(socket)).start();
} catch (IOException e) {
System.out.println("Could not connect to: " + ip + " : " + port);
System.exit(0);
}
}
public static class MessageHandler implements Runnable {
Socket socket;
public MessageHandler(Socket socket) {
this.socket = socket;
}
#Override
public void run() {
PrintWriter out = null;
BufferedReader in = null;
try {
out = new PrintWriter(socket.getOutputStream(), true);
in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
}catch(IOException e) {
e.printStackTrace();
}
String fromServer;
try {
while ((fromServer = in.readLine()) != null) {
System.out.println(fromServer);
fromServer.trim();
if(fromServer.contains(" ")) {
String split[] = fromServer.split(" ", 2);
System.out.println(split.length == 2 ? split[1] + " Is a two part message" : "Strange message");
} else {
if(fromServer.equals("NAME")) {
System.out.println("Server asks for name | gives given name to server");
out.println(name);
}
if(fromServer.equals("SUCCESS")) {
System.out.println("Message succesfully sent");
}
if(fromServer.equals("ERROR")) {
System.out.println("Something went wrong when sending the message");
}
if(fromServer.equals("OCCUPIED")) {
System.out.println("Name was already used. Start the program with another name");
socket.close();
System.exit(0);
}
if(fromServer.equals("DISCONNECTED")) {
System.out.println("The reciever was not connected");
}
if(fromServer.equals("REGISTERED")) {
System.out.println("Successfully logged in to server");
}
}
}
} catch(IOException e) {
e.printStackTrace();
}
}
}
public static class InputHandler implements Runnable {
Socket socket;
public InputHandler(Socket socket) {
this.socket = socket;
}
#Override
public void run() {
String fromUser = null;
PrintWriter out = null;
try {
out = new PrintWriter(socket.getOutputStream(), true);
} catch (IOException e) {
e.printStackTrace();
}
BufferedReader kbi = new BufferedReader(new InputStreamReader(System.in));
try {
while ((fromUser = kbi.readLine()) != null) {
out.println(fromUser);
}
} catch (IOException e) {
e.printStackTrace();
}
}
}
}
This is my server class
import java.io.*;
import java.net.*;
import java.util.*;
public class MyServer {
public static int port;
public static int maxConnections;
public static final String NAME = "Server";
private static Map<String, Socket> socketsMap = new HashMap<>();
public static void main(String args[]) {
if(args.length == 2) {
try{
port = Integer.parseInt(args[0]);
maxConnections = Integer.parseInt(args[1]);
}catch(Exception e){
System.out.println("Usage: Java Myserver <port> <maxConnections>");
System.exit(0);
}
} else {
System.out.println("Usage: Java Myserver <port> <maxConnections>");
System.exit(0);
}
new Thread(new ConnectionHandler(port, maxConnections)).start();
}
public static class ConnectionHandler implements Runnable {
private ServerSocket server;
private Socket socket = null;
public int currentSocket = 0;
public int port = 0;
public int maxConnections = 0;
public ConnectionHandler(int port, int maxConnections) {
this.maxConnections = maxConnections;
this.port = port;
}
#Override
public void run() {
try {
server = new ServerSocket(7777);
} catch (IOException e1) {
System.out.println("Server could not connect to port: " + port);
e1.printStackTrace();
System.exit(0);
}
while(currentSocket++ < maxConnections) {
try {
socket = server.accept();
PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
out.println("NAME");
String message;
while((message = in.readLine()) != null) {
if(!socketsMap.containsKey(message) || message.equals("SERVER")) {
out.println("REGISTERED");
socketsMap.put(message, socket);
System.out.println(message + " has logged on server");
new Thread(new SocketHandler(socketsMap.get(message))).start();
} else {
out.println("OCCUPIED");
System.out.println(socket.getInetAddress().toString() + " has tried to login with existing name " + message);
socket.close();
currentSocket--;
}
socket = null;
break;
}
} catch (IOException e) {
System.out.println("Something went wrong with client connection");
}
}
}
}
public static class SocketHandler implements Runnable {
Socket socket = null;
public SocketHandler(Socket socket) {
this.socket = socket;
}
#Override
public void run() {
try {
BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
PrintWriter respond = new PrintWriter(socket.getOutputStream(), true);
String input;
while((input = in.readLine() ) != null) {
String[] info = input.split(" ", 2);
if(info.length != 2){
System.out.println("Message computed wrong");
respond.println("ERROR");
break;
}
if(info[0].equals("SERVER")) {
//TODO: Write a server command handler class
} else if ( socketsMap.containsKey(info[0]) ){
Socket reciever = socketsMap.get(info[0]);
PrintWriter out = new PrintWriter(reciever.getOutputStream());
respond.println("SUCCESS");
System.out.println(info[0] + " send the message: "+ info[1]);
out.println("MESSAGE" + " " + info[1]);
break;
} else {
System.out.println("Reciever is not connected");
respond.println("DISCONNECTED");
}
}
}catch(IOException e) {
e.printStackTrace();
}
}
}
}
Maybe you can run it on your computers and see what i mean?

#Override
public void run() {
try {
server = new ServerSocket(7777);
} catch (IOException e1) {
System.out.println("Server could not connect to port: " + port);
e1.printStackTrace();
System.exit(0);
}
change the code above to
#Override
public void run() {
try {
server = new ServerSocket(port);
} catch (IOException e1) {
System.out.println("Server could not connect to port: " + port);
e1.printStackTrace();
System.exit(0);
}

Java socket - simple program doesn't work

I've spent lot of time to find out where is the problem but with no success. Server is launching correctly, but when I launch Client I get "Unexpected Error" exception. I've changed ports too with no effects. What should I do to make this working?
/* Server.java */
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;
public class Server
{
private static final int PORT = 50000;
static boolean flaga = true;
private static ServerSocket serverSocket;
private static Socket clientSocket;
public static void main(String[] args) throws IOException
{
serverSocket = null;
try
{
serverSocket = new ServerSocket(PORT);
}
catch(IOException e)
{
System.err.println("Could not listen on port: "+PORT);
System.exit(1);
}
System.out.print("Wating for connection...");
Thread t = new Thread(new Runnable()
{
public void run()
{
try
{
while(flaga)
{
System.out.print(".");
Thread.sleep(1000);
}
}
catch(InterruptedException ie)
{
//
}
System.out.println("\nClient connected on port "+PORT);
}
});
t.start();
clientSocket = null;
try
{
clientSocket = serverSocket.accept();
flaga = false;
}
catch(IOException e)
{
System.err.println("Accept failed.");
t.interrupt();
System.exit(1);
}
final PrintWriter out = new PrintWriter(clientSocket.getOutputStream(),true);
final BufferedReader in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
t = new Thread(new Runnable()
{
public void run()
{
try
{
Thread.sleep(5000);
while(true)
{
out.println("Ping");
System.out.println(System.currentTimeMillis()+" Ping sent");
String input = in.readLine();
if(input.equals("Pong"))
{
System.out.println(System.currentTimeMillis()+" Pong received");
}
else
{
System.out.println(System.currentTimeMillis()+" Wrong answer");
out.close();
in.close();
clientSocket.close();
serverSocket.close();
break;
}
Thread.sleep(5000);
}
}
catch(Exception e)
{
System.err.println(System.currentTimeMillis()+" Unexpected Error");
}
}
});
t.start();
}
}
and the Client class
/* Client.java */
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.Socket;
public class Client
{
private static final int PORT = 50000;
private static final String HOST = "localhost";
public static void main(String[] args) throws IOException
{
Socket socket = null;
try
{
socket = new Socket(HOST, PORT);
}
catch(Exception e)
{
System.err.println("Could not connect to "+HOST+":"+PORT);
System.exit(1);
}
final PrintWriter out = new PrintWriter(socket.getOutputStream(),true);
final BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
Thread t = new Thread(new Runnable()
{
public void run()
{
long start = System.currentTimeMillis();
while (true)
{
try
{
String input = in.readLine();
if (input != null)
{
System.out.println(System.currentTimeMillis() + " Server: " + input);
}
if (input.equals("Ping"))
{
if(System.currentTimeMillis()-start>30000)
{
out.println("Pon g");
System.out.println(System.currentTimeMillis() + " Client: Pon g");
break;
}
out.println("Pong");
System.out.println(System.currentTimeMillis() + " Client: Pong");
}
}
catch (IOException ioe)
{
//
}
}
}
});
t.start();
out.close();
in.close();
socket.close();
}
}
Here is the output on running
Wating for connection............
Client connected on port 50000
1368986914928 Ping sent
java.lang.NullPointerException
at Server$2.run(Server.java:84)
at java.lang.Thread.run(Thread.java:722)

You're making a big mistake with those catch blocks that are empty or print out your useless message.
You'll get more information if you print or log the stack trace. It's simply a must.
You need some intro instruction - have a look at this and see how it's different from yours.
http://docs.oracle.com/javase/tutorial/networking/sockets/clientServer.html

It shows your out object is null. Instead of input.equals("Pong") use input != null && input.equals("Pong") in line 84 of Server.java. I guess you would have received Pong received but in later stages when you are listening to nothing you could have got this NPE.