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How to stream data from one part of a Java program to another?

I've learned in Java how to stream data over a network connection using ServerSocket and Socket, such as:
Client.java:
Socket socket = new Socket(address, port);
int i;
while ((i = System.in.read()) != -1)
socket.getOutputStream().write(i);
Server.java:
ServerSocket server = new ServerSocket(port);
Socket socket = server.accept();
int i;
while ((i = socket.getInputStream().read()) != -1)
System.out.println(i);
This would simply have Client blocking on System.in.read() at one end, and Server blocking on socket.getInputStream().read() at the other, and the bytes get passed when ENTER is pressed in the Client program.
How would I accomplish something similar within a single program, without using Sockets? For example, if I had Thread A waiting on keyboard input which is then streamed to Thread B which is able to "consume" the bytes at an arbitrary time in the future, just as Server (above) is able to consume bytes from socket.getInputStream() at some arbitrary time?
Is PipedInput/OutputStream the right solution for this, or ByteArrayInput/OutputStream, or something else? Or am I overthinking it?

Yes, you can use PipedInputStream/PipedOutputStream for "streaming" data "locally" in your JVM. You create one PipedInputStream and one PipedOutputStream instance, connect them with the connect() method and start sending/receiving bytes. Check the following example:
PipedInputStream pipedIn = new PipedInputStream();
PipedOutputStream pipedOut = new PipedOutputStream();
pipedIn.connect(pipedOut);
Thread keyboardReadingThread = new Thread() {
#Override
public void run() {
System.out.println("Enter some data:");
Scanner s = new Scanner(System.in);
String line = s.nextLine();
System.out.println("Entered line: "+line);
byte[] bytes = line.getBytes(StandardCharsets.UTF_8);
try {
pipedOut.write(bytes);
pipedOut.flush();
pipedOut.close();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
System.out.println("Keyboard reading thread terminated");
}
};
keyboardReadingThread.start();
Thread streamReadingThread = new Thread() {
#Override
public void run() {
try {
int bytesRead = 0;
byte[] targetBytes = new byte[100];
System.out.println("Read data from the PipedInputStream instance");
while ((bytesRead = pipedIn.read(targetBytes)) != -1) {
System.out.println("read "+bytesRead+" bytes");
String s = new String(targetBytes, 0, bytesRead, StandardCharsets.UTF_8);
System.out.println("Received string: "+s);
}
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
System.out.println("Streaming reading thread terminated");
}
};
streamReadingThread.start();
keyboardReadingThread.join();
streamReadingThread.join();
First the two piped stream instances are connected. After that two threads will read from the keyboard and read from the PipedInputStream instance. When you run your application you will get an output similar to this (with Some example input for testing being the keyboard input):
Enter some data:
Read data from the PipedInputStream instance
Some example input for testing
Entered line: Some example input for testing
Keyboard reading thread terminated
read 30 bytes
Received string: Some example input for testing
Streaming reading thread terminated
Also notice that the threads are not synchronized in any way, so the System.out.println() statements might get executed in a different order.

This is mostly an extension of the answer #VGR gave in the comments.
If the entirety of your "Network" exists within the same, single JVM, then you don't need anything like sockets at all - you can just use Objects and methods.
The entire point of Sockets was to allow the JVM to perform actions outside of itself (typically with another JVM somewhere in the outside world).
So unless you are trying to interact with objects outside of your current JVM, it is as simple as this.
public class ClientServerExample
{
public static void main(String[] args)
{
Server server = new Server();
Client client = new Client();
client.sendMessage("Hello Server", server);
}
static class Server
{
String respond(String input)
{
String output = "";
System.out.println("Server received the following message -- {" + input + "}");
//do something
return output;
}
}
static class Client
{
void sendMessage(String message, Server server)
{
System.out.println("Client is about to send the following message to the server -- {" + message + "}");
String response = server.respond(message);
System.out.println("Client received the following response from the server -- {" + response + "}");
//maybe do stuff with the response
}
}
}
Here is the result from running it.
Client is about to send the following message to the server -- {Hello Server}
Server received the following message -- {Hello Server}
Client received the following response from the server -- {}
Note that server doesn't return anything because I didn't do anything in the server. Replace that comment with some code of your own and you will see the results.
EDIT - to better explain a real world example, where a server will respond to requests in FIFO, here is a modified version of the above example.
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CompletableFuture;
public class ClientServerExample
{
public static void main(String[] args)
{
System.out.println("===========STARTING SYNCHRONOUS COMMUNICATION============");
synchronousCommunication();
System.out.println("===========FINISHED SYNCHRONOUS COMMUNICATION============");
System.out.println("===========STARTING ASYNCHRONOUS COMMUNICATION============");
asynchronousCommunication();
System.out.println("===========FINISHED ASYNCHRONOUS COMMUNICATION============");
}
public static void synchronousCommunication()
{
Server server = new Server();
Client client = new Client();
String response = "";
response = client.sendMessage("Good morning Server!", server).join();
System.out.println("Client received the following response from the server -- {" + response + "}");
response = client.sendMessage("Good evening Server!", server).join();
System.out.println("Client received the following response from the server -- {" + response + "}");
}
public static void asynchronousCommunication()
{
Server server = new Server();
Client client = new Client();
List<CompletableFuture<String>> responses = new ArrayList<>();
responses.add(client.sendMessage("Good morning Server!", server));
responses.add(client.sendMessage("Good evening Server!", server));
for (CompletableFuture<String> eachResponse : responses)
{
System.out.println("Client received the following response from the server -- {" + eachResponse.join() + "}");
}
}
static class Server
{
CompletableFuture<String> respond(final String input)
{
System.out.println("Server received the following message -- {" + input + "}");
return
CompletableFuture.supplyAsync(
() ->
{
try
{
//sleep for 2 seconds, to represent arbitrary delay in receiver processing
Thread.sleep(2000);
return input.contains("morning") ? "Good morning to you too!" : "Good evening to you too!";
}
catch (Exception e)
{
throw new IllegalStateException("What happened?", e);
}
});
}
}
static class Client
{
CompletableFuture<String> sendMessage(String message, Server server)
{
System.out.println("Client is about to send the following message to the server -- {" + message + "}");
return server.respond(message);
}
}
}
Both of these examples are performing a FIFO approach to data processing. They receive the request, calculate a response, and then send back a CompletableFuture, which is basically an Object that contains the response that will arrive once the Server gets around to it, sort of like a Promise in Javascript.
For the synchronous example, we see that a client message is sent, and then processed before the next one is sent. As a result, we have a minor delay between the 2 (about 2 seconds).
For the asynchronous example, we see that both client messages are sent, and their CompletableFutures are put into a batch list, which is converted to normal strings once all requests have been sent.
The synchronous example takes around 10 seconds.
The asynchronous example takes around 5 seconds.
Both of these are different ways of performing FIFO in the way that you described. They both are examples where multiple clients send a request to the server, and then the server finishes them when they get around to it. That 5 seconds delay is meant to represent the idea of "getting around to it". In reality, getting around to it usually means that the server has so much on it's plate that it will take a long time before it has a chance to give a full response.
Let me know if you need another example to better help you understand.

Socket server stream messages in stream are mixed

I am implementing multiplayer game with server socket mechanism.
Game is running in one loop mechanism.
Server only broadcast any message to every player connected.
Server creates thread for every connected player
Messages are JSONs and they are being sent every 100ms from client side.
Messages are being broadcasted by server and read by client without any interval.
This is server loop:
while (true) {
try {
System.out.println("LISTENING...");
Socket clientSocket = serverSocket.accept();
System.out.println("GOT CONNECTION");
PlayerThread playerThread = new PlayerThread(clientSocket);
playerThread.start();
} catch (IOException e) {
LOGGER.log(Level.SEVERE, "Exception occured on server");
break;
}
}
This is PlayerThread loop:
while (true) {
try {
String inputMessage = in.readUTF();
System.out.println("GOT: " + inputMessage);
JsonNode jsonNode = objectMapper.readTree(inputMessage);
String senderName = jsonNode.get("senderName").asText();
if (!players.contains(this)) {
playerName = senderName;
players.add(this);
}
for (PlayerThread p : players) {
p.getOut().writeUTF(inputMessage);
}
And finally listening to messages:
public void listen() {
if (connected) {
try {
if (in.available() > 0) {
String inputMessage = in.readUTF();
System.out.println("GOT MESSAGE: " + inputMessage);
handleMessages(inputMessage);
}
} catch (IOException e) {
LOGGER.log(Level.INFO, "Connection lost");
connected = false;
}
} else
LOGGER.log(Level.INFO, "Player is not connected");
}
Method above is run in main game loop. It checks if there's something in inputStream and then reads it.
This is how correct message looks like:
GOT MESSAGE: {"type":"UPDATE_STATE","content":null,"senderName":"GRACZ 52","posX":10.0,"posY":5.0}
It works ok when there are 2 players, but the more players connect the more probably is to get message like this (or similiar broken messages):
GOT MESSAGE: 0} U{"type":"UPDATE_STATE","content":null,"senderName":"GRACZ 65","posX":10.0,"posY":5.0}
or
GOT MESSAGE: {"type":"UPDATE_STATE","content":null,"senderName":"GRACZ 24","pos
There different errors in message like Y letter, half-message or multiple messages in one row. Why such thing happen? It looks like when there are more players and they write into output stream in server side, this stream is not read yet so they are appending and appending. But it doesn't explain why there are broken and most imporant, how to resolve it?
I can move reading stream to another thread because in.readUTF() locks process but I wanted to keep it synchronized in main game loop and I don't think this will help (am I wrong?)

You need to synchronize your write loop on an object that's common between all PlayerThreads so that messages don't interleave.
synchronized(/*Should be a "global" server object*/) {
for (PlayerThread p : players) {
p.getOut().writeUTF(inputMessage);
}
}

Creating a monitor of messages between applications with Java

I am trying to create a monitor of messages between two applications. The idea is this monitor works in the middle of simple client/server application, and log the messages to the standard output. This program must be against of fails of the client/server (disconnections, time out's, etc). In the code, i call the client as "origin" and the server as "destiny". The current problem is when the server dies my program doesn't know and when a new message from client comes, this error appears "Software caused connection abort: socket write error". When the server comes up to life again, this error continues persisting. I think when i ask in the code "if ( !socketToDestiny.isConnected() )", it is not really connected. I am sure the problem is how i manage the "close" at the stream too.
This is the code of the program, i hope you could help me.
package interceptorprocess;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.net.InetSocketAddress;
import java.net.ServerSocket;
import java.net.Socket;
import java.net.SocketException;
import java.net.SocketTimeoutException;
public class GenericInterceptorProcess implements Runnable
{
private final String prefix_log_messages = "[CONNECTOR]";
//COMMUNICATION'S ORIGIN'S VARIABLES
ServerSocket serverSocketLocal;
Socket socketForLocal;
DataInputStream streamFromOrigin;
DataOutputStream streamToOrigen;
int len_message_from_origen;
byte[] buffer_msg_origin = new byte[4096];
byte[] message_origin = null;
//COMMUNICATION'S DESTINY'S VARIABLES
Socket socketToDestiny;
DataInputStream streamFromDestiny;
DataOutputStream streamToDestiny;
int len_message_from_destiny;
byte[] buffer_msg_destiny = new byte[4096];
byte[] message_destiny;
#Override
public void run()
{
//OCCASIONAL USE
String aux;
try
{
logger("STARTING SERVER --- PORT NUMBER: " + "1234");
//CREATING THE LOCAL SERVER SOCKET
serverSocketLocal = new ServerSocket(1234);
//CREATING THE DESTINITY CONNECTION WITH 15 TIMEOUT'S SECONDS
socketToDestiny = new Socket();
socketToDestiny.setSoTimeout(15000);
//THIS LOOP MAINTAINS THE CONNECTIVITY WITH ONE CLIENT AT TIME
while ( true )
{
logger("WAITING FOR A CONNECTION OF A CLIENT...");
socketForLocal = serverSocketLocal.accept();
//CREATING THE ORIGIN'S STREAMS
streamFromOrigin = new DataInputStream(socketForLocal.getInputStream());
streamToOrigen = new DataOutputStream(socketForLocal.getOutputStream());
logger("CONNECTED CLIENT: " + socketForLocal.getRemoteSocketAddress() );
//THIS LOOP MAINTAINS THE MESSAGES'S CHANGES
while ( true )
{
logger("WAITING FOR A MESSAGE..");
len_message_from_origen = streamFromOrigin.read(buffer_msg_origin);
if ( len_message_from_origen < 0 )
{
closeOriginStream();
break;
}
message_origin = new byte[len_message_from_origen];
//SAVE THE ORIGIN'S MESSAGE INTO AN ARRAY WHO HAS THE EXACT SIZE OF THIS MESSAGE
System.arraycopy(buffer_msg_origin, 0, message_origin, 0, len_message_from_origen);
aux = new String(message_origin);
logger("RECEIVED MESSAGE FROM ORIGIN: " + aux);
//TRY TO CONNECT TO DESTINY
try
{
if ( !socketToDestiny.isConnected() )
socketToDestiny.connect(new InetSocketAddress("10.10.200.200",1234),5000);
}
catch(IOException ex)
{
logger("CONNECTION REJECTED BY DESTINY: " + ex.getMessage());
continue;
}
//CREATING THE DESTINY'S STREAMS
streamFromDestiny = new DataInputStream(socketToDestiny.getInputStream());
streamToDestiny = new DataOutputStream(socketToDestiny.getOutputStream());
logger("SENDING MESSAGE TO DESTINY: " + aux);
//I HAD TO PUT THIS BLOCK BECAUSE IF THE DESTINY APPLICATIONS FAILS
//OR NOT ANSWER, THE PROGRAM MUST KEEP LISTENING THE FOLLOWING MESSAGES
try
{
//SENDING MESSAGE TO DESTINY
streamToDestiny.write(message_origin);
//READING THE ANSWER MESSAGE
logger("READING MESSAGE FROM DESTINY...");
len_message_from_destiny = streamFromDestiny.read(buffer_msg_destiny);
}
//IN ONE OF THE FOLLOWINGS TWO CATCHS I GET THE ERROR
catch (SocketTimeoutException ex)
{
logger("IT DIDN'T COULD RETRIEVE A MESSAGE FROM DESTINY: " + ex.getMessage());
continue;
}
catch (SocketException ex)
{
//THE "socketToDestiny.isConnected()" ALWAYS RETURNS TRUE SINCE THE FIRST SUCCESSFULLY
//CONNECTION, AFTER THAT, IF THE SOCKET IS DISCONNECTED, IT REMAINS RETURNING "true".
//THUS, I HAD TO MAKE THE NEXT CODE BLOCK
streamFromDestiny.close();
streamToDestiny.close();
socketToDestiny.close();
socketToDestiny = new Socket();
socketToDestiny.setSoTimeout(confs.timeout_destiny);
socketToDestiny.connect(new InetSocketAddress(confs.destiny_ip,confs.destiny_port),confs.timeout_connections);
streamFromDestiny = new DataInputStream(socketToDestiny.getInputStream());
streamToDestiny = new DataOutputStream(socketToDestiny.getOutputStream());
logger("TRYING TO RECONNECT WITH DESTINY AND SEND THE MESSAGE... ");
logger("READING MESSAGE FROM DESTINY AFTER ERROR...");
len_message_from_destiny = streamFromDestiny.read(buffer_msg_destiny);
}
message_destiny = new byte[len_message_from_destiny];
//SAVE THE DESTINY'S MESSAGE INTO AN ARRAY WHO HAS THE EXACT SIZE OF THIS MESSAGE
System.arraycopy(buffer_msg_destiny, 0, message_destiny, 0, len_message_from_destiny);
aux = new String(message_destiny);
logger("RECEIVED MESSAGE FROM DESTINY " + aux);
//SENDING THE ANSWER BACK TO THE ORIGIN
logger("SENDING BACK THE MESSAGE TO ORIGIN...");
streamToOrigen.write(message_destiny);
logger("MESSAGE DELIVERED SUCCESSFULLY!");
} //INTERNAL LOOP OF MESSAGES
} //INTERNAL LOOP OF CLIENTS
} //TRY
catch(IOException ex )
{
logger("THE SERVICE DIED: " + ex.getMessage() );
ex.printStackTrace();
} //CATCH
} //RUN
private void closeDestinyStream() throws IOException
{
streamFromDestiny.close();
streamToDestiny.close();
}
private void closeOriginStream() throws IOException
{
streamFromOrigin.close();
streamToOrigen.close();
}
private void closeAll() throws IOException
{
closeDestinyStream();
closeOriginStream();
}
private void logger(String message)
{
System.out.println(Utilidades.date() + " " + prefix_log_messages + " " + message);
}
}
Regards!
Sorry for my english, i am not a native speaker.

According to Java API,
public boolean isConnected()
Returns the connection state of the socket.
Note: Closing a socket doesn't clear its connection state, which means this method will return true for a closed socket (see isClosed()) if it was successfuly connected prior to being closed.
Returns:
true if the socket was successfuly connected to a server
Since:
1.4
Note that even if you close the socket, isConnect() still returns true, so it's very likely that your isConnect() usage is not right.
According to Sumit Singh's answer to this question,
How do I check if a Socket is currently connected in Java?
socket.isConnected() returns always true once the client connects (and even after the disconnect) weird !!
So, even after disconnecting, isConnected() will return true. So my hypothesis (like yours) is that although you caught the SocketWrite exception, the isConnected() still returns true. I cannot test this out, because its not a working example. However, you can do something like this:
try {
//write message to server
} catch ( SocketException e ) {
//we lost the connection, right? then print if the socket is connected
System.out.println( socketToDestiny.isConnected() );
}
and see what the output is. If the output is still true, then we have discovered the problem. If my hypothesis is correct, I would propose that you try to reconnect in the catch statement. For example:
try {
//write message to server
} catch ( SocketException e ) {
//we lost the connection, so let's try to reconnect
while ( true ) {
try {
socketToDestiny.connect( ...ip address ... );
break;
} catch ( IOException e2 ) {
//keep trying to reconnect!
continue;
}
}
}

How to add heartbeat messaging on top of this Java code( for KnockKnockClient/Server)?

I'm studying the following basic Java socket code( source ). It's a Knock-Knock-Joke client/server app.
In the Client, we set up the socket as usual:
try {
kkSocket = new Socket("localhost", 4444);
out = new PrintWriter(kkSocket.getOutputStream(), true);
in = new BufferedReader(new InputStreamReader(kkSocket.getInputStream()));
} catch( UnknownHostException uhe ){ /*...more error catching */
And then later, we just read and write to Server:
BufferedReader stdIn = new BufferedReader(new InputStreamReader(System.in));
String fromServer;
String fromUser;
while ((fromServer = in.readLine()) != null) {
System.out.println("Server: " + fromServer);
if (fromServer.equals("bye."))
break;
fromUser = stdIn.readLine();
if (fromUser != null){
System.out.println("Client: " + fromUser);
out.println(fromUser);
}
And on the server, we have the corresponding code, to get the joke punch-line.
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;
I want to attach a heartbeat to the whole thing, which will print out to the console whenever it detects that the other side died. Because what happens now if I kill the other side is an exception - like this one below:
So if I am running both KnockKnockClient and KnockKnockServer, then I shut down KnockKnockServer, what should happen is that on the Client I see this outputted:
>The system has detected that KnockKnockServer was aborted
I'm looking for any tips. So far I've mainly been trying to run a daemon thread that periodially creates new connections to the other side. But I'm confused about what condition to check for(but I think it's just a boolean value?). Is that the right approach? I just found out online there's a library called JGroups for multicast networking - would that be a better way? I'm looking for any tips.
My server-code so far(sorry it's messy)
&
Client-side
thanks

But the exception you are getting is exactly this! It's telling you that the other side just died. Just catch the exception and print to the console, that "The system has detected that KnockKnockServer was aborted".
You are using TCP connection and TCP has built-in heartbeat (keepalive) mechanism that will do this for you. Just set setKeepAlive() on the socket. That being said - It is possible to control keepalive frequency per each connection, but I do not know how to do that in java.
http://tldp.org/HOWTO/TCP-Keepalive-HOWTO/overview.html
https://stackoverflow.com/a/1480259/706650

you have a Synchronous communication. for having the heartbeat message, use an asynchronous communication. there will be 2 threads. one will read from the socket and another will keep writing to the socket. If you use asynchronous communication, the server will be sending a message every 10 seconds. the client thread will be reading messages from the server and if there is no message, it means the server is down. in your case, the server either sends back the message to client(if client has some message) or send an automatic reply.your server code can be modified like this.
Create a server thread that will keep sending messages to client every 10 seconds.
public class receiver extends Thread{
public static bool hearbeatmessage=true;
Socket clientSocket=new Socket();
PrintWriter out=new PrintWriter();
public receiver(Socket clientsocket){
clientSocket=clientsocket;
out = new PrintWriter(clientSocket.getOutputStream(), true);
}
public void run(){
while(true)
{
if(heartbeatmessage){
thread.sleep(10000);
out.println("heartbeat");
}
}
}
}
In your server code:
KnockKnockProtocol kkp = new KnockKnockProtocol();
outputLine = kkp.processInput(null);
out.println(outputLine);
receiver r=new reciver(clientSocket);
r.run(); /*it will start sending hearbeat messages to clients */
while ((inputLine = in.readLine()) != null) {
outputLine = kkp.processInput(inputLine);
reciver.hearbeatMessage=false; /* since you are going to send a message to client now, sending the heartbeat message is not necessary */
out.println(outputLine);
reciver.hearbeatMessage=true; /*start the loop again*/
if (outputLine.equals("Bye."))
break;
The client code will also be modified, a thread will keep reading messages from the socket and if it has not received message for more than 11 seconds(1 second extra), it will declare the server is not available.
Hope this helps. There might be some flaw in the logic too. Let me know.

The following are best practices which we apply on a daily base when interfacing with hardware (using sockets).
Good practice 1 : SoTimeout
This property enables a read timeout. The goal of this is to avoid the issue that Tom had. He wrote something in the line of : "you will need to wait till the next client message arrives". Well, this offers a solution to that problem. And it's also the key to implementing a heartbeat and many other checks.
By default, the InputStream#read() method will wait forever, until a message arrives. The setSoTimeout(int timeout) changes this behaviour. It will apply a timeout now. When it timeouts it will throw the SocketTimeoutException. Just catch the exception, check a couple of things and continue reading (repeat). So basically, you put your reading method in a loop (and probably even in a dedicated thread).
// example: wait for 200 ms
connection.setSoTimeout(200);
You can use these interruptions (caused by the timeout) to validate the status: E.g. how long has it been since I received my last message.
Here is an example to implement the loop:
while (active)
{
try
{
// some function that parses the message
// this method uses the InputStream#read() method internally.
code = readData();
if (code == null) continue;
lastRead = System.currentTimeMillis();
// the heartbeat message itself should be ignored, has no functional meaning.
if (MSG_HEARTBEAT.equals(code)) continue;
//TODO FORWARD MESSAGE TO ACTION LISTENERS
}
catch (SocketTimeoutException ste)
{
// in a typical situation the soTimeout should be about 200ms
// the heartbeat interval is usually a couple of seconds.
// and the heartbeat timeout interval a couple of seconds more.
if ((heartbeatTimeoutInterval > 0) &&
((System.currentTimeMillis() - lastRead) > heartbeatTimeoutInterval))
{
// no reply to heartbeat received.
// end the loop and perform a reconnect.
break;
}
// simple read timeout
}
}
Another use of this timeout: It can be used to cleanly stop your session by setting active = false. Use the timeout to check if this field is true. If that's the case, then break the loop. Without the SoTimeout logic this would not be possible. You would either be forced to do a socket.close() or to wait for the next client message (which clearly makes no sense).
Good practice 2 : Built-in Keep-Alive
connection.setKeepAlive(true);
Well basically this is pretty much what your heart-beat logic does. It automatically sends a signal after a period of inactivity and checks for a reply. The keep-alive interval is operating system dependent though, and has some shortcomings.
Good practice 3 : Tcp No-Delay
Use the following setting when you are often interfacing small commands that need to be handled quickly.
try
{
connection.setTcpNoDelay(true);
}
catch (SocketException e)
{
}

I think you are over complicating things.
From the client side:
If the client gets an IOException for the connection reset, then this means the server is dead. Instead of printing the stack trace just do what ever you need to do once you know that the server is down. You already know the server is down due to the exception.
From the server side:
Either start a timer and if you don't get a request for a time more than the interval assume that the client is down.
OR start a background server thread at the client (making the client and server peers) and have the server send a "dummy" hearbeat request (server now acts as a client). If you get exception the client is down.

Figured I'd take a crack at this... I started with the KnockKnockServer and KnockKnockClient that are on the Java site (in your original question).
I didn't add any threading, or heartbeats; I simply changed the KnockKnockClient to the following:
try { // added try-catch-finally block
while ((fromServer = in.readLine()) != null) {
System.out.println("Server: " + fromServer);
if (fromServer.equals("Bye."))
break;
fromUser = stdIn.readLine();
if (fromUser != null) {
System.out.println("Client: " + fromUser);
out.println(fromUser);
}
}
} catch (java.net.SocketException e) { // catch java.net.SocketException
// print the message you were looking for
System.out.println("The system has detected that KnockKnockServer was aborted");
} finally {
// this code will be executed if a different exception is thrown,
// or if everything goes as planned (ensure no resource leaks)
out.close();
in.close();
stdIn.close();
kkSocket.close();
}
This seems to do what you want (even though I modified the original Java website example, rather than your code - hopefully you'll be able to see where it plugs in). I tested it with the case you described (shut down the server while the client is connected).
The downside to this is that, while the client is waiting for user input, you don't see that the server has died; you have to enter client input, and then you'll see that the server has died. If this is not the behavior you want, please post a comment (perhaps that was the whole point of the question - it just seemed like you might have been going down a longer road than you needed in order to get to where you wanted to be).

Here's a slight modification to the client. It doesn't use an explicit heartbeat, but as long as you keep reading from the server, you'll immediately detect the disconnect anyway.
This is because readLine will immediately detect any read errors.
// I'm using an anonymous class here, so we need
// to have the reader final.
final BufferedReader reader = in;
// Decouple reads from user input using a separate thread:
new Thread()
{
public void run()
{
try
{
String fromServer;
while ((fromServer = reader.readLine()) != null)
{
System.out.println("Server: " + fromServer);
if (fromServer.equals("Bye."))
{
System.exit(0);
}
}
}
catch (IOException e) {}
// When we get an exception or readLine returns null,
// that will be because the server disconnected or
// because we did. The line-break makes output look better if we
// were in the middle of writing something.
System.out.println("\nServer disconnected.");
System.exit(0);
}
}.start();
// Now we can just read from user input and send to server independently:
while (true)
{
String fromUser = stdIn.readLine();
if (fromUser != null)
{
System.out.println("Client: " + fromUser);
out.println(fromUser);
}
}
In this case, we allow client writes even when we're waiting for reply from the server. For a more stable application, we'd want to lock the input while we're waiting for a reply by adding a semaphore controlling when we start reading.
These are the modifications we would make to control the input:
final BufferedReader reader = in;
// Set up a shared semaphore to control client input.
final Semaphore semaphore = new Semaphore(1);
// Remove the first permit.
semaphore.acquireUninterruptibly();
new Thread()
... code omitted ...
System.out.println("Server: " + fromServer);
// Release the current permit.
semaphore.release();
if (fromServer.equals("Bye."))
... code omitted ...
while (true)
{
semaphore.acquireUninterruptibly();
String fromUser = stdIn.readLine();
... rest of the code as in the original ...

I think #Bala's answer is correct on server side. I'd like to give a supplementary on client side.
On client side, you should:
use an variable to keep the timestamp of the last message from server;
start a thread which runs periodically(every 1 second, e.g.) to compare current timestamp and the last message timestamp, if it is longer than desired timeout(10 seconds, e.g.), a disconnection should be reported.
Following are some code snippet:
The TimeoutChecker class(thread):
static class TimeoutChecker implements Runnable {
// timeout is set to 10 seconds
final long timeout = TimeUnit.SECONDS.toMillis(10);
// note the use of volatile to make sure the update to this variable thread-safe
volatile long lastMessageTimestamp;
public TimeoutChecker(long ts) {
this.lastMessageTimestamp = ts;
}
#Override
public void run() {
if ((System.currentTimeMillis() - lastMessageTimestamp) > timeout) {
System.out.println("timeout!");
}
}
}
Start the TimeoutChecker after connection is established:
try {
kkSocket = new Socket("localhost", 4444);
// create TimeoutChecker with current timestamp.
TimeoutChecker checker = new TimeoutChecker(System.currentTimeMillis());
// schedule the task to run on every 1 second.
ses.scheduleAtFixedRate(, 1, 1,
TimeUnit.SECONDS);
out = new PrintWriter(kkSocket.getOutputStream(), true);
in = new BufferedReader(new InputStreamReader(kkSocket.getInputStream()));
} catch( UnknownHostException uhe ){ /*...more error catching */
The ses is a ScheduledExecutorService:
ScheduledExecutorService ses = Executors.newScheduledThreadPool(1);
And remember to update the timestamp when receiving messages from server:
BufferedReader stdIn = new BufferedReader(new InputStreamReader(System.in));
String fromServer;
String fromUser;
while ((fromServer = in.readLine()) != null) {
// update the message timestamp
checker.lastMessageTimestamp = System.currentTimeMillis();
System.out.println("Server: " + fromServer);
if (fromServer.equals("bye."))
break;

Adel,was looking at your code http://pastebin.com/53vYaECK
Can you try the following solution. not sure whether it will work.
instead of creating a bufferedreader with the inputstream once,
we can create an instance of BufferedReader eachtime.
when the kkSocket.getInputStream is null, it comes out of the while loop and set completeLoop to false, so that we exit the while loop.
it has 2 while loops and the objects are created each time.
if the connection is open but does not have data in it inputstream will not be null,
BufferedReader.readLine would be null.
bool completeLoop=true;
while(completeLoop) {
while((inputstream is=kkSocket.getInputStream())!=null) /*if this is null it means the socket is closed*/
{
BufferedReader in = new BufferedReader( new InputStreamReader(is));
while ((fromServer = in.readLine()) != null) {
System.out.println("Server: " + fromServer);
if (fromServer.equals("Bye."))
break;
fromUser = stdIn.readLine();
if (fromUser != null) {
System.out.println("Client: " + fromUser);
out.println(fromUser);
}
}
}
completeLoop=false;
System.out.println('The connection is closed');
}

Trying to get data from textfield to ouputstream for a chat program in Java

i'm working on a simple GUI chat program in Java. The goal is for the user to choose whether to host a server or to connect as a client. All of this works. The problem I'm having is letting either the client or the server chat. ideally, the user or the server can type into the textField and hit enter (or press the send button), and then the message will be sent to every client that is connected. During execution, the server runs an infinite while loop where it waits for more clients. The problem I'm having is two-fold:
1) I'm not sure if the way I'm passing the string to the inputstream is right, and 2) I don't know when I can have the server receive and then re-send the data, since it waits at server.accept().
here's the run method:
public void run()
{
conversationBox.appendText("Session Start.\n");
inputBox.requestFocus();
while (!kill)
{
if (isServer)
{
conversationBox.appendText("Server starting on port " + port + "\n");
conversationBox.appendText("Waiting for clients...\n");
startServer();
}
if (isClient)
{
conversationBox.appendText("Starting connection to host " + host + " on port " + port + "\n");
startClient();
}
}
}
here's the startClient method:
public void startClient()
{
try
{
Socket c = new Socket(host, port);
in = new Scanner(c.getInputStream());
out = new PrintWriter(c.getOutputStream());
while (true)
{
if (in.hasNext())
{
Chat.conversationBox.appendText("You Said: " + message);
out.println("Client Said: " + message);
out.flush();
}
}
}
catch (Exception e)
{
e.printStackTrace();
}
}
and here's the startServer method:
public void startServer()
{
try
{
server = new ServerSocket(port);
while (true)
{
s = server.accept();
conversationBox.appendText("Client connected from " + s.getLocalAddress().getHostName() + "\n");
}
}
catch (Exception e)
{
conversationBox.appendText("An error occurred.\n");
e.printStackTrace();
isServer = false;
reEnableAll();
}
}
And finally, here's the part of actionPerformed where I get the data and (attempt) to write it to the outputstream:
if (o == sendButton || o == inputBox)
{
if(inputBox.getText() != "")
{
out.println(inputBox.getText());
inputBox.setText("");
}
}
I guess my question is: How can I rearrange my methods so that the server can wait for text from the client and then send it back to all the clients? And, how do I send the text from the client to the server?

Among the problems with this code:
You keep creating clients and servers. Surely you should only do one of each?
You are performing blocking network operations on the event thread instead of in a separate thread.
You are looping at EOS via while (true) ... if in.hasNext(). This should be while (in.hasNext()) ...
You are accepting a socket and not apparently doing anything with it. It looks like you can only handle one client at a time. You should start a new thread to handle each accepted socket.