Develop Reference

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

Java Sockets: Sending Stdin and GUI data from Client to Server, Server not reading and responding as expected

Apologies as I asked something similar last night, but I have narrowed my problem down. I am wondering how to make my Java TCP Socket Server read in the data sent using the printWriter(out) in the Client code from a GUI as it does from the command line stdin.
I have the following classes as an example and everything works fine until the GUI comes into the equation. The data is being sent over to the Server from the GUI as I can echo it on the server side, but it is not being read and parsed properly as the stdin is. Nothing is being sent back to the client. I have tried flushing, using different streams and adding line separators all over the place to no avail. There is also a Protocol class that handles the data on the Server side.
public class KnockKnockServer {
public static void main(String[] args) throws IOException {
ServerSocket serverSocket = null;
try {
serverSocket = new ServerSocket(4444);
System.out.println("Waiting for client...");
} catch (IOException e) {
System.err.println("Could not listen on port: 4444.");
System.exit(1);
}
Socket clientSocket = null;
try {
clientSocket = serverSocket.accept();
} catch (IOException e) {
System.err.println("Accept failed.");
System.exit(1);
}
PrintWriter out = new PrintWriter(clientSocket.getOutputStream(), true);
BufferedReader in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
String inputLine, outputLine;
KnockKnockProtocol kkp = new KnockKnockProtocol();
outputLine = kkp.processInput(null);
out.println(outputLine);
while ((inputLine = in.readLine()) != null) {
System.out.println(inputLine);
outputLine = kkp.processInput(inputLine);
out.println(outputLine);
if (outputLine.equals("Bye.")) {
break;
}
}
out.close();
in.close();
clientSocket.close();
serverSocket.close();
}
}
.
public class KnockKnockClient {
public static PrintWriter out = null;
public static String sendAnswer;
public static void Client() {
//JButton Action Listener
saveAnswer.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
ButtonModel b = group.getSelection();
if (b.getActionCommand() == "A") { sendAnswer = radioA.getText(); }
String data = "รท" + sendAnswer;
out.println(data);
}
});
}
public static void main(String[] args) throws IOException {
KnockKnockClient.Client();
Socket kkSocket = null;
//PrintWriter out = null;
BufferedReader in = null;
try {
kkSocket = new Socket("localhost", 4444);
out = new PrintWriter(kkSocket.getOutputStream(), true);
in = new BufferedReader(new InputStreamReader(kkSocket.getInputStream()));
} catch (UnknownHostException e) {
System.err.println("Don't know about host: localhost.");
System.exit(1);
} catch (IOException e) {
System.err.println("Couldn't get I/O for the connection to: localhost.");
System.exit(1);
}
BufferedReader stdIn = new BufferedReader(new InputStreamReader(System.in));
String fromServer, 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);
}
}
out.close();
in.close();
stdIn.close();
kkSocket.close();
}
}

TCP stream being read by Java

I would like to create a Java server socket application that receives a TCP packet and reads the content of it. Based on the contents of the packet it will perform several actions. I managed to get to the point where it reads some content and prints a string System.out.println(sb.toString());
But (a) not all the content is printed and (b) I am not sure how to process the content as they arrive in network order. An example would be to receive an HTTP packet and from the header to report the "Content-Length" or the "User-Agent". Any example would be appreciated.
public static void main(String[ ] args){
PrintWriter out = null;
BufferedReader in = null;
int bufferSize = 0;
try{
String message = args[0];
int count = 0;
ServerSocket connectionSocket = null;
try {
connectionSocket = new ServerSocket(4444);
System.out.println("Server started");
} catch (IOException e) {
System.err.println("Could not listen on port: 4444.");
System.exit(1);
}
Socket clientSocket = null;
try {
while(true){
count++;
clientSocket = connectionSocket.accept();
System.out.println("TCP packet received… " + count);
InputStream is = clientSocket.getInputStream();
out = new PrintWriter(clientSocket.getOutputStream());
in = new BufferedReader(new InputStreamReader(is));
StringBuilder sb = new StringBuilder();
String line = null;
while ((line = in.readLine()) != null) {
sb.append(line + "\n");
}
System.out.println(sb.toString());
clientSocket.close();
}
} catch (IOException e) {
System.err.println("Accept failed.");
System.exit(1);
}
}
catch(Exception e){
e.printStackTrace();
}
}

Difficulty with socket communication from Server to Client

I have a relatively simple program where I try establish Client Server connection and at the same time I use threads in the client side to allow for multiple connections.
I run the server and then the server invokes the client constructor and passes the port connection to the client and the thread is started on the client side.
The problem I have is that when I run the server side it doesn't want to go beyond the constructor call. It seems to get stuck at the constructor.
Sorry all this sounds a bit confusing
Any thoughts perhaps
this is the server side
ServerMultipleThreads()
{
System.out.println("Starting the server first...");
try
{
ServerSoc = new ServerSocket(7777);
listening = true;
}
catch(Exception e)
{
System.out.println(e.toString());
System.exit(1);
}
System.out.println("The server has started running");
while(listening)
{
try
{
//creating the client socket and starting the new client session
new ClientSession(ServerSoc.accept());
System.out.println("The clientSession was called");
in = new DataInputStream(clientSocket.getInputStream());
BufferedReader is = new BufferedReader(new InputStreamReader(in));
os = new PrintStream(clientSocket.getOutputStream());
while(true)
{
line = is.readLine();
PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter("myFile,txt")), true);
out.println(line);
}
}
catch(IOException ioe)
{
System.out.println(ioe.toString());
}
}
}
and this is on client side
ClientSession(Socket s)
{
clientSocket = s;
try
{
out = new PrintWriter(clientSocket.getOutputStream(), true);
in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
out.println("Welcome");
}
catch(IOException exe)
{
System.out.println(exe.toString());
}
//starting the thread
while(runner == null)
{
runner = new Thread(this);
runner.start();
}
}
public void run()
{
while(runner == Thread.currentThread())
{
BufferedReader buf = new BufferedReader(new InputStreamReader(System.in));
String stdIn;
try
{
while((stdIn = buf.readLine()) != null)
{
out.println(stdIn);
}
}
catch(IOException exe)
{
exe.toString();
}
try
{
Thread.sleep(10);
}
catch(InterruptedException e){}
}
Kind regards
Arian

That is because ServerSocket.accept() blocks until it receives a client request.
You need to have a client calling the server, something like this:
Socket socket = new Socket(host, port);
InputStream in = socket.getInputStream();
// write some data...

Server and ServerSocket in one Application: not working

I am trying to write a small program, that opens a server, creates a client that connects to this server and receives a message from it.
This is the Code so far
public static void main(String[] args) {
final ServerSocket serverSocket;
try {
serverSocket = new ServerSocket(12345);
Thread t = new Thread(){
public void run(){
try {
Socket server = serverSocket.accept();
PrintWriter writer = new PrintWriter(server.getOutputStream(), true);
writer.write("Hello World");
writer.flush();
} catch (IOException e) {
e.printStackTrace();
}
}
};
t.start();
Socket client = new Socket("localhost", 12345);
BufferedReader reader = new BufferedReader(new InputStreamReader(client.getInputStream()));
String message = reader.readLine();
System.out.println("Received " + message);
} catch (IOException e1) {
e1.printStackTrace();
}
}
If i run program it keeps waiting in readLine() - so obviously the client does not receive the message from the server.
Has anyone got an idea why this isn' working?

Your reading thread is waiting for a newline in the data stream. Just change the server to use:
writer.write("Hello World\r\n");
and you'll get the result you were expecting. Alternatively, you can just close the server socket, and then readLine will return when it reaches the end of the data stream.

You should put the readline in a loop as follows:
public static void main(String[] args) {
final ServerSocket serverSocket;
try {
serverSocket = new ServerSocket(12345);
Thread t = new Thread() {
public void run() {
try {
Socket server = serverSocket.accept();
PrintWriter writer = new PrintWriter(server.getOutputStream(), true);
writer.write("Hello World");
writer.flush();
} catch (IOException e) {
e.printStackTrace();
}
}
};
t.start();
Socket client = new Socket("localhost", 12345);
BufferedReader reader = new BufferedReader(new InputStreamReader(client.getInputStream()));
// Check this --------------------------------------------------->
String message = null;
while ((message = in.readLine()) != null) {
System.out.println("Received " + message);
break; //This break will exit the loop when the first message is sent by the server
}
} catch (IOException e1) {
e1.printStackTrace();
}
}
You can read this documentation for further explanation: http://download.oracle.com/javase/tutorial/networking/sockets/