Develop Reference

Code of new thread after accepting the connection in TCP server isn't executed

I have the following tcp server:
public class Server {
private Connection db;
private Statement statement;
private ServerSocket socket;
public static void main(String[] args) {
Server server = new Server();
server.initializeServer();
System.out.println("Server initialized");
server.listenConnections();
}
private void initializeServer() {
try {
db = DriverManager.getConnection("jdbc:mysql://localhost:3306/courseworkschema" +
"?verifyServerCertificate=false" +
"&useSSL=false" +
"&requireSSL=false" +
"&useLegacyDatetimeCode=false" +
"&amp" +
"&serverTimezone=UTC",
"Sergei",
"12345");
statement = db.createStatement();
socket = new ServerSocket(1024);
} catch (SQLException | IOException e) {
e.printStackTrace();
}
}
private void listenConnections() {
System.out.println("Listening connections ... ");
while (true) {
try {
Socket client = socket.accept();
new Thread(() -> {
System.out.println("Client accepted");
try {
OutputStream outputStream = client.getOutputStream();
InputStream inputStream = client.getInputStream();
String clientAction;
String queryContent;
boolean flag = true;
while (flag) {
byte[] msg = new byte[100];
int k = inputStream.read(msg);
clientAction = new String(msg, 0, k);
clientAction = clientAction.trim();
msg = new byte[100];
k = inputStream.read(msg);
queryContent = new String(msg, 0, k);
queryContent = queryContent.trim();
System.out.println(clientAction);
System.out.println(queryContent);
if (clientAction.equalsIgnoreCase("END")) {
flag = false;
}
else if (clientAction.equalsIgnoreCase("LOGIN")) {
System.out.println("Login action");
}
}
} catch (IOException e) {
e.printStackTrace();
}
});
} catch (IOException e) {
e.printStackTrace();
}
}
}
}
This server is created to communicate with database. Here's the way how I try to connect to this serverL
public class LoginController {
private LoginWindow window;
private Socket socket;
private InputStream is;
private OutputStream os;
public LoginController() {
connectToServer();
}
public void logInUser(String login, String password) {
if (!login.isEmpty() && !password.isEmpty()) {
sendDataToServer("LOGIN");
sendDataToServer("");
} else {
window.showMessageDialog("Fill the fields!", JOptionPane.ERROR_MESSAGE);
}
}
public void attachView(LoginWindow window) {
this.window = window;
}
private void connectToServer() {
try {
socket = new Socket("127.0.0.1", 1024);
System.out.println("Connected");
} catch (IOException e) {
e.printStackTrace();
}
}
private void sendDataToServer(String res) {
try {
os = socket.getOutputStream();
os.write(res.getBytes());
} catch (IOException e) {
e.printStackTrace();
}
}
}
When I run the server and then client, I have such logs in server:
Server initialized
Listening connections ...
Process finished with exit code -1
So, I can't understand why server doesn't wait and accept a connection from client, but closes after initializing and listening. So, what's the matter? I will appreciate any help. Thanks in advance!
UPD
When I run my app it started to work but I found out that code in Thread block isn't executed. I even can't understand, why does it happen

In your private void listenConnections() you are creating a Thread object but you are not telling it to start after its created thus it wont execute.
Your thread creation line should look something like this:
new Thread(() -> {
//your code
}).start();
From the javadocs:
https://docs.oracle.com/javase/7/docs/api/java/lang/Thread.html#start()
public void start()
Causes this thread to begin execution; the Java Virtual Machine calls
the run method of this thread. The result is that two threads are
running concurrently: the current thread (which returns from the call
to the start method) and the other thread (which executes its run
method).
It is never legal to start a thread more than once. In particular, a
thread may not be restarted once it has completed execution.
Throws: IllegalThreadStateException - if the thread was already
started.
See Also: run(), stop()

Multi-Client Server in Java, Maybe threads not running properly or i dont know , cant find out why

I'm programming a server in java that broadcasts the Date() function each second to the clients. The problem is that it worked for only one client but as soon as I started making modifications for multi-client support it broadcasts the Date() only once and then stops, as if the function is being called only once. I can not find what I'm doing wrong so I will just paste the code and hopefully someone will spot the mistake. I searched online but only to end up more confused than I started. For Client program I use the tellnet terminal app for windows.
public class Server
{
private ServerSocket SERVER;
private int PORT;
private Socket CLIENT;
public Server()
{
PORT = 8818;
try
{
SERVER = new ServerSocket(PORT);
System.out.println("Server started on port: " + PORT);
}
catch(IOException e)
{
System.out.println(e.getMessage());
}
}
public void On() throws IOException
{
while(true)
{
CLIENT = SERVER.accept();
new ClientHandler(CLIENT).start();
}
}
}
#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
public class ClientHandler extends Thread
{
private Socket CLIENT;
private OutputStream out;
public ClientHandler(Socket CLIENT)
{
System.out.println("Accepted Connection from: " + CLIENT.getInetAddress());
this.CLIENT = CLIENT;
}
public void run()
{
try
{
out = CLIENT.getOutputStream();
out.write(("Time now is: " + new Date() + "\n").getBytes());
sleep(1000);
}
catch(Exception e)
{
System.out.println(CLIENT.getInetAddress() + " has left the session");
try
{
out.close();
CLIENT.close();
}
catch(IOException j)
{
System.out.println("Unexpected Error");
System.exit(-1);
}
}
}
}

Your modifications nearly worked - below is a running version with only minor modifications from your code.
A part of your modifications you unintentionally removed the while loop in the run function, which means the Date() function is in fact only called once. To see this, remove the while loop in run() and after the Date is printed (in the telnet window) the message "Done with the run function." is printed.
I added an identifier to each client which is printed with the date. The static cnt class field makes sure that each client has a different id.
I started separate clients in separate command prompt terminals using
telnet localhost 8818 such that they were running at the same time. At the bottom is the output from the 3rd client.
I did switch the code to camelCase convention (start variables with a lower case letter and capitalizing each new word) since all CAPS is usually reserved for constants and the change made the code easier to read for me.
public class Server
{
private ServerSocket server;
private int port;
private Socket client;
public Server()
{
port = 8818;
try
{
server = new ServerSocket(port);
System.out.println("Server started on port: " + port);
}
catch(IOException e)
{
System.out.println(e.getMessage());
}
}
public void on() throws IOException
{
while(true)
{
client = server.accept();
new ClientHandler(client).start();
}
}
}
public class ClientHandler extends Thread {
private Socket client;
private OutputStream out;
private int id;
private static int cnt=0;
public ClientHandler(Socket client) {
System.out.println("Accepted Connection from: " + client.getInetAddress());
this.client = client;
id=cnt;
cnt++;
}
public void run() {
try {
out = client.getOutputStream();
while (true) {
out.write(("Client " + id + ": Time now is: " + new Date() + "\n").getBytes());
sleep(1000);
}
} catch (Exception e) {
System.out.println(client.getInetAddress() + " has left the session");
try {
out.close();
client.close();
} catch (IOException j) {
System.out.println("Unexpected Error");
System.exit(-1);
}
}
System.out.println("Done with the run function.");
}
}
public static void main(String[] args) throws IOException {
// TODO Auto-generated method stub
Server s = new Server();
s.on();
}
}

How to stop() Thread when it created on server side for client in java

when client leave the server connection how can i stop the Thread of server which is create for the communication between client and server and please tell me how can i send a single client message to all clients which are connected with that server thanks in advance :).
ChatServer.java
import java.net.*;
import java.io.*;
public class ChatServer implements Runnable
{
private ServerSocket server = null;
private Thread thread = null;
private ChatServerThread client = null;
public ChatServer(int port)
{
try
{ System.out.println("Binding to port " + port + ", please wait ...");
server = new ServerSocket(port);
System.out.println("Server started: " + server);
start();
}
catch(IOException ioe)
{ System.out.println(ioe); }
}
public void run()
{
while (thread != null)
{
try
{ System.out.println("Waiting for a client ...");
addThread(server.accept());
}
catch(IOException ie)
{ System.out.println("Acceptance Error: " + ie); }
}
}
public void addThread(Socket socket)
{ System.out.println("Client accepted: " + socket);
client = new ChatServerThread(this, socket);
try
{ client.open();
client.start();
}
catch(IOException ioe)
{ System.out.println("Error opening thread: " + ioe); }
}
public void start()
{ if (thread == null)
{ thread = new Thread(this);
thread.start();
}
}
public void stop()
{ if (thread != null)
{ thread.stop();
thread = null;
}
}
public static void main(String args[])
{ ChatServer server = null;
int x=2111;
if (x<1)
System.out.println("Usage: java ChatServer port");
else
server = new ChatServer(x);
}
}
ChatServerThread
import java.net.*;
import java.io.*;
public class ChatServerThread extends Thread
{ private Socket socket = null;
private ChatServer server = null;
private int ID = -1;
private DataInputStream streamIn = null;
public ChatServerThread(ChatServer _server, Socket _socket)
{ server = _server; socket = _socket; ID = socket.getPort();
}
public void run()
{ System.out.println("Server Thread " + ID + " running."+Thread.activeCount());
while (true)
{ try
{ System.out.println(streamIn.readUTF());
}
catch(IOException ioe) { }
}
}
public void open() throws IOException
{ streamIn = new DataInputStream(new BufferedInputStream(socket.getInputStream()));
}
public void close() throws IOException
{
if (socket != null) socket.close();
if (streamIn != null) streamIn.close();
}
}

In ChatServerThread:
public void run()
try {
// thread code
} catch(InterruptedException e) {
Thread.currentThread().interrupt();
} finally {
// cleanup code, if required
}
}
and in ChatServer:
public void stop() {
if (thread != null) {
thread.interrupt();
thread = null;
}
}
This won't stop the thread, but it signals the thread to stop what it's doing - it might or might not stop right then, or even at all, depending on what code the thread is currently executing.

Socket Client/Server stop method in java

This is the simple client/server socket app for my faculty project. First, the Server class should be run, and then if Client class runs - it prints out the IP address of the local machine and the port that's been used.
I can't figure out one thing:
How and WHERE to create a method in class that will close(stop) the Server? And
how to make this like an event or something, for example if client
sends "stop" it should somehow stop the server...
SERVER.JAVA
import java.io.*;
import java.net.*;
public class Server {
public static void main(String[] args) {
System.out.println("The server has been summoned.\n");
System.out.println("The server is waiting for client to come...");
try {
ServerSocket servertest = new ServerSocket(2014);
while (true) {
try {
Socket ser = servertest.accept();
new ThreadSer(ser).start();
} catch (IOException e) {}
}
} catch (IOException e) {System.err.println(e);}
}
public static class ThreadSer extends Thread {
private Socket s;
public ThreadSer(Socket s) {
this.s = s;
}
#Override
public void run() {
try {
String response = "This is the IP: " + s.getLocalAddress() + " that has come via port: "
+ s.getLocalPort() + "\r\n";
OutputStream out = s.getOutputStream();
out.write(response.getBytes());
} catch (IOException e) { System.err.println(e); }
}}}
CLIENT.JAVA
import java.io.*;
import java.net.*;
public class Client {
public static void main(String[] args) throws UnknownHostException, IOException {
Socket socket = new Socket("localhost", 2014);
new OutputThread(socket.getInputStream()).start();
}
public static class OutputThread extends Thread {
private InputStream inputstream;
public OutputThread(InputStream inputstream) {
this.inputstream = inputstream;
}
#Override
public void run() {
BufferedReader input = new BufferedReader(new InputStreamReader(inputstream));
while (true) {
try {
String line = input.readLine();
System.out.println(line);
} catch (IOException exception) {
exception.printStackTrace();
break;
}
}
}}}

You should constantly ask for the inputstream of the client.. put it in the loop that always accept for the client input..
example:
public static class ThreadSer extends Thread {
private Socket s;
public ThreadSer(Socket s) {
this.s = s;
}
#Override
public void run() {
try {
String response = "This is the IP: " + s.getLocalAddress() + " that has come via port: "
+ s.getLocalPort() + "\r\n";
ObjectInputStream input = new ObjectInputStream(s.getInputStream());
while(true)
{
Object object = input.readObject();
if(object instanceof String)
{
String command = ((String) object).trim();
if(command.equals("stop"))
break;
}
}
s.close();
} catch (IOException e) { System.err.println(e); }
}}}

Having multiple clients connecting to Java Server

I'm creating a simple socket server using Java. I am able to connect one client at a time, I tried to implement Threads to handle multiple client. In my Server constructor I created a thread that handles the ServerSocket, and should keep listening for new clients. Once a socket is connected then I tried to create another thread to handle the client Socket. But I am still not able to connect more than one client. The second client I try to connect will not get IO streams.
public class Server extends JFrame {
private JTextField enterField;
private JTextArea displayArea;
private ObjectOutputStream output;
private ObjectInputStream input;
private ServerSocket server;
private Socket connection;
private int counter = 1;
public Server() {
super("Server");
enterField = new JTextField();
enterField.setEditable(false);
enterField.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent event) {
sendData(event.getActionCommand());
enterField.setText("");
}
});
add(enterField, BorderLayout.NORTH);
displayArea = new JTextArea();
add(new JScrollPane(displayArea));
setSize(300, 150);
setLocation(500, 500);
setVisible(true);
new Thread(new Runnable() {
public void run() {
try {
server = new ServerSocket(50499, 100);
displayMessage("Listening on Port: "
+ server.getLocalPort() + "\n");
for (;;) {
Socket nextClient = server.accept();
displayMessage("Client Connected");
new ClientThread(nextClient).start();
nextClient = null;
}
} catch (IOException exception) {
exception.printStackTrace();
}
}
}).start();
}
private void closeConnection() {
displayMessage("\nTerminating connection\n");
setTextFieldEditable(false);
try {
output.close();
input.close();
connection.close();
} catch (IOException ioException) {
ioException.printStackTrace();
}
}
private void displayMessage(final String string) {
SwingUtilities.invokeLater(new Runnable() {
#Override
public void run() {
displayArea.append(string);
}
});
}
private void setTextFieldEditable(final boolean editable) {
SwingUtilities.invokeLater(new Runnable() {
#Override
public void run() {
enterField.setEditable(editable);
}
});
}
private void sendData(String message) {
try {
output.writeObject("SERVER>>> " + message);
output.flush();
displayMessage("\nSERVER>>> " + message);
} catch (IOException ioException) {
displayArea.append("\nError Writing Object");
}
}
private class ClientThread extends Thread {
public ClientThread(Socket socket) throws IOException {
try {
connection = socket;
output = new ObjectOutputStream(socket.getOutputStream());
output.flush();
input = new ObjectInputStream(socket.getInputStream());
displayMessage("Got I/O Stream\n");
displayMessage("Connection " + counter + " received from: "
+
connection.getInetAddress().getHostName());
counter++;
String message = "Connection Sucessful";
sendData(message);
setTextFieldEditable(true);
do {
message = (String) input.readObject();
displayMessage("\n" + message);
} while (!message.endsWith(">>> TERMINATE"));
} catch (ClassNotFoundException classNotFoundException) {
displayMessage("\nUnknown object type recieved");
} finally {
closeConnection();
}
}
}
}

You're doing the connection stuff inside the ClientThread constructor. Therefore the new ClientThread(...) never returns until you send a TERMINATEcommand. Put the logic inside the run() method.
private class ClientThread extends Thread {
private Socket socket;
// The queue, thread-safe for good measure
private Queue<String> queue = new ConcurrentLinkedQueue<String>();
public ClientThread(Socket socket) throws IOException {
this.socket = socket;
}
public void send(String message) {
if (message != null) {
this.sendQueue.add(message);
}
}
public void run() {
try {
connection = socket;
output = new ObjectOutputStream(socket.getOutputStream());
output.flush();
input = new ObjectInputStream(socket.getInputStream());
displayMessage("Got I/O Stream\n");
displayMessage("Connection " + counter + " received from: "
+ connection.getInetAddress().getHostName());
counter++;
String message = "Connection Sucessful";
sendData(message);
setTextFieldEditable(true);
do {
// Purge the queue and send all messages.
while ((String msg = queue.poll()) != null) {
sendData(msg);
}
message = (String) input.readObject();
displayMessage("\n" + message);
} while (!message.endsWith(">>> TERMINATE"));
} catch (ClassNotFoundException classNotFoundException) {
displayMessage("\nUnknown object type recieved");
} finally {
closeConnection();
}
}
}
Tipically, you'll send messages to the connection from other threads:
ClientThread client = new ClientThread(newClient);
client.start();
client.send("Hi there");
Personally, I would have used a non-blocking (NIO) networking library such as Netty or Mina to implement this sort of stuff. There's some learning to do to use them but I think its worth it. Non-blocking means that you don't dedicate a separate thread for each connection but rather you're notified when something is received in the socket.

You did not provide implementation of run method for Thread Class.
Put processing logic of ClientThread inside run method
To keep your code modularised and more manageable make your ClientThread class public and share only necessary resources between those thread.
public class ClientThread extends Thread { //make class public
private Socket socket;
// define all other references
ObjectOutputStream output = null;
//...
public ClientThread(Socket socket) throws IOException {
this.socket = socket;
}
public void run() {
try {
connection = socket;
output = new ObjectOutputStream(socket.getOutputStream());
output.flush();
input = new ObjectInputStream(socket.getInputStream());
displayMessage("Got I/O Stream\n");
displayMessage("Connection " + counter + " received from: "
+
connection.getInetAddress().getHostName());
counter++;
String message = "Connection Sucessful";
sendData(message);
setTextFieldEditable(true);
do {
message = (String) input.readObject();
displayMessage("\n" + message);
} while (!message.endsWith(">>> TERMINATE"));
} catch (ClassNotFoundException classNotFoundException) {
displayMessage("\nUnknown object type recieved");
} finally {
closeConnection();
}
}
}}