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How to effectively ping and discover all clients on a network?

The problem I am having is acquiring all reachable clients on a network.The below method returns some clients when called. In most cases other android clients.However for the PC it fails when firewall is on.Is there a more effective way to get all clients in Java/android purely or will I need to use android NDK?Any help from experts in this domain will be appreciated.Thanks in advance.
/***
* ping_JavaStyle(final int j)
* uses multi threads to enhance performance
* while pinging from 0>j<=255
* #param j
*/
private void ping_JavaStyle(final int j)
{
new Thread(new Runnable() { // new thread for parallel execution
public void run() {
try {
String testIp = prefix + String.valueOf(j);
InetAddress address = InetAddress.getByName(testIp);
String output = address.toString().substring(1);
if (address.isReachable(3000)) {
System.out.println(output + " is on the network");
ipList.add(testIp);
} else {
if (retest(testIp, 139)) {
ipList.add(testIp);
} else {
System.out.println("Not Reachable: " + output);
}
}
} catch (Exception e) {
e.printStackTrace();
}
}
}).start();
}

After Researching some more, got this working.With help of this repo:https://github.com/stealthcopter/AndroidNetworkTools
Below code solves the problem:
** RunnableTask.Java
* Created by Kirk on 10/29/2017.
*/
public class RunnableTask implements Callable<Boolean> {
private String testIp = "";
private Boolean is_Reachable = false;
public RunnableTask(String testIp) {
this.testIp = testIp;
}
#Override
public Boolean call() throws Exception {
try {
PingResult pingResult = Ping.onAddress(this.testIp).setTimes(1).setTimeOutMillis(1500).doPing();
if (pingResult.isReachable) {
is_Reachable = true;
}
} catch (UnknownHostException e) {
e.printStackTrace();
}
return is_Reachable;
}
}
And use in the caller method:
private static final int NTHREDS = 255;
//.......
ExecutorService executor = Executors.newFixedThreadPool(NTHREDS);
List<Future<Boolean>> thread_Values_list = new ArrayList<>();
for (int i = 1; i <= 255; i++) {
final int j = i;
try {
try {
String testIp = prefix + String.valueOf(j);
RunnableTask worker = new RunnableTask(testIp);
Future<Boolean> submit = executor.submit(worker);
thread_Values_list.add(submit);
} catch (Exception e) {
e.printStackTrace();
}
} catch (Exception e) {
}
}
for (Future<Boolean> finishedThread : thread_Values_list) {
String reachable_Ip = "";
try {
if (finishedThread.get()) {
reachable_Ip = prefix + String.valueOf(finishThread_counter);
ipList.add(reachable_Ip);
}
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
finishThread_counter++;
}
executor.shutdown();
}

returning value from async onComplete

gettedID is a public variable and gets a value after onComplete is done. How can I listen to gettedID value in order to execute some code after it is filled up, without using a separate thread?
VKRequest request = VKApi.wall().get(VKParameters.from("domain", "bitcoin", VKApiConst.OFFSET, offset, VKApiConst.COUNT, count));
request.executeWithListener(new VKRequest.VKRequestListener() {
#Override
public void onError(VKError error) {
System.out.println("error: " + error);
}
#Override
public void onComplete(VKResponse response) {
super.onComplete(response);
try {
json = response.json.getJSONObject("response").getJSONArray("items");
for (int i = 0; i < json.length(); i++) {
returnArray.put(json.getJSONObject(i).get("id").toString());
}
gettedID = returnArray;
//Log.d("BTCR", "gettedID: " + returnArray.toString());
} catch (JSONException e) {
e.printStackTrace();
}
}
});

Server side auto disconnect user connection and array index outofbounds error

server will run the array which disconnect user after client press connect button
public void run() {
String message, connect = "Connect", disconnect = "Disconnect", chat = "Chat" ;
String[] data;
try {
while ((message = reader.readLine()) != null) {
outputTextArea.append("Received: " + message + "\n");
data = message.split(":");
for (String token:data) {
outputTextArea.append(token + "\n");
}
if (data[2].equals(connect)) {
tellEveryone((data[0] + ":" + data[1] + ":" + chat));
userAdd(data[0]);
} else if (data[2].equals(disconnect)) {
tellEveryone((data[0] + ":has disconnected." + ":" + chat));
userRemove(data[0]);
} else if (data[2].equals(chat)) {
tellEveryone(message);
} else {
outputTextArea.append("No Conditions were met. \n");
}
} // end while
} // end try
catch (Exception ex) {
outputTextArea.append("Lost a connection. \n");
ex.printStackTrace();
clientOutputStreams.remove(client);
} // end catch
} // end run()
} // end class ClientHandler
public void userAdd (String data) {
String message, add = ": :Connect", done = "Server: :Done", name = data;
outputTextArea.append("Before " + name + " added. \n");
onlineUsers.add(name);
outputTextArea.append("After " + name + " added. \n");
String[] tempList = new String[(onlineUsers.size())];
onlineUsers.toArray(tempList);
for (String token:tempList) {
message = (token + add);
tellEveryone(message);
}
tellEveryone(done);
}
public void userRemove (String data) {
String message, add = ": :Connect", done = "Server: :Done", name = data;
onlineUsers.remove(name);
String[] tempList = new String[(onlineUsers.size())];
onlineUsers.toArray(tempList);
for (String token:tempList) {
message = (token + add);
tellEveryone(message);
}
tellEveryone(done);
}
public void tellEveryone(String message) {
// sends message to everyone connected to server
Iterator it = clientOutputStreams.iterator();
while (it.hasNext()) {
try {
PrintWriter writer = (PrintWriter) it.next();
writer.println(message);
writer.flush();
outputTextArea.setCaretPosition(outputTextArea.getDocument().getLength());
} // end try
catch (Exception ex) {
outputTextArea.append("Error telling everyone. \n");
} // end catch
} // end while
} // end tellEveryone()
Client Side:
ArrayList<String> userlist = new ArrayList();
public class IncomingReader implements Runnable{
public void run(){
String stream;
String[] data;
String done = "Done", connect = "connect", disconnect = "Disconnect", chat ="Chat";
try {
while ((stream = reader.readLine()) != null){
data = stream.split("!");
if (data[2].equals(chat)) {
chatTextArea.append(data[0]+":"+ data[1]+"\n");
chatTextArea.setCaretPosition(chatTextArea.getDocument().getLength());
} else if (data[2].equals(connect)){
chatTextArea.removeAll();
userAdd(data[0]);
} else if (data[2].equals(disconnect)){
userRemove(data[0]);
} else if (data[2].equals(done)){
onlineuserlist.setText("");
writeUsers();
userlist.clear();
}
}
}catch(Exception ex){
}
}
}
private void userAdd(String data) {
userlist.add(data);
}
private void userRemove(String data) {
chatTextArea.append(data +"has disconnected.\n");
}
private void writeUsers() {
String[] tempList = new String[(userlist.size())];
userlist.toArray(tempList);
for (String token:tempList) {
onlineuserlist.append(token +"\n");
}
}
public void sendDisconnect(){
String bye =(username + ": :Disconnect");
try{
writer.println(bye);
writer.flush();
} catch (Exception ex){
chatTextArea.append("could not send Disconnect Message.\n");
}
}
public void Disconnect(){
try{
chatTextArea.append("Disconnected.\n");
sock.close();
} catch (Exception ex){
chatTextArea.append("Failed to disconnect. \n");
}
isConnected = false;
usernameField.setEditable(true);
onlineuserlist.setText("");
}
}
after start the server and client press the connect button, it will show which user has connected but it also disconnect the connection and got this error.
java.lang.ArrayIndexOutOfBoundsException: 2
at chatsystemserver.ServerSide$ClientHandler.run(ServerSide.java:55)
at java.lang.Thread.run(Thread.java:745)

It would appear that you don't have an array of size == 3 when you do you split. You might want to do some array size checking before you access "data[2]".

java chat program via internet [closed]

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well, what i want to do is basically, to make a client server chat program that works over internet, ive done a basic one that works flawlessly over lan, but cant get it right over the internet..
Server :
public class Server extends javax.swing.JFrame {
HashMap<String,PrintWriter> map = new HashMap<String,PrintWriter>();
ArrayList clientOutputStreams = new ArrayList();
ArrayList<String> onlineUsers = new ArrayList();
int port = 5080;
Socket clientSock = null;
public class ClientHandler implements Runnable {
BufferedReader reader;
Socket sock;
PrintWriter client;
public ClientHandler(Socket clientSocket, PrintWriter user) {
// new inputStreamReader and then add it to a BufferedReader
client = user;
try {
sock = clientSocket;
InputStreamReader isReader = new InputStreamReader(sock.getInputStream());
reader = new BufferedReader(isReader);
System.out.println("first");
} // end try
catch (Exception ex) {
System.out.println("error beginning StreamReader");
} // end catch
} // end ClientHandler()
#Override
public void run() {
System.out.println("run method is running");
String message;
String[] data;
String connect = "Connect";
String disconnect = "Disconnect";
String chat = "Chat";
try {
while ((message = reader.readLine()) != null) {
ta1.append(message + "\n");
ta1.repaint();
System.out.println("Received: " + message);
data = message.split("#");
for (String token : data) {
System.out.println(token);
}
System.out.println(data[data.length - 1] + " datalast");
if (data[2].equals(connect)) {
tellEveryone((data[0] + "#" + data[1] + "#" + chat));
userAdd(data[0]);
map.put(data[0], client);
} else if (data[2].equals(disconnect)) {
System.out.println("barpppppppppp");
tellEveryone((data[0] + "#has disconnected." + "#" + chat));
userRemove(data[0]);
map.remove(data[0]);
} else if (data[2].equals(chat)) {
tellEveryone(message);
} else {
System.out.println("No Conditions were met.");
}
} // end while
} // end try
catch (Exception ex) {
System.out.println("lost a connection");
System.out.println(ex.getMessage());
clientOutputStreams.remove(client);
} // end catch
} // end run()
}
public void go() {
// clientOutputStreams = new ArrayList();
try {
ServerSocket serverSock = new ServerSocket(port);
System.out.println("ServerSocket Created !");
System.out.println("Started listening to port " + port);
while (true) {
// set up the server writer function and then begin at the same
// the listener using the Runnable and Thread
clientSock = serverSock.accept();
PrintWriter writer = new PrintWriter(clientSock.getOutputStream());
ta1.append(writer + " ");
ta1.repaint();
System.out.println(writer);
clientOutputStreams.add(writer);
//data_of_names_and_output_streams.add(writer.toString());
// use a Runnable to start a 'second main method that will run
// the listener
Thread listener = new Thread(new Server.ClientHandler(clientSock, writer));
listener.start();
System.out.println("Server Thread for 'new player' was started");
System.out.println("got a connection");
} // end while
} // end try
catch (Exception ex) {
System.out.println("error making a connection");
} // end catch
} // end go()
public void userAdd(String data) {
String message;
String add = "# #Connect", done = "Server# #Done";
onlineUsers.add(data);
String[] tempList = new String[(onlineUsers.size())];
onlineUsers.toArray(tempList);
for (String token : tempList) {
message = (token + add);
tellEveryone(message);
System.out.println(message);
}
tellEveryone(done);
}
public void userRemove(String data) {
System.out.println(onlineUsers.size() + " is size of online users");
System.out.println(clientOutputStreams.size() + " is size of ous");
String message;
String add = "# #Connect", done = "Server# #Done";
onlineUsers.remove(data);
String[] tempList = new String[(onlineUsers.size())];
onlineUsers.toArray(tempList);
for (String token : tempList) {
message = (token + add);
tellEveryone(message);
}
tellEveryone(done);
}
public void tellEveryone(String message) {
System.out.println(onlineUsers.size() + " is size of online users");
System.out.println(clientOutputStreams.size() + " is size of ous");
// jButton1.doClick();
// sends message to everyone connected to server
Iterator it = clientOutputStreams.iterator();
if (message.length() < 250) {
System.out.println("inside it");
while (it.hasNext()) {
try {
PrintWriter writer = (PrintWriter) it.next();
writer.println(message);
// l1.setText(message);
System.out.println("Sending " + message);
writer.flush();
} // end try
catch (Exception ex) {
System.out.println("error telling everyone");
} // end catch
}
} else {
try {
clientSock.close();
} catch (IOException ex) {
Logger.getLogger(Server.class.getName()).log(Level.SEVERE, null, ex);
}
}
}
/**
* Creates new form Server
*/
public Server() {
initComponents();
ta1.repaint();
}
public static void main(String args[]) throws Exception {
UIManager.setLookAndFeel("com.sun.java.swing.plaf.windows.WindowsLookAndFeel");
/* Create and display the form */
java.awt.EventQueue.invokeLater(new Runnable() {
public void run() {
new Server().setVisible(true);
}
});
new Server().go();
}
} //end form
Client : jbutton1 is setting up connection,jbutton2 sends the message.
public class Client extends javax.swing.JFrame {
boolean sent, receive;
SimpleDateFormat sdf;
String ip;
String username;
Socket sock;
BufferedReader reader;
PrintWriter writer;
ArrayList<String> userList = new ArrayList();
Boolean isConnected = false;
DefaultListModel dlm;
public Client() {
initComponents();
dlm = (DefaultListModel) l1.getModel();
ip = JOptionPane.showInputDialog("Enter the IP of the server to connect");
}
public class IncomingReader implements Runnable {
public void run() {
String stream;
String[] data;
String done = "Done", connect = "Connect", disconnect = "Disconnect", chat = "Chat", battlerequest = "battlerequest";
try {
while ((stream = reader.readLine()) != null) {
data = stream.split("#");
System.out.println(stream + " ------------------------ data");
if (data[2].equals(chat)) {
sdf = new SimpleDateFormat("HH:mm:ss");
t.append("(" + sdf.format(new Date()) + ") " + data[0] + ": " + data[1] + "\n");
//t.setText("<html><b>hi" + 3 + 3 + "</b></html>");
} else if (data[2].equals(connect)) {
t.removeAll();
userAdd(data[0]);
} else if (data[2].equals(disconnect)) {
userRemove(data[0]);
} else if (data[2].equals(done)) {
dlm.removeAllElements();
writeUsers();
userList.clear();
} else {
System.out.println("no condition met - " + stream);
}
}
} catch (Exception ex) {
System.out.println(ex.getMessage() + " hi");
}
}
}
public void ListenThread() {
Thread IncomingReader = new Thread(new Client.IncomingReader());
IncomingReader.start();
}
public void userAdd(String data) {
userList.add(data);
}
public void userRemove(String data) {
t.setText(t1.getText() + data + " has disconnected.\n");
}
public void writeUsers() {
String[] tempList = new String[(userList.size())];
userList.toArray(tempList);
for (String token : tempList) {
//ul.append( token + "\n");
dlm.addElement(token);
// ul.setText(ul.getText() + token + '\n');
}
}
public void sendDisconnect() {
String bye = (username + "# #Disconnect");
try {
writer.println(bye); // Sends server the disconnect signal.
writer.flush(); // flushes the buffer
} catch (Exception e) {
t.append("Could not send Disconnect message.\n");
}
}
public void Disconnect() {
try {
t.append("Disconnected.\n");
sock.close();
} catch (Exception ex) {
t.append("Failed to disconnect. \n");
}
isConnected = false;
n.setEditable(true);
dlm.removeAllElements();
// ul.setText("");
}
private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {
if (isConnected == false && !n.getText().equals("")) {
username = n.getText();
n.setEditable(false);
try {
sock = new Socket(ip, 5080);
InputStreamReader streamreader = new InputStreamReader(sock.getInputStream());
reader = new BufferedReader(streamreader);
writer = new PrintWriter(sock.getOutputStream());
writer.println(username + "#has connected.#Connect"); // Displays to everyone that user connected.
writer.flush(); // flushes the buffer
isConnected = true;
jLabel4.setText(n.getText());
//t.append( "<html><font color = \"black\"><b>Server : Welcome,</b></font></html>"+username);
//t1.setText("<html><font color=\"red\">yo</font></html>");
// Used to see if the client is connected.
} catch (Exception ex) {
t.append("Cannot Connect! Try Again. \n");
n.setEditable(true);
}
ListenThread();
} else if (isConnected == true) {
t.append("You are already connected. \n");
} else if (n.getText().equals("")) {
t.append("Enter a valid name \n");
}
}
private void jButton2ActionPerformed(java.awt.event.ActionEvent evt) {
String nothing = "";
if ((t1.getText()).equals(nothing)) {
t1.setText("");
t1.requestFocus();
} else {
try {
writer.println(username + "#" + t1.getText() + "#" + "Chat");
writer.flush(); // flushes the buffer
} catch (Exception ex) {
t.append("Message was not sent. \n");
}
t1.setText("");
t1.requestFocus();
}
t1.setText("");
t1.requestFocus(); // TODO add your handling code here:
}
private void dicsActionPerformed(java.awt.event.ActionEvent evt) {
sendDisconnect();
Disconnect(); // TODO add your handling code here:
}
i have also port forwarded the ports i am going to use - ie. 5080
now when my friend opens the client program from his computer from his home, i tell him to enter the ip as 192.168.1.2 coz thats what is saved when i open cmd and type ipconfig....
sometimes i think that the ip address i gave him is wrong coz 192.168.1.2 is i guess lan or internal ip address, so then, so do i do ? where do i get the correct ip address ? or is something else wrong in my code ?

192.168.1.2 is a non-routable IP. Click here to get your current external IP (unless your IP address is static, it may change periodically).
If you were to sign up for a dynamic dns service (here for example), then you could give your friend a "domain name" (e.g. something.dnsdynamic.com) and the service would update when your IP address changes.

Java server socket sending multiple messages issue

We have a java socket program where the server gets data from many devices and works fine. At times the server needs to send some command to the devices. When it sends individual commands it works fine. The problem comes when it sends multiple commands, only the first one is successful. We cant figure out why the rest fails. Below is the snippet showing how the message is sent. Should I set a delay after a message is sent?
public static void main(String[] args) {
new sServer7888();
}
sServer7888() {
try{
final ServerSocket serverSocketConn = new ServerSocket(7888);
while (true){
try{
Socket socketConn1 = serverSocketConn.accept();
new Thread(new ConnectionHandler(socketConn1)).start();
}
catch(Exception e){
e.printStackTrace(System.out);
}
}
}
catch (Exception e) {
e.printStackTrace(System.out);
}
}
class ConnectionHandler implements Runnable {
private Socket receivedSocketConn1;
ConnectionHandler(Socket receivedSocketConn1) {
this.receivedSocketConn1=receivedSocketConn1;
}
public void run() {
while ((nextChar=readIn1.read()) != -1) {
completeMessage += (char) nextChar;
if (nextChar == '*')
{
String[] splitResult = completeMessage .split(",");
String header=splitResult[0].trim().substring(0,4);
if((header.equals("$ACK")){
//update the message sent from the server as already acknowledge.
}
else{
//run query to find if there are any message to be sent out to the devices
while(rsOC1.next()){
commandText = rsOC1.getString("commandText");
writeOut1.write(commandText);
writeOut1.write("\r\n");
writeOut1.flush();
}
//now process the normal message receive from the devices.
}
completeMessage="";
}
}
}
}

If your device is sending ACK on Every message and Server is able to receive it then you can proceed in following way with your server side program.
EDIT
I have updated the code as per the requirement analysis. Let me know if any discrepancy is found after implementing it.
Thread.sleep(1000) is not the reliable solution for above case because
we are not knowing how long the device might take to execute previous
command sent by Server .
public void run()
{
int i = -1;
ArrayList<String> list = new ArrayList<String>();
while ((nextChar=readIn1.read()) != -1)
{
boolean isCompleteMessage = readMessage(nextChar);
if (isCompleteMessage)
{
String[] splitResult = completeMessage .split(",");
String header=splitResult[0].trim().substring(0,4);
if((header.equals("$ACK"))
{
String id = null;
if (i != -1)
{
id = list.get(i);
id = id.substring(0,id.indexOf("^"));
}
//update the message sent from the server as already acknowledge using id extracted above.
if ( i == 0)
{
list.remove(i);
if (list.size() == 0)
{
i = -1;
}
else
{
commandText = list.get(i);
writeOut1.write(commandText.substring((commandText.indexOf("^")) + 1));
writeOut1.write("\r\n");
writeOut1.flush();
}
}
}
else
{
//process here the normal message receive from the devices.
if (i == -1)
{
list = getRecords();
if (list.size() > 0)
{
i = 0;
commandText = list.get(i);
writeOut1.write(commandText.substring((commandText.indexOf("^")) + 1));
writeOut1.write("\r\n");
writeOut1.flush();
}
}
else
{
commandText = list.get(i);
writeOut1.write(commandText.substring((commandText.indexOf("^")) + 1));
writeOut1.write("\r\n");
writeOut1.flush();
}
}
completeMessage = "";
}
}
}
public boolean readMessage(int nextChar)
{
completeMessage += (char)nextChar;
if (((char)nextChar) == '*')
{
return true;
}
else
{
return false;
}
}
//Retreive all commands from database and returns the ArrayList containing those commands.
public ArrayList<String> getRecords()
{
ArrayList<String> list = new ArrayList<String>();
Statement stat = null;
ResultSet rsOC1 = null;
try
{
stat = con.createStatement();
rsOC1 = stat.executeQuery("Query for message retrieval from database");
while (rsOC1.next())
{
String sElement = rs0C1.getString("commandID") + "^" + rs0C1.getString("commandText");
list.add(sElement);
}
}
catch (Exception ex){}
finally
{
if (rs0C1 != null)
{
try
{
rs0C1.close();
} catch () {}
}
if (stat != null)
{
try
{
stat.close();
} catch () {}
}
return list;
}
}