Develop Reference

Java: How can I create an application that will only start if an instance of it doesn't exist and call the instance if it does exist?

I am trying to create a program with Java that can only have one instance of it running at a time.
I am using Sockets and ServerSockets to try to achieve this.
How the program is supposed to work is:
The main method will check if any parameters have been passed, it will try to write the first parameter to the server, if it fails that means, that means that this is the only running instance, so it will open the ServerSocket and then start the frame. If it doesn't fail then the application is already running so it should send the string and the other instance should be able to read it and process it.
Here's the main method:
public static void main(String[] args) {
String fileName = null;
if (args.length >= 1) {
fileName = args[0];
}
if (Singleton.sendSignal(fileName)) {
Frame.getFrame().open(fileName);
Singleton.checkInput();
}
}
And here's the server class:
public class Singleton {
private static final int portNumber = 4243;
private static ServerSocket serverSocket;
private static Socket clientSocket;
private static Socket echoSocket;
public static boolean sendSignal() {
try {
echoSocket = new Socket(InetAddress.getLocalHost().getHostName(), portNumber);
PrintWriter out = new PrintWriter(echoSocket.getOutputStream(), true);
out.write("Open\n");
out.close();
close();
return false;
} catch (Exception e) {
close();
return true;
}
}
public static void checkInput() {
try {
renewReader();
} catch (Exception e) {
close();
}
}
public static void renewReader() throws Exception {
serverSocket = new ServerSocket(portNumber);
clientSocket = serverSocket.accept();
BufferedReader in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
String inputLine = in.readLine();
if (inputLine.equals("Open")) {
Widget.getInstance().setVisible(true);
}
close();
renewReader();
}
public static void close() {
try {
serverSocket.close();
} catch (Exception e) {
}
try {
clientSocket.close();
} catch (Exception e) {
}
try {
echoSocket.close();
} catch (Exception e) {
}
}
}
Although half of this code works (only one instance runs at a time), only the first set of data are being passed and then the program stops reading. How can I make the socket listen until the program is closed?

I your checkInput() method, you are accepting for client connection once here. Try something like this:
public static void checkInput()
{
//do something here
serverSocket = new ServerSocket(portNumber);
//wait for request from client.
Socket clientSocket = serverSocket.accept();
//
// do your processing here
// call checkInput method again.
checkInput();
}
As soon as another instance it started, server will accept the request, do the processing and then again starts waiting for more requests (for this we called cehckInput again).
Also in your main() add this:
public static void main(String[] args)
{
String fileName = null;
if (args.length >= 1) {
fileName = args[0];
}
if (Singleton.sendSignal(fileName))
{
Frame.getFrame().open(fileName);
// start the server in a thread so that main method can continue on
new Thread()
{
public void run()
{
Singleton.checkInput();
}
}.start();
}
// do your other tasks.
}
On upon termination of program, your sockets will auto close. Also if you want to explicitly close the sockets, you can add a shutdown hook to close it.
A simple hook looks like this.
Runtime.getRuntime().addShutdownHook(your thread that will close sockets);

Java - Iterate through IP Address List and call thread with IP Address as argument

I'm relatively new to Java and I'm writing an application to interrogate an Apache HTTP server's access_log file; with this, I want to submit the IP Addresses individually (probably via the Apache HTTPClient library) to another Java instance on another server (as the Web server does not have FTP enabled) to pull some log files. At the moment I've managed to bumble my way through modifying a 'tail -f' equivalent class to suit the programs needs and then manipulate that data to get the IP Addresses that I need to do something with - I even managed to make the 'tail' class threaded so it could address multiple periods of time!
With that said, I want to use a for loop to iterate through each entry in my computerRebootList String Array and with each address create a thread to perform some more work but all I can think of is this;
for (String tmpIpAddress : computerRebootList ) {
ComputerIpHandler handler = new ComputerIpHandler();
}
and then create another class named ComputerIpHandler like so;
public class KioskIpHandler implements Runnable {
static final Logger logger = LogManager.getLogger( ComputerIpHandler.class );
#Override public void run() {
//do some code
}
public static void main(String computerIp) {
Thread mainThread = new Thread(new ComputerIpHandler());
mainThread.start();
try {
logger.info("log some stuff");
mainThread.join();
logger.info("yay it's done");
}
catch (InterruptedException errInterrupted) {
logger.error(errInterrupted.getMessage());
logger.error(errInterrupted.getStackTrace());
}
}
}
I read somewhere about ensuring that I need to manage resource limitations so I would have to create a maximum number of threads - arguably I could send something like 10 IPs to this class and then have the rest of the addresses 'queue' until the one has returned... I'm just not confident or fluent enough to be able to conceptualise these ideas.
EDIT: I omitted that I am restricted to Java 1.6 as this is the maximum compatible version of the JRE that we can use on this server - not sure if that hinders this effort somewhat...
Can anyone point me in the right direction?

Check ScheduledThreadPoolExecutor and ScheduledExecutorService classes in package java.util.concurrent in java API. Those and some other classes in that package would manage all resources for you. They are available in java since version 1.5

I recommend using Java's built in FTP connection platform to make a thread for continually receiving data on a specified port, until it receives a termination key.
Basically, one class will create a ServerSocket (open socket on server) and upon connection with another socket (the client socket) it would create a new thread for receiving information.
public class Server {
public ServerSocket ss;
public Socket clientSocket;
public Thread receiveingThread;
public BufferedReader inFromClient = null;
public boolean running = false;
public Server(int port) {
try {
ss = new ServerSocket(port);
startReceiving();
} catch (Exception e) {
e.printStackTrace();
}
}
public synchronized void startReceiving() {
receiveingThread = new Thread("Recieve") {
public void run() {
String dataFromClient = new String("");
while (running) {
try {
inFromClient = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
dataFromClient = inFromClient.readLine();
} catch (Exception e) {
e.printStackTrace();
}
if (dataFromClient.equals("TERMINATOR_KEY") {
stopRecieving();
}
else if(!dataFromClient.equals("")) {
//add item to array
}
}
}
};
receiveingThread.start();
}
public synchronized void stopReceiving() {
try {
running = false;
receivingThread.join();
ss.close();
clientSocket.close();
} catch (Exception e) {
e.printStackTrace();
}
System.exit(0);
}
public static void main(String[] args) {
new Server(yourPortHere);
}
}
then the client class would look something like:
public class Client {
public Socket socket;
public Thread send;
public Client(string serverPublicIP, int port) {
try {
socket = new Socket(serverPublicIP, port);
send("some IP address");
} catch (Exception e) {
e.printStackTrace();
}
}
public void send(String toSend) {
send = new Thread("Send") {
public void run() {
PrintWriter outToServer;
try {
outToServer = new PrintWriter(new OutputStreamWriter(socket.getOutputStream()));
outToServer.print(toSend);
outToServer.flush();
} catch (Exception e) {
e.printStackTrace();
}
finally {
outToServer.close();
}
}
};
send.start();
}
public static void main(String[] args) {
new Client("127.0.0.1", yourPortHere);
}
}
This is the link for the start of socket tutorials on the oracle site.
This is the Oracle Documentation for java.net.ServerSocket
This is the Oracle Documentation for java.net.Socket

ObjectInputStream consumes too much memory

I have a Socket that sends a list of Objects every few seconds to a client through ObjectOutputStream. On the server side, after every writeObject(myList) i execute flush then reset. Using VisualVM to check for memory usage, on the server there's no memory leaks, but on the client it seems that the previously read Lists are kept in memory. I tried to execute reset on the ObjectInputStream on the client side but looks like ObjectInputStream does not support this method (it throws a java.io.IOException: mark/reset not supported).
This is my server socket:
public class ConsultaBombas {
public static void inicializarServidorSocket() {
try {
ServerSocket serverSocket = new ServerSocket(5963);
Thread thread = new Thread(() -> {
while (!serverSocket.isClosed()) {
try {
final Socket socket = serverSocket.accept();
new ThreadComunicacao(socket).start();
} catch (Exception e) {
e.printStackTrace();
}
}
});
thread.setName("Consulta bombas (Inicializador)");
thread.start();
} catch (Exception e) {
e.printStackTrace();
}
}
static class ThreadComunicacao extends Thread {
private Socket socket;
public ThreadComunicacao(Socket socket) {
this.socket = socket;
setName("Consulta bombas (Comunicação) com início: " + new SimpleDateFormat("dd/MM/yyyy HH:mm:ss").format(new Date()));
}
#Override
public void run() {
try {
ObjectOutputStream out = new ObjectOutputStream(socket.getOutputStream());
while (!socket.isClosed()) {
List<Bomba> bombas = new DaoBomba().findAll();
out.writeObject(bombas);
out.flush();
out.reset();
Thread.sleep(1000);
}
} catch (SocketException e) {
if (e.getLocalizedMessage() != null && e.getLocalizedMessage().equalsIgnoreCase("Connection reset by peer: socket write error")) {
System.out.println("Cliente desconectou...");
} else {
e.printStackTrace();
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
}
An this is the client (started with start() method):
public class ConsultaBombasClient {
private Socket socket;
private Thread threadConsulta;
public ConsultaBombasClient(BombasListener bombasListener, String maquinaDestino) {
threadConsulta = new Thread(() -> {
try {
Thread.currentThread().setName("Consulta Bombas");
System.out.println("Endereço bagual: "+maquinaDestino);
socket = new Socket(maquinaDestino, 5963);
ObjectInputStream in = new ObjectInputStream(socket.getInputStream());
Object leitura;
while ((leitura = in.readObject()) != null) {
List<Bomba> bombas = (List<Bomba>) leitura;
bombasListener.run(bombas);
}
} catch (Exception e) {
e.printStackTrace();
throw new RuntimeException(e);
}
});
threadConsulta.setDaemon(true);
}
public void start() {
threadConsulta.start();
}
public interface BombasListener {
void run(List<Bomba> bombas);
}
}
What am i doing wrong?

garbage collection is not immediate, do you have any real memory troubles? Have you tried running the client with low -Xmx value, did you receive the OutOfMemoryError?
– user3707125
You're right, after some time when the memory gets close to the maximum heap size, it clears the objects from memory. I wasn't seeing this because i have a lot of RAM in my pc but with Xmx50m i could see this working as you said. – Mateus Viccari

Clearly bombasListener.run(), whatever it may be, is not releasing the supplied list.
NB ObjectInputStream.readObject() does not return null at end of stream. It is therefore incorrect to use this test as a termination condition for a read loop.

Continuously read objects from an ObjectInputStream in Java

I have a problem using an ObjectInputStream and I have been struggling with it for 2 days now. I tried to search for a solution but unfortunately found no fitting answer.
I am trying to write a client/server application in which the client sends objects (in this case a configuration class) to the server. The idea is that connection keeps alive after sending the object so it is possible to send a new object if necessary.
Here are the important parts of my client code:
mSocket = new Socket("192.168.43.56", 1234);
mObjectIn = new ObjectInputStream(mSocket.getInputStream());
mObjectOut = new ObjectOutputStream(mSocket.getOutputStream());
mObjectOut.writeObject(stubConfig);
mObjectOut.flush();
In the above code, I left out some try/catch blocks to keep the code readable for you.
The server side looks as follows:
mHostServer = new ServerSocket(port);
mSocket = mHostServer.accept();
// create streams in reverse oreder
mObjectOut = new ObjectOutputStream(mConnection.getOutputStream());
mObjectOut.flush();
mObjectIn = new ObjectInputStream(mConnection.getInputStream());
while (mIsSocketConnected)
{
StubConfig = (StubConfiguration)mObjectIn.readObject();
}
What I want to achieve is that as long at the socketconnection is alive, the server is listening for incoming config objects.
When I run my program however, I got an EOFException in the while loop at server side. I receive the first config object without any problems in the first iteration of the while loop but after that I get an EOFException every time readObject() is called.
I am looking for a way to solve this. Can anyone put me in the good direction?
EDIT: What I read about the EOFException is that it is thrown when you want to read from a stream when the end of it is reached. That means that for some reason the stream ended after the object has been send. Is there a way to reinitialize the streams or so??

EOFException is thrown by readObject() when the peer has closed the connection. There can never be more data afterwards. Ergo you can't have written multiple objects at all: you closed the connection instead.

try using this
Server side
1.Server running on a separate thread
public class ServeurPresence implements Runnable {
public final static int PORT = 20000 ;
public final static String HOSTNAME = "localhost" ;
public static enum Action {CONNEXION, MSG, DECONNEXION,USER, FINCLASSEMENT};
ServerSocket serveur ;
static List<String> names ;
*/
public ServeurPresence()
{
System.out.println("Start Server...");
try
{
serveur = new ServerSocket(PORT) ;
new Thread(this).start();
//javax.swing.SwingUtilities.invokeLater(new Runnable() { public void run() { createAndShowGUI();} } );
}
catch (IOException e)
{
e.printStackTrace();
}
}
/**
* #param args
*/
public static void main(String[] args)
{
new ServeurPresence();
}
#Override
public void run()
{
System.out.println("server runs");
while(true)
{
try {
Socket sock = serveur.accept();
ServiceClientsThread thread= new ServiceClientsThread(sock);
thread.start();
}
catch (IOException e)
{
System.out.println("Error with socket");
e.printStackTrace();
}
}
}
}
2. A Thread to handle each Client:ServiceClientThread
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.net.Socket;
import java.util.ArrayList;
import java.util.List;
public class ServiceClientsThread extends Thread{
private Socket sock ;
ServiceClientsThread(Socket sock)
{
//super();
this.sock=sock;
}
#Override
public void run()
{
DataInputStream is ;
DataOutputStream os ;
String name =null ;
try {
is = new DataInputStream(sock.getInputStream()) ;
os = new DataOutputStream(sock.getOutputStream()) ;
ServeurPresence.Action act ;
do {
// read Action
act = ServeurPresence.Action.valueOf(is.readUTF()) ; // read string -> enum
System.out.println("action :"+act);
switch (act) {
case CONNEXION :
name = is.readUTF(); //read client name
System.out.println("Name :"+name);
os.writeUTF("Hi");//send welcome msg
break ;
case MSG :
String msg = is.readUTF();
os.writeUTF("OK");//response
break ;
case DECONNEXION :
System.out.println(name+" is logged out");
break ;
}
} while (act!=ServeurPresence.Action.DECONNEXION) ;
// the end
is.close();
os.close();
sock.close();
} catch (IOException e)
{
System.out.println("Error with "+name+" socket");
e.printStackTrace();
}
}
}
3. Client side
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.net.InetAddress;
import java.net.Socket;
import java.net.UnknownHostException;
public class Client {
/**
*
*/
Client(String name)
{
System.out.println("Start Client...");
try {
Socket sock = new Socket(ServeurPresence.HOSTNAME,ServeurPresence.PORT) ;
DataOutputStream os = new DataOutputStream(sock.getOutputStream()) ;
DataInputStream is = new DataInputStream(sock.getInputStream()) ;
System.out.println("Send "+name+" to server");
// CONNECTION : Action then value
os.writeUTF(ServeurPresence.Action.CONNEXION.name()) ; // send action : write enum -> String
os.writeUTF(name) ; // send the name
//read server welcome msg
String msg = is.readUTF();
System.out.println("Welcome msg: "+msg);
/* Your actions here : see example below */
try
{
Thread.currentThread().sleep(4000);
os.writeUTF(ServeurPresence.Action.MSG.name()) ; // send action : write enum -> String
os.writeUTF("My message here") ; // send msg
Thread.currentThread().sleep(4000);
msg = is.readUTF();//server response message
}
catch (InterruptedException e)
{
e.printStackTrace();
}
/************************************************/
//CLOSE
os.writeUTF(ServeurPresence.Action.DECONNEXION.name()) ; // send action
System.out.println("Log out");
os.close();
sock.close();
}
catch (UnknownHostException e)
{
System.out.println(ServeurPresence.HOSTNAME+ " unknown");
e.printStackTrace();
}
catch (IOException e)
{
System.out.println("Impossible to connect to "+ServeurPresence.HOSTNAME+ ":"+ServeurPresence.PORT);
e.printStackTrace();
}
}
}
4. In your case use readObject()/writeObject() instead of readUTF()/writeUTF() to write your config objects

Try this and let me know how it goes:
while (1==1)
{
StubConfig = (StubConfiguration)mObjectIn.readObject();
Thread.sleep(100); //Saves CPU usage
}

Very late answer, but just for future reference. I have been having problems sending Objects via sockets because the method flush() is not working properly.
I solved this problem just by switching flush() to reset().

Pinging computer by name in Java

I am in the process of writing a simple program that extracts computer names from MySQL Database then stores those names into a String array list (this part works fine). After that I wrote a class and a method that takes a String as a parameter (which will be the computer name) and tries to ping it. Here is the code for that class:
public class Ping
{
public void pingHost(String hostName)
{
try
{
InetAddress inet = InetAddress.getByName(hostName);
boolean status = inet.isReachable(5000);
if (status)
{
System.out.println(inet.getHostName() + " Host Reached\t" + inet.getHostAddress());
}
else
{
System.out.println(inet.getHostName() + " Host Unreachable");
}
}
catch (UnknownHostException e)
{
System.err.println(e.getMessage() + " Can't Reach Host");
}
catch (IOException e)
{
System.err.println(e.getMessage() + " Error in reaching the Host");
}
}
The problem is that I keep getting UnknownHostException thrown for most computers even if I can ping them manually or if I hard code the computer name as the "hostName".
Here is what my main looks like:
public static void main(String[] args) throws InstantiationException, IllegalAccessException, ClassNotFoundException, SQLException
{
ArrayList <String> list = new ArrayList<String>();
MySQLConnect myConnection = new MySQLConnect();
myConnection.importData(list);
Ping pingComputer = new Ping();
pingComputer.pingHost(list.get(87));
}
Right now I'm just trying to experiment with one computer which is throwing UnknownHostException but can be pinged manually. Anyone have any idea why this is happening?
EDIT...
Just to explain this a little bit more. For example in main, if I pass these parameters to pingHost:
pingComputer.pingHost("ROOM-1234");
It pings fine and returns correct host name/address. But list.get(87) returns same host name "ROOM-1234" but throws UnknownHostException? This has got me really confused and not sure why its not working.
EDIT
Wow finally figured it out. Reason ping was working when I was passing the string directly like so "ROOM-1234", was because there were no white spaces and getting is from array like so list.get(87) returned same thing but when I checked charLength, it returned a different value :) So I just ended up using trim to get rid of white spaces and now itworks great.
pingComputer.pingHost(list.get(87).trim());
Thanks for all the suggestions!

Dear Actually the code you are using is to check whether the host is reachable or not.
Use following class to ping windows pc use ping method but for other than windows pc use isreachable.
package com.utils;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.InetAddress;
import java.net.UnknownHostException;
public class Ping {
public Boolean IsReachable(String ipaddress) {
try {
final InetAddress host = InetAddress.getByName(ipaddress);
try {
return host.isReachable(3000);
} catch (IOException e) {
e.printStackTrace();
}
} catch (UnknownHostException e) {
e.printStackTrace();
}
return false;
}
public Boolean ping(String ipaddress) {
Runtime runtime = Runtime.getRuntime();
String cmds = "ping " + ipaddress;
System.out.println(cmds);
Process proc;
try {
proc = runtime.exec(cmds);
proc.getOutputStream().close();
InputStream inputstream = proc.getInputStream();
InputStreamReader inputstreamreader = new InputStreamReader(inputstream);
BufferedReader bufferedreader = new BufferedReader(inputstreamreader);
String line;
while ((line = bufferedreader.readLine()) != null) {
if (line.contains("Reply from " + ipaddress + ":")) {
return true;
}
}
} catch (IOException e) {
e.printStackTrace();
}
return false;
}
}
And use as below code
public static void main(String[] args) {
ArrayList<String> list = new ArrayList<String>();
MySQLConnect myConnection = new MySQLConnect();
myConnection.importData(list);
Ping ping = new Ping();
if (ping.ping(list.get(87)) {
System.out.prinln("Online / Host is reachable");
} else {
System.out.prinln("Offline /Host is unreachable");
}
}
But I would suggest ping by ip address is better than pinging with computer name.