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POJava single instance software with socket. issue in closing socket under windows
 

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  1. POJava single instance software with socket. issue in closing socket under windows
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    <p>I need to force my Java application to run with a single instance. I found on <a href="http://www.rbgrn.net/content/43-java-single-application-instance" rel="nofollow noreferrer">this link</a> this very nice piece of code that solve the problem using socket instead of using the file system.</p> <p>here the as i adjusted:</p> <p>package cern.ieplc.controller; import java.net.InetAddress; import java.net.InetSocketAddress; import java.net.ServerSocket; import java.net.Socket; import java.net.UnknownHostException;</p> <p>import org.apache.log4j.Logger;</p> <p>import java.io.BufferedReader; import java.io.IOException; import java.io.InputStreamReader; import java.io.OutputStream;</p> <pre><code>public class ApplicationInstanceManager { public interface ApplicationInstanceListener { public void newInstanceCreated(); } private static final Logger log = Logger.getLogger(CustomLock.class); private static ApplicationInstanceListener subListener; /** Randomly chosen, but static, high socket number */ public static final int SINGLE_INSTANCE_NETWORK_SOCKET = 44331; /** Must end with newline */ public static final String SINGLE_INSTANCE_SHARED_KEY = "$$NewInstance$$\n"; private static ServerSocket socket; /** * Registers this instance of the application. * * @return true if first instance, false if not. */ public static boolean registerInstance() { // returnValueOnError should be true if lenient (allows app to run on network error) or false if strict. boolean returnValueOnError = true; // try to open network socket // if success, listen to socket for new instance message, return true // if unable to open, connect to existing and send new instance message, return false try { socket = new ServerSocket(SINGLE_INSTANCE_NETWORK_SOCKET, 10, InetAddress.getByAddress(new byte[]{127,0,0,1})); socket.setReuseAddress(true);//allows the socket to be bound even though a previous connection is in a timeout state. socket.bind(new InetSocketAddress(SINGLE_INSTANCE_NETWORK_SOCKET)); log.debug("Listening for application instances on socket " + SINGLE_INSTANCE_NETWORK_SOCKET); Thread instanceListenerThread = new Thread(new Runnable() { public void run() { boolean socketClosed = false; while (!socketClosed) { if (socket.isClosed()) { socketClosed = true; } else { try { Socket client = socket.accept(); BufferedReader in = new BufferedReader(new InputStreamReader(client.getInputStream())); String message = in.readLine(); if (SINGLE_INSTANCE_SHARED_KEY.trim().equals(message.trim())) { log.debug("Shared key matched - new application instance found"); fireNewInstance(); } in.close(); client.close(); } catch (IOException e) { socketClosed = true; } } } } }); instanceListenerThread.start(); // listen } catch (UnknownHostException e) { log.error(e.getMessage(), e); return returnValueOnError; } catch (IOException e) { log.debug("Port is already taken. Notifying first instance."); try { Socket clientSocket = new Socket(InetAddress.getByAddress(new byte[]{127,0,0,1}), SINGLE_INSTANCE_NETWORK_SOCKET); OutputStream out = clientSocket.getOutputStream(); out.write(SINGLE_INSTANCE_SHARED_KEY.getBytes()); out.close(); clientSocket.close(); log.debug("Successfully notified first instance."); return false; } catch (UnknownHostException e1) { log.error(e.getMessage(), e); return returnValueOnError; } catch (IOException e1) { log.error("Error connecting to local port for single instance notification"); log.error(e1.getMessage(), e1); return returnValueOnError; } } return true; } public static void setApplicationInstanceListener(ApplicationInstanceListener listener) { subListener = listener; } private static void fireNewInstance() { if (subListener != null) { subListener.newInstanceCreated(); } } public static void closeInstance() { if (socket != null) { try { socket.close(); } catch (IOException e) { log.error("Error while closing the socket"); } } } } </code></pre> <p>I tryed the code and it works really well under Linux. if i close the application (even trying to kill it) the socket is immediatly released and i can launch a new application! Unfortunatelly under windows thinks are not so easy. once the resource is allocated is never released. if i close the software i will not be able to launch it again till i close my section.</p> <p>Any idea about how fix nicelly the code to make it works under windows. I tought i could use a shut down hook to catch at least the normal shutting down. Do not really know instead wat to do in case he process terminates in an unexpected way.</p> <p>Here i attach a print screen done over the SW TCPView that shoes how the port is kept open by java: <img src="https://i.stack.imgur.com/8wtx9.png" alt="enter image description here"></p> <p>I tryed implementing a much simpler version. still the same problem. under windows the resources are not released.</p> <p>Here is the second code:</p> <pre><code>import java.net.ServerSocket; import javax.swing.JOptionPane; import javax.swing.JFrame; import java.io.IOException; import java.net.BindException; class MyApplication{ public static ServerSocket serverSocket; public static void main(String as[]) { try { //creating object of server socket and bind to some port number serverSocket = new ServerSocket(15486); ////do not put common port number like 80 etc. ////Because they are already used by system JFrame jf = new JFrame(); jf.setVisible(true); jf.setSize(200, 200); } catch (BindException exc) { JOptionPane.showMessageDialog(null, "Another instance of this application is already running.", "Error", JOptionPane.ERROR_MESSAGE); System.exit(0); } catch (IOException exc) { JOptionPane.showMessageDialog(null, "Another instance of this application is already running.", "Error", JOptionPane.ERROR_MESSAGE); System.exit(0); } } } </code></pre> <p>There is somethink that does not clse properly. It does not work if i put in the shutdown hook the followin code as well:</p> <p>// shut down server </p> <pre><code>try{ serverSocket.close(); }catch (IOException e) { e.printStackTrace(); } </code></pre> <p>Thanks in advance</p>
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    1. COA word of warning: above code contains a bug. Luckily one that is easy to fix. You should use InetAddress.getByAddress(new byte[]{127, 0, 0, 1}); instead of .getLocalHost(). Reason is simply that that the getLocalHost() methods return value can change if machine gets new IP address from DHCP-server. See .getLocalHost():s documentation for more details.
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    2. COWhat is the issue with using a lock file?
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    3. COwell. beside the fact that i do not really like the fact that in case of crashing the file stays there while socket will be closed, i cannot use it becase i have no write priviledges in my sw. (does not depend on me) . using a socket in the worst case will work after a reboot while the file will have to be somehow removed
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