Data commit

Task/Determine-if-only-one-instance-is-running/Java/determine-if-only-one-instance-is-running.java

@@ -0,0 +1,35 @@
+import java.io.IOException;
+import java.net.InetAddress;
+import java.net.ServerSocket;
+import java.net.UnknownHostException;
+
+public class SingletonApp
+{
+    private static final int PORT = 65000;  // random large port number
+    private static ServerSocket s;
+
+    // static initializer
+    static {
+        try {
+            s = new ServerSocket(PORT, 10, InetAddress.getLocalHost());
+        } catch (UnknownHostException e) {
+            // shouldn't happen for localhost
+        } catch (IOException e) {
+            // port taken, so app is already running
+            System.out.print("Application is already running,");
+            System.out.println(" so terminating this instance.");
+            System.exit(0);
+        }
+    }
+
+    public static void main(String[] args) {
+        System.out.print("OK, only this instance is running");
+        System.out.println(" but will terminate in 10 seconds.");
+        try {
+            Thread.sleep(10000);
+            if (s != null && !s.isClosed()) s.close();
+        } catch (Exception e) {
+            System.err.println(e);
+        }
+    }
+}