java - How to write only once to file from a thread? -

i want write end of file every time file modified , i'm using code :

public class main {      public static final string directory_to_watch = "d:\\test";      public static void main(string[] args) {         path towatch = paths.get(directory_to_watch);         if (towatch == null) {             throw new unsupportedoperationexception();         }         try {             watchservice mywatcher = towatch.getfilesystem().newwatchservice();             filewatcher filewatcher = new filewatcher(mywatcher);             thread t = new thread(filewatcher, "filewatcher");             t.start();             towatch.register(mywatcher, standardwatcheventkinds.entry_modify);             t.join();         } catch (ioexception e) {             // todo auto-generated catch block             e.printstacktrace();         } catch (interruptedexception e) {             // todo auto-generated catch block             e.printstacktrace();         }      } } 

and thread class :

public class filewatcher implements runnable{     private watchservice mywatcher;     private path towatch;     string content = "dong\n";     int counter = 0;      public filewatcher (watchservice mywatcher, path towatch) {     this.mywatcher = mywatcher;     this.towatch = towatch; }     @override     public void run() {         try {             watchkey key = mywatcher.take();             while (key != null) {                 (watchevent event : key.pollevents()) {                     //system.out.printf("received %s event file: %s\n", event.kind(), event.context());                     //system.out.println(counter);                     mywatcher = null;                     file file = new file(main.directory_to_watch + "\\" + event.context());                     filewriter fw = new filewriter(file.getabsolutefile(), true);                     fw.write(counter + content);                     fw.close();                     counter++;                     mywatcher = towatch.getfilesystem().newwatchservice();                     towatch.register(mywatcher, standardwatcheventkinds.entry_modify); //                  bufferedwriter bwwriter = new bufferedwriter(fw); //                  bwwriter.write(content); //                  bwwriter.close();                 }                 key.reset();                 key = mywatcher.take();             }         } catch (interruptedexception e) {             // todo auto-generated catch block             e.printstacktrace();         }          catch (ioexception e) {             // todo auto-generated catch block             e.printstacktrace();         }      }  } 

i want in file :

acasc 0dong dwqcacesv 1dong terert 2dong 

however, i'm getting this, because writes many times in file:

acasc 0dong 1dong ... 50123dong 

if use system.out.println(counter); works want (prints number of file changes correctly), goes wild on fw.write(counter + content);

your thread's write causing further changes file. self feeding loop.