File changed listener in Java

Question

I d like to be notified when a file has been changed in the file system  I have found nothing but a thread that polls the lastModified File property and clearly this solution is not optimal

User · Answer

I ve written a log file monitor before  and I found that the impact on system performance of polling the attributes of a single file  a few times a second  is actually very small   Java 7  as part of NIO 2 has added the WatchService API     The WatchService API is designed for applications that need to be notified about file change events

User · Answer

There is a lib called jnotify that wraps inotify on linux and has also support for windows  Never used it and I don t know how good it is  but it s worth a try I d say

User · Answer

Polling the last modified file property is a simple yet effective solution though  Just define a class extending my FileChangedWatcher and implement the onModified   method   import java io File   public abstract class FileChangedWatcher       private File file       public FileChangedWatcher String filePath                file   new File filePath              public void watch   throws InterruptedException               long currentModifiedDate   file lastModified             while  true                        long newModifiedDate   file lastModified                 if  newModifiedDate    currentModifiedDate                                currentModifiedDate   newModifiedDate                  onModified                               Thread sleep 100                        public String getFilePath                 return file getAbsolutePath               protected abstract void onModified

User · Answer

You may also consider the Apache Commons JCI  Java Compiler Interface    Although this API seems to be focused on dynamic compilation of classes  it also includes classes in its API that monitors file changes   Example  http   commons apache org jci usage html

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quot More NIO features quot  has file watch functionality  with implementation dependent upon the underlying OS  Should be in JDK7  Update  Was added to Java SE 7  Chris Janicki offers a link to the relevant Java tutorial

User · Answer

Spring Integration provides a nice mechanism for watching Directories and files  http   static springsource org spring-integration reference htmlsingle  files  Pretty sure it s cross platform  I ve used it on mac  linux  and windows

User · Answer

Spring Integration provides a nice mechanism for watching Directories and files  http   static springsource org spring-integration reference htmlsingle  files  Pretty sure it s cross platform  I ve used it on mac  linux  and windows

User · Answer

If you are willing to part with some money  JNIWrapper is a useful library with a Winpack  you will be able to get file system events on certain files  Unfortunately windows only   See https   www teamdev com jniwrapper   Otherwise  resorting to native code is not always a bad thing especially when the best on offer is a polling mechanism as against a native event   I ve noticed that Java file system operations can be slow on some computers and can easily affect the application s performance if not handled well

User · Answer

quot More NIO features quot  has file watch functionality  with implementation dependent upon the underlying OS  Should be in JDK7  Update  Was added to Java SE 7  Chris Janicki offers a link to the relevant Java tutorial

User · Answer

I use the VFS API from Apache Commons  here is an example of how to monitor a file without much impact in performance   DefaultFileMonitor

User · Answer

There is a lib called jnotify that wraps inotify on linux and has also support for windows  Never used it and I don t know how good it is  but it s worth a try I d say

User · Answer

I run this snippet of code every time I go to read the properties file  only actually reading the file if it has been modified since the last time I read it  Hope this helps someone   private long timeStamp  private File file   private boolean isFileUpdated  File file       this file   file    this timeStamp   file lastModified       if  this timeStamp    timeStamp         this timeStamp   timeStamp        Yes  file is updated     return true          No  file is not updated   return false      Similar approach is used in Log4J FileWatchdog

User · Answer

I run this snippet of code every time I go to read the properties file  only actually reading the file if it has been modified since the last time I read it  Hope this helps someone   private long timeStamp  private File file   private boolean isFileUpdated  File file       this file   file    this timeStamp   file lastModified       if  this timeStamp    timeStamp         this timeStamp   timeStamp        Yes  file is updated     return true          No  file is not updated   return false      Similar approach is used in Log4J FileWatchdog

User · Answer

You can listen file changes using a FileReader  Plz see the example below     File content change listener  private String fname  private Object lck   new Object         public void run         try               BufferedReader br   new BufferedReader  new FileReader  fname              String s          StringBuilder buf   new StringBuilder            while  true                         s   br readLine                if  s    null                                 synchronized  lck                                         lck wait  500                                                else                              System out println   s       s                                       catch  Exception e                 e printStackTrace

User · Answer

You may also consider the Apache Commons JCI  Java Compiler Interface    Although this API seems to be focused on dynamic compilation of classes  it also includes classes in its API that monitors file changes   Example  http   commons apache org jci usage html

User · Answer

I use the VFS API from Apache Commons  here is an example of how to monitor a file without much impact in performance   DefaultFileMonitor

User · Answer

Polling the last modified file property is a simple yet effective solution though  Just define a class extending my FileChangedWatcher and implement the onModified   method   import java io File   public abstract class FileChangedWatcher       private File file       public FileChangedWatcher String filePath                file   new File filePath              public void watch   throws InterruptedException               long currentModifiedDate   file lastModified             while  true                        long newModifiedDate   file lastModified                 if  newModifiedDate    currentModifiedDate                                currentModifiedDate   newModifiedDate                  onModified                               Thread sleep 100                        public String getFilePath                 return file getAbsolutePath               protected abstract void onModified

User · Answer

I ve written a log file monitor before  and I found that the impact on system performance of polling the attributes of a single file  a few times a second  is actually very small   Java 7  as part of NIO 2 has added the WatchService API     The WatchService API is designed for applications that need to be notified about file change events

User · Answer

I use the VFS API from Apache Commons  here is an example of how to monitor a file without much impact in performance   DefaultFileMonitor

User · Answer

There is a commercial cross-desktop library for files and folders watching called JxFileWatcher  It can be downloaded from here  http   www teamdev com jxfilewatcher   Also you can see it in action online   http   www teamdev com jxfilewatcher onlinedemo

User · Answer

quot More NIO features quot  has file watch functionality  with implementation dependent upon the underlying OS  Should be in JDK7  Update  Was added to Java SE 7  Chris Janicki offers a link to the relevant Java tutorial

User · Answer

Since JDK 1 7  the canonical way to have an application be notified of changes to a file is using the WatchService API  The WatchService is event-driven  The official tutorial provides an example        Copyright  c  2008  2010  Oracle and or its affiliates  All rights reserved        Redistribution and use in source and binary forms  with or without    modification  are permitted provided that the following conditions    are met          - Redistributions of source code must retain the above copyright        notice  this list of conditions and the following disclaimer          - Redistributions in binary form must reproduce the above copyright        notice  this list of conditions and the following disclaimer in the        documentation and or other materials provided with the distribution          - Neither the name of Oracle nor the names of its        contributors may be used to endorse or promote products derived        from this software without specific prior written permission        THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS  quot AS    IS quot  AND ANY EXPRESS OR IMPLIED WARRANTIES  INCLUDING  BUT NOT LIMITED TO     THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR    PURPOSE ARE DISCLAIMED   IN NO EVENT SHALL THE COPYRIGHT OWNER OR    CONTRIBUTORS BE LIABLE FOR ANY DIRECT  INDIRECT  INCIDENTAL  SPECIAL     EXEMPLARY  OR CONSEQUENTIAL DAMAGES  INCLUDING  BUT NOT LIMITED TO     PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES  LOSS OF USE  DATA  OR    PROFITS  OR BUSINESS INTERRUPTION  HOWEVER CAUSED AND ON ANY THEORY OF    LIABILITY  WHETHER IN CONTRACT  STRICT LIABILITY  OR TORT  INCLUDING    NEGLIGENCE OR OTHERWISE  ARISING IN ANY WAY OUT OF THE USE OF THIS    SOFTWARE  EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE       import java nio file    import static java nio file StandardWatchEventKinds    import static java nio file LinkOption    import java nio file attribute    import java io    import java util            Example to watch a directory  or tree  for changes to files       public class WatchDir        private final WatchService watcher      private final Map lt WatchKey Path gt  keys      private final boolean recursive      private boolean trace   false        SuppressWarnings  quot unchecked quot       static  lt T gt  WatchEvent lt T gt  cast WatchEvent lt   gt  event            return  WatchEvent lt T gt  event                        Register the given directory with the WatchService             private void register Path dir  throws IOException           WatchKey key   dir register watcher  ENTRY CREATE  ENTRY DELETE  ENTRY MODIFY           if  trace                Path prev   keys get key               if  prev    null                    System out format  quot register   s n quot   dir                 else                   if   dir equals prev                         System out format  quot update   s - gt   s n quot   prev  dir                                                     keys put key  dir                         Register the given directory  and all its sub-directories  with the        WatchService              private void registerAll final Path start  throws IOException              register directory and sub-directories         Files walkFileTree start  new SimpleFileVisitor lt Path gt                   Override             public FileVisitResult preVisitDirectory Path dir  BasicFileAttributes attrs                  throws IOException                               register dir                   return FileVisitResult CONTINUE                                                  Creates a WatchService and registers the given directory             WatchDir Path dir  boolean recursive  throws IOException           this watcher   FileSystems getDefault   newWatchService            this keys   new HashMap lt WatchKey Path gt             this recursive   recursive           if  recursive                System out format  quot Scanning  s     n quot   dir               registerAll dir               System out println  quot Done  quot              else               register dir                         enable trace after initial registration         this trace   true                        Process all events for keys queued to the watcher             void processEvents             for                        wait for key to be signalled             WatchKey key              try                   key   watcher take                  catch  InterruptedException x                    return                             Path dir   keys get key               if  dir    null                    System err println  quot WatchKey not recognized   quot                    continue                             for  WatchEvent lt   gt  event  key pollEvents                      WatchEvent Kind kind   event kind                        TBD - provide example of how OVERFLOW event is handled                 if  kind    OVERFLOW                        continue                                        Context for directory entry event is the file name of entry                 WatchEvent lt Path gt  ev   cast event                   Path name   ev context                    Path child   dir resolve name                       print out event                 System out format  quot  s   s n quot   event kind   name    child                       if directory is created  and watching recursively  then                    register it and its sub-directories                 if  recursive  amp  amp   kind    ENTRY CREATE                         try                           if  Files isDirectory child  NOFOLLOW LINKS                                 registerAll child                                                   catch  IOException x                               ignore to keep sample readbale                                                                       reset key and remove from set if directory no longer accessible             boolean valid   key reset                if   valid                    keys remove key                       all directories are inaccessible                 if  keys isEmpty                          break                                                       static void usage             System err println  quot usage  java WatchDir  -r  dir quot            System exit -1              public static void main String   args  throws IOException              parse arguments         if  args length    0    args length  gt  2              usage            boolean recursive   false          int dirArg   0          if  args 0  equals  quot -r quot                  if  args length  lt  2                  usage                recursive   true              dirArg                          register directory and process its events         Path dir   Paths get args dirArg            new WatchDir dir  recursive  processEvents             For individual files  various solutions exist  such as   https   dzone com articles listening-to-fileevents-with-java-nio  Note that Apache VFS uses a polling algorithm  although it may offer greater functionality  Also note that the API does not offer a way to determine whether a file has been closed

User · Answer

If you are willing to part with some money  JNIWrapper is a useful library with a Winpack  you will be able to get file system events on certain files  Unfortunately windows only   See https   www teamdev com jniwrapper   Otherwise  resorting to native code is not always a bad thing especially when the best on offer is a polling mechanism as against a native event   I ve noticed that Java file system operations can be slow on some computers and can easily affect the application s performance if not handled well

User · Answer

There is a commercial cross-desktop library for files and folders watching called JxFileWatcher  It can be downloaded from here  http   www teamdev com jxfilewatcher   Also you can see it in action online   http   www teamdev com jxfilewatcher onlinedemo

User · Answer

Similar to the other answers  here s how I did it using File  Timer  and TimerTask to let this run as a background thread polling at set intervals   import java io File  import java util Timer  import java util TimerTask   public class FileModifiedWatcher     private static File file    private static int pollingInterval    private static Timer fileWatcher    private static long lastReadTimeStamp   0L     public static boolean init String  file  int  pollingInterval          file    new File  file       pollingInterval    pollingInterval     In seconds      watchFile         return true         private static void watchFile           if   null    fileWatcher               System out println  START           fileWatcher   new Timer           fileWatcher scheduleAtFixedRate new TimerTask                    Override         public void run                        if   file lastModified    gt  lastReadTimeStamp                           System out println  File Modified                           lastReadTimeStamp   System currentTimeMillis                       0  1000   pollingInterval

User · Answer

There is a lib called jnotify that wraps inotify on linux and has also support for windows  Never used it and I don t know how good it is  but it s worth a try I d say

User · Answer

Since JDK 1 7  the canonical way to have an application be notified of changes to a file is using the WatchService API  The WatchService is event-driven  The official tutorial provides an example        Copyright  c  2008  2010  Oracle and or its affiliates  All rights reserved        Redistribution and use in source and binary forms  with or without    modification  are permitted provided that the following conditions    are met          - Redistributions of source code must retain the above copyright        notice  this list of conditions and the following disclaimer          - Redistributions in binary form must reproduce the above copyright        notice  this list of conditions and the following disclaimer in the        documentation and or other materials provided with the distribution          - Neither the name of Oracle nor the names of its        contributors may be used to endorse or promote products derived        from this software without specific prior written permission        THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS  quot AS    IS quot  AND ANY EXPRESS OR IMPLIED WARRANTIES  INCLUDING  BUT NOT LIMITED TO     THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR    PURPOSE ARE DISCLAIMED   IN NO EVENT SHALL THE COPYRIGHT OWNER OR    CONTRIBUTORS BE LIABLE FOR ANY DIRECT  INDIRECT  INCIDENTAL  SPECIAL     EXEMPLARY  OR CONSEQUENTIAL DAMAGES  INCLUDING  BUT NOT LIMITED TO     PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES  LOSS OF USE  DATA  OR    PROFITS  OR BUSINESS INTERRUPTION  HOWEVER CAUSED AND ON ANY THEORY OF    LIABILITY  WHETHER IN CONTRACT  STRICT LIABILITY  OR TORT  INCLUDING    NEGLIGENCE OR OTHERWISE  ARISING IN ANY WAY OUT OF THE USE OF THIS    SOFTWARE  EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE       import java nio file    import static java nio file StandardWatchEventKinds    import static java nio file LinkOption    import java nio file attribute    import java io    import java util            Example to watch a directory  or tree  for changes to files       public class WatchDir        private final WatchService watcher      private final Map lt WatchKey Path gt  keys      private final boolean recursive      private boolean trace   false        SuppressWarnings  quot unchecked quot       static  lt T gt  WatchEvent lt T gt  cast WatchEvent lt   gt  event            return  WatchEvent lt T gt  event                        Register the given directory with the WatchService             private void register Path dir  throws IOException           WatchKey key   dir register watcher  ENTRY CREATE  ENTRY DELETE  ENTRY MODIFY           if  trace                Path prev   keys get key               if  prev    null                    System out format  quot register   s n quot   dir                 else                   if   dir equals prev                         System out format  quot update   s - gt   s n quot   prev  dir                                                     keys put key  dir                         Register the given directory  and all its sub-directories  with the        WatchService              private void registerAll final Path start  throws IOException              register directory and sub-directories         Files walkFileTree start  new SimpleFileVisitor lt Path gt                   Override             public FileVisitResult preVisitDirectory Path dir  BasicFileAttributes attrs                  throws IOException                               register dir                   return FileVisitResult CONTINUE                                                  Creates a WatchService and registers the given directory             WatchDir Path dir  boolean recursive  throws IOException           this watcher   FileSystems getDefault   newWatchService            this keys   new HashMap lt WatchKey Path gt             this recursive   recursive           if  recursive                System out format  quot Scanning  s     n quot   dir               registerAll dir               System out println  quot Done  quot              else               register dir                         enable trace after initial registration         this trace   true                        Process all events for keys queued to the watcher             void processEvents             for                        wait for key to be signalled             WatchKey key              try                   key   watcher take                  catch  InterruptedException x                    return                             Path dir   keys get key               if  dir    null                    System err println  quot WatchKey not recognized   quot                    continue                             for  WatchEvent lt   gt  event  key pollEvents                      WatchEvent Kind kind   event kind                        TBD - provide example of how OVERFLOW event is handled                 if  kind    OVERFLOW                        continue                                        Context for directory entry event is the file name of entry                 WatchEvent lt Path gt  ev   cast event                   Path name   ev context                    Path child   dir resolve name                       print out event                 System out format  quot  s   s n quot   event kind   name    child                       if directory is created  and watching recursively  then                    register it and its sub-directories                 if  recursive  amp  amp   kind    ENTRY CREATE                         try                           if  Files isDirectory child  NOFOLLOW LINKS                                 registerAll child                                                   catch  IOException x                               ignore to keep sample readbale                                                                       reset key and remove from set if directory no longer accessible             boolean valid   key reset                if   valid                    keys remove key                       all directories are inaccessible                 if  keys isEmpty                          break                                                       static void usage             System err println  quot usage  java WatchDir  -r  dir quot            System exit -1              public static void main String   args  throws IOException              parse arguments         if  args length    0    args length  gt  2              usage            boolean recursive   false          int dirArg   0          if  args 0  equals  quot -r quot                  if  args length  lt  2                  usage                recursive   true              dirArg                          register directory and process its events         Path dir   Paths get args dirArg            new WatchDir dir  recursive  processEvents             For individual files  various solutions exist  such as   https   dzone com articles listening-to-fileevents-with-java-nio  Note that Apache VFS uses a polling algorithm  although it may offer greater functionality  Also note that the API does not offer a way to determine whether a file has been closed

User · Answer

There is a lib called jnotify that wraps inotify on linux and has also support for windows  Never used it and I don t know how good it is  but it s worth a try I d say

User · Answer

Since JDK 1 7  the canonical way to have an application be notified of changes to a file is using the WatchService API  The WatchService is event-driven  The official tutorial provides an example        Copyright  c  2008  2010  Oracle and or its affiliates  All rights reserved        Redistribution and use in source and binary forms  with or without    modification  are permitted provided that the following conditions    are met          - Redistributions of source code must retain the above copyright        notice  this list of conditions and the following disclaimer          - Redistributions in binary form must reproduce the above copyright        notice  this list of conditions and the following disclaimer in the        documentation and or other materials provided with the distribution          - Neither the name of Oracle nor the names of its        contributors may be used to endorse or promote products derived        from this software without specific prior written permission        THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS  quot AS    IS quot  AND ANY EXPRESS OR IMPLIED WARRANTIES  INCLUDING  BUT NOT LIMITED TO     THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR    PURPOSE ARE DISCLAIMED   IN NO EVENT SHALL THE COPYRIGHT OWNER OR    CONTRIBUTORS BE LIABLE FOR ANY DIRECT  INDIRECT  INCIDENTAL  SPECIAL     EXEMPLARY  OR CONSEQUENTIAL DAMAGES  INCLUDING  BUT NOT LIMITED TO     PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES  LOSS OF USE  DATA  OR    PROFITS  OR BUSINESS INTERRUPTION  HOWEVER CAUSED AND ON ANY THEORY OF    LIABILITY  WHETHER IN CONTRACT  STRICT LIABILITY  OR TORT  INCLUDING    NEGLIGENCE OR OTHERWISE  ARISING IN ANY WAY OUT OF THE USE OF THIS    SOFTWARE  EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE       import java nio file    import static java nio file StandardWatchEventKinds    import static java nio file LinkOption    import java nio file attribute    import java io    import java util            Example to watch a directory  or tree  for changes to files       public class WatchDir        private final WatchService watcher      private final Map lt WatchKey Path gt  keys      private final boolean recursive      private boolean trace   false        SuppressWarnings  quot unchecked quot       static  lt T gt  WatchEvent lt T gt  cast WatchEvent lt   gt  event            return  WatchEvent lt T gt  event                        Register the given directory with the WatchService             private void register Path dir  throws IOException           WatchKey key   dir register watcher  ENTRY CREATE  ENTRY DELETE  ENTRY MODIFY           if  trace                Path prev   keys get key               if  prev    null                    System out format  quot register   s n quot   dir                 else                   if   dir equals prev                         System out format  quot update   s - gt   s n quot   prev  dir                                                     keys put key  dir                         Register the given directory  and all its sub-directories  with the        WatchService              private void registerAll final Path start  throws IOException              register directory and sub-directories         Files walkFileTree start  new SimpleFileVisitor lt Path gt                   Override             public FileVisitResult preVisitDirectory Path dir  BasicFileAttributes attrs                  throws IOException                               register dir                   return FileVisitResult CONTINUE                                                  Creates a WatchService and registers the given directory             WatchDir Path dir  boolean recursive  throws IOException           this watcher   FileSystems getDefault   newWatchService            this keys   new HashMap lt WatchKey Path gt             this recursive   recursive           if  recursive                System out format  quot Scanning  s     n quot   dir               registerAll dir               System out println  quot Done  quot              else               register dir                         enable trace after initial registration         this trace   true                        Process all events for keys queued to the watcher             void processEvents             for                        wait for key to be signalled             WatchKey key              try                   key   watcher take                  catch  InterruptedException x                    return                             Path dir   keys get key               if  dir    null                    System err println  quot WatchKey not recognized   quot                    continue                             for  WatchEvent lt   gt  event  key pollEvents                      WatchEvent Kind kind   event kind                        TBD - provide example of how OVERFLOW event is handled                 if  kind    OVERFLOW                        continue                                        Context for directory entry event is the file name of entry                 WatchEvent lt Path gt  ev   cast event                   Path name   ev context                    Path child   dir resolve name                       print out event                 System out format  quot  s   s n quot   event kind   name    child                       if directory is created  and watching recursively  then                    register it and its sub-directories                 if  recursive  amp  amp   kind    ENTRY CREATE                         try                           if  Files isDirectory child  NOFOLLOW LINKS                                 registerAll child                                                   catch  IOException x                               ignore to keep sample readbale                                                                       reset key and remove from set if directory no longer accessible             boolean valid   key reset                if   valid                    keys remove key                       all directories are inaccessible                 if  keys isEmpty                          break                                                       static void usage             System err println  quot usage  java WatchDir  -r  dir quot            System exit -1              public static void main String   args  throws IOException              parse arguments         if  args length    0    args length  gt  2              usage            boolean recursive   false          int dirArg   0          if  args 0  equals  quot -r quot                  if  args length  lt  2                  usage                recursive   true              dirArg                          register directory and process its events         Path dir   Paths get args dirArg            new WatchDir dir  recursive  processEvents             For individual files  various solutions exist  such as   https   dzone com articles listening-to-fileevents-with-java-nio  Note that Apache VFS uses a polling algorithm  although it may offer greater functionality  Also note that the API does not offer a way to determine whether a file has been closed

User · Answer

If you are willing to part with some money  JNIWrapper is a useful library with a Winpack  you will be able to get file system events on certain files  Unfortunately windows only   See https   www teamdev com jniwrapper   Otherwise  resorting to native code is not always a bad thing especially when the best on offer is a polling mechanism as against a native event   I ve noticed that Java file system operations can be slow on some computers and can easily affect the application s performance if not handled well

User · Answer

quot More NIO features quot  has file watch functionality  with implementation dependent upon the underlying OS  Should be in JDK7  Update  Was added to Java SE 7  Chris Janicki offers a link to the relevant Java tutorial

User · Answer

If you are willing to part with some money  JNIWrapper is a useful library with a Winpack  you will be able to get file system events on certain files  Unfortunately windows only   See https   www teamdev com jniwrapper   Otherwise  resorting to native code is not always a bad thing especially when the best on offer is a polling mechanism as against a native event   I ve noticed that Java file system operations can be slow on some computers and can easily affect the application s performance if not handled well

User · Answer

Java commons-io has a FileAlterationObserver  it does polling in combination with a FileAlterationMonitor  Similar to commons VFS  The advantag is that it has much less dependencies   edit  Less dependencies is not true  they are optional for VFS  But it uses java File instead of the VFS abstraction layer

User · Answer

I ve written a log file monitor before  and I found that the impact on system performance of polling the attributes of a single file  a few times a second  is actually very small   Java 7  as part of NIO 2 has added the WatchService API     The WatchService API is designed for applications that need to be notified about file change events

User · Answer

Since JDK 1 7  the canonical way to have an application be notified of changes to a file is using the WatchService API  The WatchService is event-driven  The official tutorial provides an example        Copyright  c  2008  2010  Oracle and or its affiliates  All rights reserved        Redistribution and use in source and binary forms  with or without    modification  are permitted provided that the following conditions    are met          - Redistributions of source code must retain the above copyright        notice  this list of conditions and the following disclaimer          - Redistributions in binary form must reproduce the above copyright        notice  this list of conditions and the following disclaimer in the        documentation and or other materials provided with the distribution          - Neither the name of Oracle nor the names of its        contributors may be used to endorse or promote products derived        from this software without specific prior written permission        THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS  quot AS    IS quot  AND ANY EXPRESS OR IMPLIED WARRANTIES  INCLUDING  BUT NOT LIMITED TO     THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR    PURPOSE ARE DISCLAIMED   IN NO EVENT SHALL THE COPYRIGHT OWNER OR    CONTRIBUTORS BE LIABLE FOR ANY DIRECT  INDIRECT  INCIDENTAL  SPECIAL     EXEMPLARY  OR CONSEQUENTIAL DAMAGES  INCLUDING  BUT NOT LIMITED TO     PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES  LOSS OF USE  DATA  OR    PROFITS  OR BUSINESS INTERRUPTION  HOWEVER CAUSED AND ON ANY THEORY OF    LIABILITY  WHETHER IN CONTRACT  STRICT LIABILITY  OR TORT  INCLUDING    NEGLIGENCE OR OTHERWISE  ARISING IN ANY WAY OUT OF THE USE OF THIS    SOFTWARE  EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE       import java nio file    import static java nio file StandardWatchEventKinds    import static java nio file LinkOption    import java nio file attribute    import java io    import java util            Example to watch a directory  or tree  for changes to files       public class WatchDir        private final WatchService watcher      private final Map lt WatchKey Path gt  keys      private final boolean recursive      private boolean trace   false        SuppressWarnings  quot unchecked quot       static  lt T gt  WatchEvent lt T gt  cast WatchEvent lt   gt  event            return  WatchEvent lt T gt  event                        Register the given directory with the WatchService             private void register Path dir  throws IOException           WatchKey key   dir register watcher  ENTRY CREATE  ENTRY DELETE  ENTRY MODIFY           if  trace                Path prev   keys get key               if  prev    null                    System out format  quot register   s n quot   dir                 else                   if   dir equals prev                         System out format  quot update   s - gt   s n quot   prev  dir                                                     keys put key  dir                         Register the given directory  and all its sub-directories  with the        WatchService              private void registerAll final Path start  throws IOException              register directory and sub-directories         Files walkFileTree start  new SimpleFileVisitor lt Path gt                   Override             public FileVisitResult preVisitDirectory Path dir  BasicFileAttributes attrs                  throws IOException                               register dir                   return FileVisitResult CONTINUE                                                  Creates a WatchService and registers the given directory             WatchDir Path dir  boolean recursive  throws IOException           this watcher   FileSystems getDefault   newWatchService            this keys   new HashMap lt WatchKey Path gt             this recursive   recursive           if  recursive                System out format  quot Scanning  s     n quot   dir               registerAll dir               System out println  quot Done  quot              else               register dir                         enable trace after initial registration         this trace   true                        Process all events for keys queued to the watcher             void processEvents             for                        wait for key to be signalled             WatchKey key              try                   key   watcher take                  catch  InterruptedException x                    return                             Path dir   keys get key               if  dir    null                    System err println  quot WatchKey not recognized   quot                    continue                             for  WatchEvent lt   gt  event  key pollEvents                      WatchEvent Kind kind   event kind                        TBD - provide example of how OVERFLOW event is handled                 if  kind    OVERFLOW                        continue                                        Context for directory entry event is the file name of entry                 WatchEvent lt Path gt  ev   cast event                   Path name   ev context                    Path child   dir resolve name                       print out event                 System out format  quot  s   s n quot   event kind   name    child                       if directory is created  and watching recursively  then                    register it and its sub-directories                 if  recursive  amp  amp   kind    ENTRY CREATE                         try                           if  Files isDirectory child  NOFOLLOW LINKS                                 registerAll child                                                   catch  IOException x                               ignore to keep sample readbale                                                                       reset key and remove from set if directory no longer accessible             boolean valid   key reset                if   valid                    keys remove key                       all directories are inaccessible                 if  keys isEmpty                          break                                                       static void usage             System err println  quot usage  java WatchDir  -r  dir quot            System exit -1              public static void main String   args  throws IOException              parse arguments         if  args length    0    args length  gt  2              usage            boolean recursive   false          int dirArg   0          if  args 0  equals  quot -r quot                  if  args length  lt  2                  usage                recursive   true              dirArg                          register directory and process its events         Path dir   Paths get args dirArg            new WatchDir dir  recursive  processEvents             For individual files  various solutions exist  such as   https   dzone com articles listening-to-fileevents-with-java-nio  Note that Apache VFS uses a polling algorithm  although it may offer greater functionality  Also note that the API does not offer a way to determine whether a file has been closed

User · Answer

You can listen file changes using a FileReader  Plz see the example below     File content change listener  private String fname  private Object lck   new Object         public void run         try               BufferedReader br   new BufferedReader  new FileReader  fname              String s          StringBuilder buf   new StringBuilder            while  true                         s   br readLine                if  s    null                                 synchronized  lck                                         lck wait  500                                                else                              System out println   s       s                                       catch  Exception e                 e printStackTrace

User · Answer

I use the VFS API from Apache Commons  here is an example of how to monitor a file without much impact in performance   DefaultFileMonitor

User · Answer

Java commons-io has a FileAlterationObserver  it does polling in combination with a FileAlterationMonitor  Similar to commons VFS  The advantag is that it has much less dependencies   edit  Less dependencies is not true  they are optional for VFS  But it uses java File instead of the VFS abstraction layer

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Similar to the other answers  here s how I did it using File  Timer  and TimerTask to let this run as a background thread polling at set intervals   import java io File  import java util Timer  import java util TimerTask   public class FileModifiedWatcher     private static File file    private static int pollingInterval    private static Timer fileWatcher    private static long lastReadTimeStamp   0L     public static boolean init String  file  int  pollingInterval          file    new File  file       pollingInterval    pollingInterval     In seconds      watchFile         return true         private static void watchFile           if   null    fileWatcher               System out println  START           fileWatcher   new Timer           fileWatcher scheduleAtFixedRate new TimerTask                    Override         public void run                        if   file lastModified    gt  lastReadTimeStamp                           System out println  File Modified                           lastReadTimeStamp   System currentTimeMillis                       0  1000   pollingInterval

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I ve written a log file monitor before  and I found that the impact on system performance of polling the attributes of a single file  a few times a second  is actually very small   Java 7  as part of NIO 2 has added the WatchService API     The WatchService API is designed for applications that need to be notified about file change events

[java] File changed listener in Java

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