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.

Answer 1

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:

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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<WatchKey,Path> keys;     private final boolean recursive;     private boolean trace = false;      @SuppressWarnings("unchecked")     static <T> WatchEvent<T> cast(WatchEvent<?> event) {         return (WatchEvent<T>)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("register: %s\n", dir);             } else {                 if (!dir.equals(prev)) {                     System.out.format("update: %s -> %s\n", 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<Path>() {             @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<WatchKey,Path>();         this.recursive = recursive;          if (recursive) {             System.out.format("Scanning %s ...\n", dir);             registerAll(dir);             System.out.println("Done.");         } 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("WatchKey not recognized!!");                 continue;             }              for (WatchEvent<?> 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<Path> ev = cast(event);                 Path name = ev.context();                 Path child = dir.resolve(name);                  // print out event                 System.out.format("%s: %s\n", event.kind().name(), child);                  // if directory is created, and watching recursively, then                 // register it and its sub-directories                 if (recursive && (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("usage: java WatchDir [-r] dir");         System.exit(-1);     }      public static void main(String[] args) throws IOException {         // parse arguments         if (args.length == 0 || args.length > 2)             usage();         boolean recursive = false;         int dirArg = 0;         if (args[0].equals("-r")) {             if (args.length < 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.