EJB 3.1 and NIO2: Monitoring the file system

Question

I guess most of us agree, that NIO2 is a fine thing to make use of. Presumed you want to monitor some part of the file system for incoming xml - files it is an easy task now. Bu

Answer 1

You could create a singleton that loads at startup and delegates the monitoring to an Asynchronous bean

@Singleton
@Startup
public class Initialiser {

    @EJB
    private FileSystemMonitor fileSystemMonitor;

    @PostConstruct
    public void init() {
        String fileSystemPath = ....;
        fileSystemMonitor.poll(fileSystemPath);
    }

}

Then the Asynchronous bean looks something like this

@Stateless
public class FileSystemMonitor {

    @Asynchronous
    public void poll(String fileSystemPath) {
        WatchService watcher = ....;
        for (;;) {
            WatchKey key = null;
            try {
                key = watcher.take();
                for (WatchEvent<?> event: key.pollEvents()) {
                    WatchEvent.Kind<?> kind = event.kind();
                    if (kind == StandardWatchEventKinds.OVERFLOW) {
                        continue; // If events are lost or discarded
                    }
                    WatchEvent<Path> watchEvent = (WatchEvent<Path>)event;

                    //Process files....

                }
            } catch (InterruptedException e) {
                e.printStackTrace();
                return;
            } finally {
                if (key != null) {
                    boolean valid = key.reset();
                    if (!valid) break; // If the key is no longer valid, the directory is inaccessible so exit the loop.
                }
            }
        }
    }

}

Answer 2

Might help if you specified what server you're using, but have you considered implementing a JMX based service ? It's a bit more "neutral" than EJB, is more appropriate for a background service and has fewer restrictions.