File Snooping

File Snooping

August 17, 2014

I recently needed to watch files for changes and had a chance to play around with using the WatchService functionality in JDK 7. As is generally the case with Java, the API requires you to jump through a number of hoops to do anything, but turns out that it’s possible to wrap it up into something reasonable in the end.

The WatchService can be used to watch directories and provides different types of events, such as create, modify, delete, and overflow. The first three are self-explanatory and the last is a special event that’s triggered when an event might have been discarded or lost.

What we’d like to do is create a function called watch that accepts an input path along with event handlers for each of the above events.

To create a watcher we first need to get an instance of a Path. To do that we have to call (-> path (file) (.toURI) (Paths/get)). Next, we can get an instance of the WatchService by calling (.newWatchService (FileSystems/getDefault))

Now that we have a Path and a WatchService, we can register the service with the path to listen for the events we specify.

To handle this, I ended up creating a map with the keys representing the events and the values being the event handling functions.

{:create event-handler
 :modify event-handler}
When the event is triggered we will receive an instance of the WatchEvent. So, the handler functions should accept it as the parameter.
(defn event-handler [event]
  (println (.kind event) (.context event)))
Next, I created a couple of helpers to map the keywords to the appropriate events:
(defn register-events! [dir watch-service opts]
  (.register dir watch-service
    (-> opts
       (select-keys [StandardWatchEventKinds/ENTRY_CREATE

(defn rename-event-keys [opts]
  (rename-keys opts
    {:create StandardWatchEventKinds/ENTRY_CREATE
     :modify StandardWatchEventKinds/ENTRY_MODIFY
     :delete StandardWatchEventKinds/ENTRY_DELETE
     :overflow StandardWatchEventKinds/OVERFLOW}))
The transformed map is now ready for use. The WatchService implements closeable, so we can use the with-open macro to manage it:
(defn watch [path opts]
  (let [dir (-> path (file) (.toURI) (Paths/get))
        opts (rename-event-keys opts)]
    (with-open [watch-service
                (.newWatchService (FileSystems/getDefault))]
      (register-events! dir watch-service opts)
      (watch-loop watch-service opts))))
The watch function will register the events we passed in, open the watch service and then call the watch-loop function to do the actual watching.
(defn watch-loop [watch-service opts]
  (loop []
    (let [k (.take watch-service)]      
      (doseq [event (.pollEvents k)]
        (if-let [handler (get opts (.kind event))]
          (handler event)))
      (when (.reset k) (recur)))))
The watch-loop starts each iteration by calling take on the watch service. This method blocks until it receives an event, the service is closed or it’s interrupted.

Once we receive an event we can look it up in our options map and call the handler for the event. Finally, we need to call reset on the key before we start the next iteration.

Since the take function blocks, we probably want to run it in a thread:

(defn start-watcher! [path opts]
  (doto (Thread. #(watch path opts))
    (.setDaemon true)

The above will start a background watcher thread and return it. The thread is daemonized, so that it doesn’t prevent the application from exiting. Example usage for the above can be to track when files are created or modified in the directory:

(start-watcher! “~/downlads”
  {:create #(println “file created” (-> % (.context) (.toString)))
   :modify #(println “file modified” (-> % (.context) (.toString)))})
That’s all there is to it and the full source for the example can be seen here.


As one of the comments points out, JDK will poll on OS X and the default poll interval is quite large. In order to get around this we can force high sensitivity when we register the WatchService as follows:

(defn register-events! [dir watch-service]
  (.register dir
Tags: clojure

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