The NIO.2 Watcher

11 Mar

So, I’ve been working on a side project involving the Builder tutorial. It roughly (not entirely, but roughly) works out as a machine-operated interpreter, that is, code altered by machine before being translated. After that it does something even more awesome, but it’s only capable of triggering the compilation, after alteration, through a utility that isn’t as well known as it should be.

The Watcher Utility

As of Java 7, we got the NIO.2 classes. These included Path (which most of you are probably familiar with), Files, FileSystem, asynchronous channels, and a host of other goodies. One of them was the Watch Service API.

What Watch ultimately amounts to is a device that can trigger an event any time an arbitrary subset of data is altered in some way. The easiest possible example is monitoring a directory for changes, but this is, gloriously, not exclusive. In classic Java nomenclature, one might think of it as a sort of PathEventListener, in a way; but it’s capable of a bit more than that particular name implies. It doesn’t have to be associated with Paths, and unlike most listeners, it’s less about monitoring for user-generated interrupts, and more about monitoring for system-wide circumstances, including secondary effects.

Using a Watcher

Watchers keep internal collections of keys, each one associated with a source object. This registration is typically located on the source object, at least directly. The best, and most correct, way to do this is through direct implementation of the Watchable interface. Many JDK classes, such as Path, already implement this. Once implemented, you would use the method:

Watchable.register(WatchService watchService, Kind<?>

This method registers all events, of the specified types, on the provided WatchService object. Every time one of them occurs, the key is flagged as signaled, and in its own time the WatchService will retrieve the data from that key and operate.

Note that a Path can be many things. It could be a path to a directory on your machine, which is of program concern. It could be a path to a printer tray, or a server, or even a kitchen appliance (think polling the status of an automated espresso machine). In this example, I will be showing a manner in which a directory path can register to be watched for alterations.

WatchEvent.Kind interface

This can be thought of, for old school Java programmers, as the class type of a Watch event. Most of the frequently used keys are in java.nio.file.StandardWatchEventKinds, but as an interface, it is fully customizable. They only require two methods to be overridden, that is,, which simply returns a String value representing the type of event; and WatchEvent.Kind.type(), which returns a Class that describes the location context of the event.

WatchEvent.Kind.type() may return null, and it won’t break anything; but after getting a feel for the results of StandardWatchEventKinds, you might consider implementing it. As an example, for ENTRY_CREATE, ENTRY_MODIFY, and ENTRY_DELETE, the context is a relative path between the Path being watched, and the item that has changed. (Knowing that a random item was deleted is of little if any use, without knowing which one.)

Implementing a WatchService

Most of the WatchServices you are likely to use are stock in the JDK. I’m going to start with one of them; in a later blog, I’ll probably create one from scratch, but it really is better to start simple.

For the common case of monitoring a directory, FileSystem.newWatchService() covers everything you need. It is important to get a watcher for the correct type of FileSystem, though; as many of you know, Java is capable, as of version 7, of taking advantage of the numerous file system-specific capabilities. The safest way to do it is through:

WatchService watcher = FileSystem.getDefault().newWatchService();

But there may be many points in which you intend to grab a watcher from a file system of a specific, or even custom, type. This is fine, but be aware of the extra layer of debugging.

Afterward, each path can be registered with the watch service through its Path.register(…) method. Be certain to include every variety of WatchEvent.Kind that you want to watch for. It may be tempting to simply register for every single standard type every time, but I encourage you, as a matter of practice, to consider whether you’re really concerned about each Kind before including it. They do, technically, cost a small amount of system resources; and while it may not be noticeable for small projects, when you’re dealing with massive file hierarchies it can become a concern.

When polling for changes, it is mildly more complicated than it is with Listeners. The watcher must be polled for WatchKey objects. WatchKeys are generated when a watched alteration occurs. They all have a state, which is continuously either ready, meaning the associated Watchable is valid but without events; signaled, meaning that at least one event has occurred and been registered with this WatchKey; and invalid, meaning that it is no longer sensible to consider the associated Watchable a candidate for events.

There’s more than one way to get the next signaled WatchKey, but one of the most efficient methods is WatchService.take(). This will always return a signaled WatchKey. It is a blocking method, so use it with that in mind; if no WatchKeys are yet signaled, it will wait until one is before returning.

Once you have a WatchKey, a secondary loop examines every sequential change that has occurred. (If you’re curious, if a WatchEvent occurs for a WatchKey that is already signaled, it is added to the stack and no other alterations are made; if it occurs while the WatchKey is ready, it initiates the stack and WatchKey is flipped to signaled). This is done via WatchKey.pollEvents(). For each event, you may examine the WatchEvent, and act on it accordingly.

After all is said and done, and the WatchKey has zero events left to parse, call WatchKey.reset(). This attempts to flip the WatchKey back to the ready state; if it fails (if the key is now invalid), the method returns false. This might signal, as an example, that the watched path no longer exists.


Any WachService manager must be running continuously. The antipattern approach is to simply use a while-true block; but in general, it is less hazardous to make it its own thread.

import java.nio.*;
import java.nio.file.*;
import java.nio.file.WatchEvent.Kind;

public class DirectoryWatcher implements Runnable {
    private WatchService watcher;
    private volatile boolean isRunning = false;
    public DirectoryWatcher() throws IOException {
        watcher = FileSystems.getDefault().newWatchService();
     * Begins watching provided path for changes.
     * @param path
     * @throws IOException 
    public void register(Path path) throws IOException {
        //register the provided path with the watch service
        path.register(watcher,    StandardWatchEventKinds.ENTRY_CREATE,

    public void run() {
        isRunning = true;
        while(isRunning) {
            //retrieve the next WatchKey
            try {
                WatchKey key = watcher.take();
                key.pollEvents().stream().forEach(event -> {
                    final Kind<?> kind = event.kind();
                    if(kind != StandardWatchEventKinds.OVERFLOW) {
                        final Path path = ((WatchEvent<Path>)event).context();
                        System.out.println(kind + " event occurred on '" + path + "'");
                if(!key.reset()) {
                    //the key should be valid now; but if it is not,
                    //then the directory was likely deleted.
            } catch (InterruptedException e) {
    public void stop() {
        this.isRunning = false;

Simple enough, yes?

The register(…) method may be a little redundant; however, the run() method is where the meat is. WatchKeys are retrieved with WatchService.take(); afterward, in a parallel stream, each WatchEvent associated with that key is looped through. (When an event is of type OVERFLOW, it usually means that data on the event has been lost; not optimal, but the best course of action here is to continue to the next key.)

In this instance, the event is simply reported to the terminal, but this lambda expression is where you would take arbitrary actions according to the event. It is also possible to use an external iteration to do this, if you need to change values or perform another non-lambda-kosher action.

After all events have been iterated through, WatchKey.reset() is called, and checked. In the event that it returns false, something has happened to our directory, and the thread has become a potential resource leak; so it is shut down automatically. Otherwise, the thread then yields to other threads, and repeats itself.

Here’s a small Main class that I’ve built to use this. A single path parameter will be the directory to monitor; or it will simply watch for $HOME/watchtest.

import java.nio.file.Path;
import java.nio.file.Paths;

public class Main {

    public static void main(String[] args) throws IOException {
        final String location = (args.length > 0) ? args[1] : 
            System.getProperty("user.home") + "/watchtest";
        final Path path = Paths.get(location);
        DirectoryWatcher watcher = new DirectoryWatcher();
        (new Thread(watcher)).start();
        //wait a set amount of time, then stop the program
        try {
        } catch (InterruptedException e) {


Try running it, and making a few changes to your select directory. See what it does.

And That’s It!

The next real question is how to create your own WatchService; which is totally doable. Generally, though, it isn’t necessary. The next time I come back to this subject, I’ll be going over that, possibly starting with WatchKey.Kinds. First, though, I need to get back to the project that I started this for, and I need to continue the Build Tool tutorial, so it might be a bit.

Good coding!

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Posted by on March 11, 2015 in Java, NIO.2, Programming


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