New Input/Output 2 - Beginning Java 8 Language Features - page 467

Register the Directory with the Watch Service
You need to create a Path object for the directory you want to watch and invoke its register() method to register
it with the watch service. At the time of registration, you need to specify the kinds of events for which you want to
register your directory. The register() method will return a WatchKey object as a registration token.
// Get a Path object for C:\kishori directory to watch
Path dirToWatch = Paths.get("C:\\kishori");
// Register the dirToWatch for create, modify and delete events
WatchKey token = dirToWatch.register(ws, ENTRY_CREATE, ENTRY_MODIFY, ENTRY_DELETE);
You can cancel the registration of a directory with the watch service using the cancel() method of the WatchKey .
When a directory is registered, its WatchKey is said to be in the ready state. You can register multiple directories with a
watch service. Note that the directory must exist at the time of registration.
Retrieve a WatchKey from the Watch Service Queue
When an event occurs on a registered directory, the WatchKey for that registered directory is said to be in the signaled
state and the WatchKey is queued to the watch service. Another event may occur on a registered directory when its
WatchKey is in the signaled state. If an event occurs on a directory while its WatchKey is in the signaled state, the event
is queued to the WatchKey , but the WatchKey itself is not re-queued to the watch service. A WatchKey in the signaled
state remains in this state until its reset() method is called to change its state to the ready state.
You can use the take() or poll() method of the WatchService object to retrieve and remove a signaled and
queued WatchKey . The take() method waits until a WatchKey is available. The poll() method lets you specify a
timeout for the wait. Typically, an infinite loop is used to retrieve a signaled WatchKey .
while(true) {
// Retrieve and remove the next available WatchKey from the watch service
WatchKey key = ws.take();
}
Process the Events
Once you retrieve and remove a WatchKey from the watch service queue, you can retrieve and remove all pending
events for that WatchKey . A WatchKey may have more than one pending events. The pollEvents() method of the
WatchKey retrieves and removes all its pending events. It returns a List of WatchEvent . Each element of the List
represents an event on the WatchKey . Typically, you will need to use the kind() , context() , and count() methods
of the WatchEvent object to know the details of the event. The following snippet of code shows the typical logic for
processing an event:
while(true) {
// Retrieve and remove the next available WatchKey
WatchKey key = ws.take();
// Process all events of the WatchKey
for(WatchEvent<?> event : key.pollEvents()) {
// Process each event here
}
}