Module java.base
Package java.nio.file

Interface WatchKey

public interface WatchKey
A token representing the registration of a watchable object with a WatchService.

A watch key is created when a watchable object is registered with a watch service. The key remains valid until:

  1. It is cancelled, explicitly, by invoking its cancel method, or
  2. Cancelled implicitly, because the object is no longer accessible, or
  3. By closing the watch service.

A watch key has a state. When initially created the key is said to be ready. When an event is detected then the key is signalled and queued so that it can be retrieved by invoking the watch service's poll or take methods. Once signalled, a key remains in this state until its reset method is invoked to return the key to the ready state. Events detected while the key is in the signalled state are queued but do not cause the key to be re-queued for retrieval from the watch service. Events are retrieved by invoking the key's pollEvents method. This method retrieves and removes all events accumulated for the object. When initially created, a watch key has no pending events. Typically events are retrieved when the key is in the signalled state leading to the following idiom:

    for (;;) {
        // retrieve key
        WatchKey key = watcher.take();

        // process events
        for (WatchEvent<?> event: key.pollEvents()) {

        // reset the key
        boolean valid = key.reset();
        if (!valid) {
            // object no longer registered

Watch keys are safe for use by multiple concurrent threads. Where there are several threads retrieving signalled keys from a watch service then care should be taken to ensure that the reset method is only invoked after the events for the object have been processed. This ensures that one thread is processing the events for an object at any time.