A Watchable object is registered with a watch service by invoking
its register method, returning a WatchKey
to represent the registration. When an event for an object is detected the
key is signalled, and if not currently signalled, it is queued to
the watch service so that it can be retrieved by consumers that invoke the
poll or take methods to retrieve keys
and process events. Once the events have been processed the consumer
invokes the key's reset method to reset the key which
allows the key to be signalled and re-queued with further events.
Registration with a watch service is cancelled by invoking the key's
cancel method. A key that is queued at the time that
it is cancelled remains in the queue until it is retrieved. Depending on the
object, a key may be cancelled automatically. For example, suppose a
directory is watched and the watch service detects that it has been deleted
or its file system is no longer accessible. When a key is cancelled in this
manner it is signalled and queued, if not currently signalled. To ensure
that the consumer is notified the return value from the reset
method indicates if the key is valid.
A watch service is safe for use by multiple concurrent consumers. To
ensure that only one consumer processes the events for a particular object at
any time then care should be taken to ensure that the key's reset
method is only invoked after its events have been processed. The close method may be invoked at any time to close the service causing
any threads waiting to retrieve keys, to throw ClosedWatchServiceException.
File systems may report events faster than they can be retrieved or
processed and an implementation may impose an unspecified limit on the number
of events that it may accumulate. Where an implementation knowingly
discards events then it arranges for the key's pollEvents method to return an element with an event type of OVERFLOW. This event can be used by the
consumer as a trigger to re-examine the state of the object.
When an event is reported to indicate that a file in a watched directory
has been modified then there is no guarantee that the program (or programs)
that have modified the file have completed. Care should be taken to coordinate
access with other programs that may be updating the file.
The FileChannel class defines methods
to lock regions of a file against access by other programs.
The implementation that observes events from the file system is intended
to map directly on to the native file event notification facility where
available, or to use a primitive mechanism, such as polling, when a native
facility is not available. Consequently, many of the details on how events
are detected, their timeliness, and whether their ordering is preserved are
highly implementation specific. For example, when a file in a watched
directory is modified then it may result in a single ENTRY_MODIFY event in some
implementations but several events in other implementations. Short-lived
files (meaning files that are deleted very quickly after they are created)
may not be detected by primitive implementations that periodically poll the
file system to detect changes.
If a watched file is not located on a local storage device then it is implementation specific if changes to the file can be detected. In particular, it is not required that changes to files carried out on remote systems be detected.
Implementation Note
The JDK's WatchService implementations buffer up to 512 pending
events for each registered watchable object. If this limit is exceeded,
pending events are discarded and the special
OVERFLOW event is queued. This
special event is the trigger to re-examine the state of the object, e.g.
scan a watched directory to get an updated list of the files in the
directory. The limit for the pending events can be changed from its default
with the system property
jdk.nio.file.WatchService.maxEventsPerPoll
set to a value that parses as a positive integer. This may be useful in
environments where there is a high volume of changes and where the impact
of discarded events is high.
FileSystem#newWatchService
| Modifier and Type | Method and Description |
|---|---|
| public void | |
| public WatchKey | Returns: the next watch key, ornullRetrieves and removes the next watch key, or |
| public WatchKey | |
| public WatchKey | Returns: the next watch keyRetrieves and removes next watch key, waiting if none are yet present. |
| close | back to summary |
|---|---|
| public void close() throws IOException Redeclares java. Closes this watch service. If a thread is currently blocked in the After a watch service is closed, any further attempt to invoke
operations upon it will throw
| |
| poll | back to summary |
|---|---|
| public WatchKey poll() Retrieves and removes the next watch key, or
| |
| poll | back to summary |
|---|---|
| public WatchKey poll(long timeout, TimeUnit unit) throws InterruptedException Retrieves and removes the next watch key, waiting if necessary up to the specified wait time if none are yet present.
| |
| take | back to summary |
|---|---|
| public WatchKey take() throws InterruptedException Retrieves and removes next watch key, waiting if none are yet present.
| |