@Beta public abstract class AbstractScheduledService extends Object implements Service
startUp() and shutDown() but while in
the "running" state need to perform a periodic task. Subclasses can implement startUp(),
shutDown() and also a runOneIteration() method that will be executed periodically.
This class uses the ScheduledExecutorService returned from executor() to run
the startUp() and shutDown() methods and also uses that service to schedule the
runOneIteration() that will be executed periodically as specified by its
AbstractScheduledService.Scheduler. When this service is asked to stop via stopAsync() it will cancel the
periodic task (but not interrupt it) and wait for it to stop before running the
shutDown() method.
Subclasses are guaranteed that the life cycle methods (runOneIteration(), startUp() and shutDown()) will never run concurrently. Notably, if any execution of runOneIteration() takes longer than its schedule defines, then subsequent executions may start
late. Also, all life cycle methods are executed with a lock held, so subclasses can safely
modify shared state without additional synchronization necessary for visibility to later
executions of the life cycle methods.
Here is a sketch of a service which crawls a website and uses the scheduling capabilities to rate limit itself.
class CrawlingService extends AbstractScheduledService {
private Set<Uri> visited;
private Queue<Uri> toCrawl;
protected void startUp() throws Exception {
toCrawl = readStartingUris();
}
protected void runOneIteration() throws Exception {
Uri uri = toCrawl.remove();
Collection<Uri> newUris = crawl(uri);
visited.add(uri);
for (Uri newUri : newUris) {
if (!visited.contains(newUri)) { toCrawl.add(newUri); }
}
}
protected void shutDown() throws Exception {
saveUris(toCrawl);
}
protected Scheduler scheduler() {
return Scheduler.newFixedRateSchedule(0, 1, TimeUnit.SECONDS);
}
}
This class uses the life cycle methods to read in a list of starting URIs and save the set of outstanding URIs when shutting down. Also, it takes advantage of the scheduling functionality to rate limit the number of queries we perform.
| Modifier and Type | Class and Description |
|---|---|
static class |
AbstractScheduledService.CustomScheduler
A
AbstractScheduledService.Scheduler that provides a convenient way for the AbstractScheduledService to
use a dynamically changing schedule. |
static class |
AbstractScheduledService.Scheduler
A scheduler defines the policy for how the
AbstractScheduledService should run its
task. |
Service.Listener, Service.State| Modifier | Constructor and Description |
|---|---|
protected |
AbstractScheduledService()
Constructor for use by subclasses.
|
| Modifier and Type | Method and Description |
|---|---|
void |
addListener(Service.Listener listener,
Executor executor)
Registers a
Service.Listener to be executed on the given
executor. |
void |
awaitRunning()
Waits for the
Service to reach the running state. |
void |
awaitRunning(long timeout,
TimeUnit unit)
Waits for the
Service to reach the running state for no
more than the given time. |
void |
awaitTerminated()
Waits for the
Service to reach the terminated state. |
void |
awaitTerminated(long timeout,
TimeUnit unit)
Waits for the
Service to reach a terminal state (either
terminated or failed) for no
more than the given time. |
protected ScheduledExecutorService |
executor()
Returns the
ScheduledExecutorService that will be used to execute the startUp(),
runOneIteration() and shutDown() methods. |
Throwable |
failureCause()
Returns the
Throwable that caused this service to fail. |
boolean |
isRunning()
Returns
true if this service is running. |
protected abstract void |
runOneIteration()
Run one iteration of the scheduled task.
|
protected abstract AbstractScheduledService.Scheduler |
scheduler()
Returns the
AbstractScheduledService.Scheduler object used to configure this service. |
protected String |
serviceName()
Returns the name of this service.
|
protected void |
shutDown()
Stop the service.
|
Service |
startAsync()
If the service state is
Service.State.NEW, this initiates service startup and returns
immediately. |
protected void |
startUp()
Start the service.
|
Service.State |
state()
Returns the lifecycle state of the service.
|
Service |
stopAsync()
|
String |
toString() |
protected AbstractScheduledService()
protected abstract void runOneIteration()
throws Exception
Service.State.FAILED state and this method will no
longer be called.Exceptionprotected void startUp()
throws Exception
By default this method does nothing.
Exceptionprotected void shutDown()
throws Exception
runOneIteration().
By default this method does nothing.
Exceptionprotected abstract AbstractScheduledService.Scheduler scheduler()
AbstractScheduledService.Scheduler object used to configure this service. This method will only be
called once.protected ScheduledExecutorService executor()
ScheduledExecutorService that will be used to execute the startUp(),
runOneIteration() and shutDown() methods. If this method is overridden the
executor will not be shutdown when this
service terminates or
fails. Subclasses may override this method to supply a
custom ScheduledExecutorService instance. This method is guaranteed to only be called
once.
By default this returns a new ScheduledExecutorService with a single thread thread
pool that sets the name of the thread to the service name.
Also, the pool will be shut down when the
service terminates or
fails.
protected String serviceName()
AbstractScheduledService may include the name in
debugging output.public final boolean isRunning()
Servicetrue if this service is running.public final Service.State state()
Servicepublic final void addListener(Service.Listener listener, Executor executor)
ServiceService.Listener to be executed on the given
executor. The listener will have the corresponding transition method called whenever the
service changes state. The listener will not have previous state changes replayed, so it is
suggested that listeners are added before the service starts.
addListener guarantees execution ordering across calls to a given listener but not
across calls to multiple listeners. Specifically, a given listener will have its callbacks
invoked in the same order as the underlying service enters those states. Additionally, at most
one of the listener's callbacks will execute at once. However, multiple listeners' callbacks
may execute concurrently, and listeners may execute in an order different from the one in which
they were registered.
RuntimeExceptions thrown by a listener will be caught and logged. Any exception thrown
during Executor.execute (e.g., a RejectedExecutionException) will be caught and
logged.
addListener in interface Servicelistener - the listener to run when the service changes state is completeexecutor - the executor in which the listeners callback methods will be run. For fast,
lightweight listeners that would be safe to execute in any thread, consider
MoreExecutors.directExecutor().public final Throwable failureCause()
ServiceThrowable that caused this service to fail.failureCause in interface Servicepublic final Service startAsync()
ServiceService.State.NEW, this initiates service startup and returns
immediately. A stopped service may not be restarted.startAsync in interface Servicepublic final Service stopAsync()
Servicepublic final void awaitRunning()
ServiceService to reach the running state.awaitRunning in interface Servicepublic final void awaitRunning(long timeout,
TimeUnit unit)
throws TimeoutException
ServiceService to reach the running state for no
more than the given time.awaitRunning in interface Servicetimeout - the maximum time to waitunit - the time unit of the timeout argumentTimeoutException - if the service has not reached the given state within the deadlinepublic final void awaitTerminated()
ServiceService to reach the terminated state.awaitTerminated in interface Servicepublic final void awaitTerminated(long timeout,
TimeUnit unit)
throws TimeoutException
ServiceService to reach a terminal state (either
terminated or failed) for no
more than the given time.awaitTerminated in interface Servicetimeout - the maximum time to waitunit - the time unit of the timeout argumentTimeoutException - if the service has not reached the given state within the deadline