@Beta @GwtIncompatible 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()
Returns a string representation of the object.
|
protected AbstractScheduledService()
protected abstract void runOneIteration() throws Exception
Service.State.FAILED
state and this method will no
longer be called.Exception
protected void startUp() throws Exception
By default this method does nothing.
Exception
protected void shutDown() throws Exception
runOneIteration()
.
By default this method does nothing.
Exception
protected 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 String toString()
java.lang.Object
toString
method returns a string that
"textually represents" this object. The result should
be a concise but informative representation that is easy for a
person to read.
It is recommended that all subclasses override this method.
The toString
method for class Object
returns a string consisting of the name of the class of which the
object is an instance, the at-sign character `@
', and
the unsigned hexadecimal representation of the hash code of the
object. In other words, this method returns a string equal to the
value of:
getClass().getName() + '@' + Integer.toHexString(hashCode())
public final boolean isRunning()
Service
true
if this service is running.public final Service.State state()
Service
public final void addListener(Service.Listener listener, Executor executor)
Service
Service.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 Service
listener
- 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()
Service
Throwable
that caused this service to fail.failureCause
in interface Service
@CanIgnoreReturnValue public final Service startAsync()
Service
Service.State.NEW
, this initiates service startup and returns
immediately. A stopped service may not be restarted.startAsync
in interface Service
@CanIgnoreReturnValue public final Service stopAsync()
Service
public final void awaitRunning()
Service
Service
to reach the running state.awaitRunning
in interface Service
public final void awaitRunning(long timeout, TimeUnit unit) throws TimeoutException
Service
Service
to reach the running state for no more
than the given time.awaitRunning
in interface Service
timeout
- 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()
Service
Service
to reach the terminated state.awaitTerminated
in interface Service
public final void awaitTerminated(long timeout, TimeUnit unit) throws TimeoutException
Service
Service
to reach a terminal state (either terminated
or failed
) for no more than the given time.awaitTerminated
in interface Service
timeout
- the maximum time to waitunit
- the time unit of the timeout argumentTimeoutException
- if the service has not reached the given state within the deadlineCopyright © 2010–2019. All rights reserved.