@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 stop() or stopAndWait(),
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.
|
ListenableFuture<Service.State> |
start()
Deprecated.
|
Service.State |
startAndWait()
Deprecated.
|
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.
|
ListenableFuture<Service.State> |
stop()
Deprecated.
|
Service.State |
stopAndWait()
Deprecated.
|
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.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 String toString()
java.lang.ObjecttoString 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())
@Deprecated public final ListenableFuture<Service.State> start()
ServiceService.State.NEW, this initiates service startup and returns
immediately. If the service has already been started, this method returns immediately without
taking action. A stopped service may not be restarted.start in interface ServiceFuture.get() will block until the service has finished
starting, and returns one of Service.State.RUNNING, Service.State.STOPPING or
Service.State.TERMINATED. If the service fails to start, Future.get()
will throw an ExecutionException, and the service's state will be
Service.State.FAILED. If it has already finished starting, Future.get()
returns immediately. Cancelling this future has no effect on the service.@Deprecated public final Service.State startAndWait()
Servicestart().get(), this method throws no checked exceptions, and it cannot
be interrupted.startAndWait in interface Servicepublic final boolean isRunning()
Servicetrue if this service is running.public final Service.State state()
Service@Deprecated public final ListenableFuture<Service.State> stop()
Servicestop in interface ServiceFuture.get() will block until the service has finished shutting
down, and either returns Service.State.TERMINATED or throws an
ExecutionException. If it has already finished stopping,
Future.get() returns immediately. Cancelling this future has no effect
on the service.@Deprecated public final Service.State stopAndWait()
ServiceService.State.STARTING, startup will be cancelled. If this is Service.State.NEW, it is
terminated without having been started nor stopped. Unlike calling
stop().get(), this method throws no checked exceptions.stopAndWait in interface 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.
There is no guaranteed ordering of execution of listeners, but any listener added through this method is guaranteed to be called whenever there is a state change.
Exceptions thrown by a listener will be propagated up to the executor. Any exception thrown
during Executor.execute (e.g., a RejectedExecutionException or an exception
thrown by inline execution) 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.sameThreadExecutor().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 deadlineCopyright © 2010-2014. All Rights Reserved.