@Beta public abstract class AbstractScheduledService extends Object implements Service
Base class for services that can implement 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();
}</p>
<p>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); } } }</p>
<p>protected void shutDown() throws Exception { saveUris(toCrawl); }</p>
<p>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()
Constructor for use by subclasses.
protected abstract void runOneIteration()
throws Exception
Run one iteration of the scheduled task. If any invocation of this method throws an exception, the service will transition to the Service.State.FAILED state and this method will no longer be called.
Exceptionprotected void startUp()
throws Exception
Start the service.
By default this method does nothing.
Exceptionprotected void shutDown()
throws Exception
Stop the service. This is guaranteed not to run concurrently with runOneIteration().
By default this method does nothing.
Exceptionprotected abstract AbstractScheduledService.Scheduler scheduler()
Returns the AbstractScheduledService.Scheduler object used to configure this service. This method will only be called once.
protected ScheduledExecutorService executor()
Returns the 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()
Returns the name of this service. AbstractScheduledService may include the name in debugging output.
public final boolean isRunning()
ServiceReturns true if this service is running.
public final Service.State state()
ServiceReturns the lifecycle state of the service.
public final void addListener(Service.Listener listener, Executor executor)
ServiceRegisters a 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 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()
ServiceReturns the Throwable that caused this service to fail.
failureCause in interface Servicepublic final Service startAsync()
ServiceIf the service state is Service.State.NEW, this initiates service startup and returns immediately. A stopped service may not be restarted.
startAsync in interface Servicepublic final Service stopAsync()
ServiceIf the service is starting or running, this initiates service shutdown and returns immediately. If the service is new, it is terminated without having been started nor stopped. If the service has already been stopped, this method returns immediately without taking action.
public final void awaitRunning()
ServiceWaits for the Service to reach the running state.
awaitRunning in interface Servicepublic final void awaitRunning(long timeout,
TimeUnit unit)
throws TimeoutException
ServiceWaits for the Service 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()
ServiceWaits for the Service to reach the terminated state.
awaitTerminated in interface Servicepublic final void awaitTerminated(long timeout,
TimeUnit unit)
throws TimeoutException
ServiceWaits for the Service 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