com.google.common.util.concurrent
Class AbstractScheduledService

java.lang.Object
  extended by com.google.common.util.concurrent.AbstractScheduledService
All Implemented Interfaces:
Service

@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 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.

Usage Example

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.

Since:
11.0
Author:
Luke Sandberg

Nested Class Summary
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.
 
Nested classes/interfaces inherited from interface com.google.common.util.concurrent.Service
Service.Listener, Service.State
 
Constructor Summary
AbstractScheduledService()
           
 
Method Summary
 void addListener(Service.Listener listener, Executor executor)
          Registers a Service.Listener to be executed on the given executor.
protected  ScheduledExecutorService executor()
          Returns the ScheduledExecutorService that will be used to execute the startUp(), runOneIteration() and shutDown() methods.
 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  void shutDown()
          Stop the service.
 ListenableFuture<Service.State> start()
          If the service state is Service.State.NEW, this initiates service startup and returns immediately.
 Service.State startAndWait()
          Initiates service startup (if necessary), returning once the service has finished starting.
protected  void startUp()
          Start the service.
 Service.State state()
          Returns the lifecycle state of the service.
 ListenableFuture<Service.State> stop()
          If the service is starting or running, this initiates service shutdown and returns immediately.
 Service.State stopAndWait()
          Initiates service shutdown (if necessary), returning once the service has finished stopping.
 String toString()
           
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait
 

Constructor Detail

AbstractScheduledService

public AbstractScheduledService()
Method Detail

runOneIteration

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.

Throws:
Exception

startUp

protected void startUp()
                throws Exception
Start the service.

By default this method does nothing.

Throws:
Exception

shutDown

protected void shutDown()
                 throws Exception
Stop the service. This is guaranteed not to run concurrently with runOneIteration().

By default this method does nothing.

Throws:
Exception

scheduler

protected abstract AbstractScheduledService.Scheduler scheduler()
Returns the AbstractScheduledService.Scheduler object used to configure this service. This method will only be called once.


executor

protected ScheduledExecutorService executor()
Returns the ScheduledExecutorService that will be used to execute the startUp(), runOneIteration() and shutDown() methods. The executor will not be ExecutorService.shutdown() when this service stops. Subclasses may override this method to use a custom ScheduledExecutorService instance.

By default this returns a new ScheduledExecutorService with a single thread thread pool. This method will only be called once.


toString

public String toString()
Overrides:
toString in class Object

start

public final ListenableFuture<Service.State> start()
Description copied from interface: Service
If the service state is Service.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.

Specified by:
start in interface Service
Returns:
a future for the startup result, regardless of whether this call initiated startup. Calling Future.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.

startAndWait

public final Service.State startAndWait()
Description copied from interface: Service
Initiates service startup (if necessary), returning once the service has finished starting. Unlike calling start().get(), this method throws no checked exceptions, and it cannot be interrupted.

Specified by:
startAndWait in interface Service
Returns:
the state of the service when startup finished.

isRunning

public final boolean isRunning()
Description copied from interface: Service
Returns true if this service is running.

Specified by:
isRunning in interface Service

state

public final Service.State state()
Description copied from interface: Service
Returns the lifecycle state of the service.

Specified by:
state in interface Service

stop

public final ListenableFuture<Service.State> stop()
Description copied from interface: Service
If 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.

Specified by:
stop in interface Service
Returns:
a future for the shutdown result, regardless of whether this call initiated shutdown. Calling Future.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.

stopAndWait

public final Service.State stopAndWait()
Description copied from interface: Service
Initiates service shutdown (if necessary), returning once the service has finished stopping. If this is Service.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.

Specified by:
stopAndWait in interface Service
Returns:
the state of the service when shutdown finished.

addListener

public final void addListener(Service.Listener listener,
                              Executor executor)
Description copied from interface: Service
Registers 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.

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.

Specified by:
addListener in interface Service
Parameters:
listener - the listener to run when the service changes state is complete
executor - the executor in which the the listeners callback methods will be run. For fast, lightweight listeners that would be safe to execute in any thread, consider MoreExecutors.sameThreadExecutor().


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