Package com.google.common.util.concurrent

Class AbstractScheduledService

  • All Implemented Interfaces:
    Service
    @Beta
public abstract class AbstractScheduledService
extends java.lang.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

    • Method Detail

      • toString

        public java.lang.String toString()
        Overrides:
        toString in class java.lang.Object

      • start

        @Deprecated
public final ListenableFuture<Service.State> start()
        Deprecated.
        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

        @Deprecated
public final Service.State startAndWait()
        Deprecated.
        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

        @Deprecated
public final ListenableFuture<Service.State> stop()
        Deprecated.
        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

        @Deprecated
public final Service.State stopAndWait()
        Deprecated.
        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,
                              java.util.concurrent.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 listeners callback methods will be run. For fast, lightweight listeners that would be safe to execute in any thread, consider MoreExecutors.sameThreadExecutor().
        Since:
        13.0

      • failureCause

        public final java.lang.Throwable failureCause()
        Description copied from interface: Service
        Returns the Throwable that caused this service to fail.
        Specified by:
        failureCause in interface Service
        Since:
        14.0

      • startAsync

        public final Service startAsync()
        Description copied from interface: Service
        If the service state is Service.State.NEW, this initiates service startup and returns immediately. A stopped service may not be restarted.
        Specified by:
        startAsync in interface Service
        Returns:
        this
        Since:
        15.0

      • stopAsync

        public final Service stopAsync()
        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:
        stopAsync in interface Service
        Returns:
        this
        Since:
        15.0

      • awaitRunning

        public final void awaitRunning​(long timeout,
                               java.util.concurrent.TimeUnit unit)
                        throws java.util.concurrent.TimeoutException
        Description copied from interface: Service
        Waits for the Service to reach the running state for no more than the given time.
        Specified by:
        awaitRunning in interface Service
        Parameters:
        timeout - the maximum time to wait
        unit - the time unit of the timeout argument
        Throws:
        java.util.concurrent.TimeoutException - if the service has not reached the given state within the deadline
        Since:
        15.0

      • awaitTerminated

        public final void awaitTerminated​(long timeout,
                                  java.util.concurrent.TimeUnit unit)
                           throws java.util.concurrent.TimeoutException
        Description copied from interface: Service
        Waits for the Service to reach a terminal state (either terminated or failed) for no more than the given time.
        Specified by:
        awaitTerminated in interface Service
        Parameters:
        timeout - the maximum time to wait
        unit - the time unit of the timeout argument
        Throws:
        java.util.concurrent.TimeoutException - if the service has not reached the given state within the deadline
        Since:
        15.0