Package com.google.common.util.concurrent

Class AbstractScheduledService

  • All Implemented Interfaces:
    Service
    @GwtIncompatible
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.

    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

    • 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

      • toString

        public String toString()
        Description copied from class: java.lang.Object
        Returns a string representation of the object. In general, the 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())
        Overrides:
        toString in class Object
        Returns:
        a string representation of the object.

      • 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.

        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.

        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.directExecutor().
        Since:
        13.0

      • stopAsync

        @CanIgnoreReturnValue
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