Module javafx.graphics
Package javafx.concurrent

Class ScheduledService<V>

java.lang.Object
javafx.concurrent.Service<V>
javafx.concurrent.ScheduledService<V>
Type Parameters:
V - The computed value of the ScheduledService
All Implemented Interfaces:
Worker<V>, EventTarget
public abstract class ScheduledService<V> extends Service<V>

The ScheduledService is a Service that will automatically restart itself after a successful execution, and under some conditions will restart even in case of failure. A new ScheduledService begins in the READY state, just as a normal Service. After calling start or restart, the ScheduledService will enter the SCHEDULED state for the duration specified by delay.

Once RUNNING, the ScheduledService will execute its Task. On successful completion, the ScheduledService will transition to the SUCCEEDED state, and then to the READY state and back to the SCHEDULED state. The amount of time the ScheduledService will remain in this state depends on the amount of time between the last state transition to RUNNING, and the current time, and the period. In short, the period defines the minimum amount of time from the start of one run and the start of the next. If the previous execution completed before period expires, then the ScheduledService will remain in the SCHEDULED state until the period expires. If on the other hand the execution took longer than the specified period, then the ScheduledService will immediately transition back to RUNNING.

If, while RUNNING, the ScheduledService's Task throws an error or in some other way ends up transitioning to FAILED, then the ScheduledService will either restart or quit, depending on the values for backoffStrategy, restartOnFailure, and maximumFailureCount.

If a failure occurs and restartOnFailure is false, then the ScheduledService will transition to FAILED and will stop. To restart a failed ScheduledService, you must call restart manually.

If a failure occurs and restartOnFailure is true, then the the ScheduledService may restart automatically. First, the result of calling backoffStrategy will become the new cumulativePeriod. In this way, after each failure, you can cause the service to wait a longer and longer period of time before restarting. Once the task completes successfully, the cumulativePeriod is reset to the value of period.

ScheduledService defines static EXPONENTIAL_BACKOFF_STRATEGY and LOGARITHMIC_BACKOFF_STRATEGY implementations, of which LOGARITHMIC_BACKOFF_STRATEGY is the default value for backoffStrategy. After maximumFailureCount is reached, the ScheduledService will transition to FAILED in exactly the same way as if restartOnFailure were false.

If the period or delay is changed while the ScheduledService is running, the new values will be taken into account on the next iteration. For example, if the period is increased, then the next time the ScheduledService enters the SCHEDULED state, the new period will be used. Likewise, if the delay is changed, the new value will be honored on the next restart or reset/start.

The ScheduledService is typically used for use cases that involve polling. For example, you may want to ping a server on a regular basis to see if there are any updates. Such as ScheduledService might be implemented like this: 

 ScheduledService<Document> svc = new ScheduledService<Document>() {
     protected Task<Document> createTask() {
         return new Task<Document>() {
             protected Document call() {
                 // Connect to a Server
                 // Get the XML document
                 // Parse it into a document
                 return document;
             }
         };
     }
 };
 svc.setPeriod(Duration.seconds(1));
This example will ping the remote server every 1 second.

Timing for this class is not absolutely reliable. A very busy event thread might introduce some timing lag into the beginning of the execution of the background Task, so very small values for the period or delay are likely to be inaccurate. A delay or period in the hundreds of milliseconds or larger should be fairly reliable.

The ScheduledService in its default configuration has a default period of 0 and a default delay of 0. This will cause the ScheduledService to execute the task immediately upon Service.start(), and re-executing immediately upon successful completion.

For this purposes of this class, any Duration that answers true to Duration.isUnknown() will treat that duration as if it were Duration.ZERO. Likewise, any Duration which answers true to Duration.isIndefinite() will be treated as if it were a duration of Double.MAX_VALUE milliseconds. Any null Duration is treated as Duration.ZERO. Any custom implementation of a backoff strategy callback must be prepared to handle these different potential values.

The ScheduledService introduces a new property called lastValue. The lastValue is the value that was last successfully computed. Because a Service clears its value property on each run, and because the ScheduledService will reschedule a run immediately after completion (unless it enters the cancelled or failed states), the value property is not overly useful on a ScheduledService. In most cases you will want to instead use the value returned by lastValue.

Implementation Note:
The Service.ready(), Service.scheduled(), Service.running(), succeeded(), Service.cancelled(), and failed() methods are implemented in this class. Subclasses which also override these methods must take care to invoke the super implementation.
Since:
JavaFX 8.0

  • Property Details

  • Field Details

    • EXPONENTIAL_BACKOFF_STRATEGY

      public static final Callback<ScheduledService<?>,Duration> EXPONENTIAL_BACKOFF_STRATEGY
      A Callback implementation for the backoffStrategy property which will exponentially backoff the period between re-executions in the case of a failure. This computation takes the original period and the number of consecutive failures and computes the backoff amount from that information.

      If the service is null, then Duration.ZERO is returned. If the period is 0 then the result of this method will simply be Math.exp(currentFailureCount). In all other cases, the returned value is the same as period + (period * Math.exp(currentFailureCount)).

    • LOGARITHMIC_BACKOFF_STRATEGY

      public static final Callback<ScheduledService<?>,Duration> LOGARITHMIC_BACKOFF_STRATEGY
      A Callback implementation for the backoffStrategy property which will logarithmically backoff the period between re-executions in the case of a failure. This computation takes the original period and the number of consecutive failures and computes the backoff amount from that information.

      If the service is null, then Duration.ZERO is returned. If the period is 0 then the result of this method will simply be Math.log1p(currentFailureCount). In all other cases, the returned value is the same as period + (period * Math.log1p(currentFailureCount)).

    • LINEAR_BACKOFF_STRATEGY

      public static final Callback<ScheduledService<?>,Duration> LINEAR_BACKOFF_STRATEGY
      A Callback implementation for the backoffStrategy property which will linearly backoff the period between re-executions in the case of a failure. This computation takes the original period and the number of consecutive failures and computes the backoff amount from that information.

      If the service is null, then Duration.ZERO is returned. If the period is 0 then the result of this method will simply be currentFailureCount. In all other cases, the returned value is the same as period + (period * currentFailureCount).

  • Constructor Details

    • ScheduledService

      public ScheduledService()
      Constructor for subclasses to call.

  • Method Details

    • getDelay

      public final Duration getDelay()
      Gets the value of the delay property.
      Property description:
      The initial delay between when the ScheduledService is first started, and when it will begin operation. This is the amount of time the ScheduledService will remain in the SCHEDULED state, before entering the RUNNING state, following a fresh invocation of Service.start() or Service.restart().
      Returns:
      the value of the delay property
      See Also:

    • setDelay

      public final void setDelay(Duration value)
      Sets the value of the delay property.
      Property description:
      The initial delay between when the ScheduledService is first started, and when it will begin operation. This is the amount of time the ScheduledService will remain in the SCHEDULED state, before entering the RUNNING state, following a fresh invocation of Service.start() or Service.restart().
      Parameters:
      value - the value for the delay property
      See Also:

    • delayProperty

      public final ObjectProperty<Duration> delayProperty()
      The initial delay between when the ScheduledService is first started, and when it will begin operation. This is the amount of time the ScheduledService will remain in the SCHEDULED state, before entering the RUNNING state, following a fresh invocation of Service.start() or Service.restart().
      Returns:
      the delay property
      See Also:

    • getPeriod

      public final Duration getPeriod()
      Gets the value of the period property.
      Property description:
      The minimum amount of time to allow between the start of the last run and the start of the next run. The actual period (also known as cumulativePeriod) will depend on this property as well as the backoffStrategy and number of failures.
      Returns:
      the value of the period property
      See Also:

    • setPeriod

      public final void setPeriod(Duration value)
      Sets the value of the period property.
      Property description:
      The minimum amount of time to allow between the start of the last run and the start of the next run. The actual period (also known as cumulativePeriod) will depend on this property as well as the backoffStrategy and number of failures.
      Parameters:
      value - the value for the period property
      See Also:

    • periodProperty

      public final ObjectProperty<Duration> periodProperty()
      The minimum amount of time to allow between the start of the last run and the start of the next run. The actual period (also known as cumulativePeriod) will depend on this property as well as the backoffStrategy and number of failures.
      Returns:
      the period property
      See Also:

    • getBackoffStrategy

      public final Callback<ScheduledService<?>,Duration> getBackoffStrategy()
      Gets the value of the backoffStrategy property.
      Property description:
      Computes the amount of time to add to the period on each failure. This cumulative amount is reset whenever the the ScheduledService is manually restarted.
      Returns:
      the value of the backoffStrategy property
      See Also:

    • setBackoffStrategy

      public final void setBackoffStrategy(Callback<ScheduledService<?>,Duration> value)
      Sets the value of the backoffStrategy property.
      Property description:
      Computes the amount of time to add to the period on each failure. This cumulative amount is reset whenever the the ScheduledService is manually restarted.
      Parameters:
      value - the value for the backoffStrategy property
      See Also:

    • backoffStrategyProperty

      public final ObjectProperty<Callback<ScheduledService<?>,Duration>> backoffStrategyProperty()
      Computes the amount of time to add to the period on each failure. This cumulative amount is reset whenever the the ScheduledService is manually restarted.
      Returns:
      the backoffStrategy property
      See Also:

    • getRestartOnFailure

      public final boolean getRestartOnFailure()
      Gets the value of the restartOnFailure property.
      Property description:
      Indicates whether the ScheduledService should automatically restart in the case of a failure in the Task.
      Returns:
      the value of the restartOnFailure property
      See Also:

    • setRestartOnFailure

      public final void setRestartOnFailure(boolean value)
      Sets the value of the restartOnFailure property.
      Property description:
      Indicates whether the ScheduledService should automatically restart in the case of a failure in the Task.
      Parameters:
      value - the value for the restartOnFailure property
      See Also:

    • restartOnFailureProperty

      public final BooleanProperty restartOnFailureProperty()
      Indicates whether the ScheduledService should automatically restart in the case of a failure in the Task.
      Returns:
      the restartOnFailure property
      See Also:

    • getMaximumFailureCount

      public final int getMaximumFailureCount()
      Gets the value of the maximumFailureCount property.
      Property description:
      The maximum number of times the ScheduledService can fail before it simply ends in the FAILED state. You can of course restart the ScheduledService manually, which will cause the current count to be reset.
      Returns:
      the value of the maximumFailureCount property
      See Also:

    • setMaximumFailureCount

      public final void setMaximumFailureCount(int value)
      Sets the value of the maximumFailureCount property.
      Property description:
      The maximum number of times the ScheduledService can fail before it simply ends in the FAILED state. You can of course restart the ScheduledService manually, which will cause the current count to be reset.
      Parameters:
      value - the value for the maximumFailureCount property
      See Also:

    • maximumFailureCountProperty

      public final IntegerProperty maximumFailureCountProperty()
      The maximum number of times the ScheduledService can fail before it simply ends in the FAILED state. You can of course restart the ScheduledService manually, which will cause the current count to be reset.
      Returns:
      the maximumFailureCount property
      See Also:

    • getCurrentFailureCount

      public final int getCurrentFailureCount()
      Gets the value of the currentFailureCount property.
      Property description:
      The current number of times the ScheduledService has failed. This is reset whenever the ScheduledService is manually restarted.
      Returns:
      the value of the currentFailureCount property
      See Also:

    • currentFailureCountProperty

      public final ReadOnlyIntegerProperty currentFailureCountProperty()
      The current number of times the ScheduledService has failed. This is reset whenever the ScheduledService is manually restarted.
      Returns:
      the currentFailureCount property
      See Also:

    • getCumulativePeriod

      public final Duration getCumulativePeriod()
      Gets the value of the cumulativePeriod property.
      Property description:
      The current cumulative period in use between iterations. This will be the same as period, except after a failure, in which case the result of the backoffStrategy will be used as the cumulative period following each failure. This is reset whenever the ScheduledService is manually restarted or an iteration is successful. The cumulativePeriod is modified when the ScheduledService enters the scheduled state. The cumulativePeriod can be capped by setting the maximumCumulativePeriod.
      Returns:
      the value of the cumulativePeriod property
      See Also:

    • cumulativePeriodProperty

      public final ReadOnlyObjectProperty<Duration> cumulativePeriodProperty()
      The current cumulative period in use between iterations. This will be the same as period, except after a failure, in which case the result of the backoffStrategy will be used as the cumulative period following each failure. This is reset whenever the ScheduledService is manually restarted or an iteration is successful. The cumulativePeriod is modified when the ScheduledService enters the scheduled state. The cumulativePeriod can be capped by setting the maximumCumulativePeriod.
      Returns:
      the cumulativePeriod property
      See Also:

    • getMaximumCumulativePeriod

      public final Duration getMaximumCumulativePeriod()
      Gets the value of the maximumCumulativePeriod property.
      Property description:
      The maximum allowed value for the cumulativePeriod. Setting this value will help ensure that in the case of repeated failures the back-off algorithm doesn't end up producing unreasonably large values for cumulative period. The cumulative period is guaranteed not to be any larger than this value. If the maximumCumulativePeriod is negative, then cumulativePeriod will be capped at 0. If maximumCumulativePeriod is NaN or null, then it will not influence the cumulativePeriod.
      Returns:
      the value of the maximumCumulativePeriod property
      See Also:

    • setMaximumCumulativePeriod

      public final void setMaximumCumulativePeriod(Duration value)
      Sets the value of the maximumCumulativePeriod property.
      Property description:
      The maximum allowed value for the cumulativePeriod. Setting this value will help ensure that in the case of repeated failures the back-off algorithm doesn't end up producing unreasonably large values for cumulative period. The cumulative period is guaranteed not to be any larger than this value. If the maximumCumulativePeriod is negative, then cumulativePeriod will be capped at 0. If maximumCumulativePeriod is NaN or null, then it will not influence the cumulativePeriod.
      Parameters:
      value - the value for the maximumCumulativePeriod property
      See Also:

    • maximumCumulativePeriodProperty

      public final ObjectProperty<Duration> maximumCumulativePeriodProperty()
      The maximum allowed value for the cumulativePeriod. Setting this value will help ensure that in the case of repeated failures the back-off algorithm doesn't end up producing unreasonably large values for cumulative period. The cumulative period is guaranteed not to be any larger than this value. If the maximumCumulativePeriod is negative, then cumulativePeriod will be capped at 0. If maximumCumulativePeriod is NaN or null, then it will not influence the cumulativePeriod.
      Returns:
      the maximumCumulativePeriod property
      See Also:

    • getLastValue

      public final V getLastValue()
      Gets the value of the lastValue property.
      Property description:
      The last successfully computed value. During each iteration, the "value" of the ScheduledService will be reset to null, as with any other Service. The "lastValue" however will be set to the most recently successfully computed value, even across iterations. It is reset however whenever you manually call reset or restart.
      Returns:
      the value of the lastValue property
      See Also:

    • lastValueProperty

      public final ReadOnlyObjectProperty<V> lastValueProperty()
      The last successfully computed value. During each iteration, the "value" of the ScheduledService will be reset to null, as with any other Service. The "lastValue" however will be set to the most recently successfully computed value, even across iterations. It is reset however whenever you manually call reset or restart.
      Returns:
      the lastValue property
      See Also:

    • succeeded

      protected void succeeded()
      A protected convenience method for subclasses, called whenever the state of the Service has transitioned to the SUCCEEDED state. This method is invoked after the Service has been fully transitioned to the new state. Implementation Note: Subclasses which override this method must call this super implementation.
      Overrides:
      succeeded in class Service<V>

    • failed

      protected void failed()
      A protected convenience method for subclasses, called whenever the state of the Service has transitioned to the FAILED state. This method is invoked after the Service has been fully transitioned to the new state. Implementation Note: Subclasses which override this method must call this super implementation.
      Overrides:
      failed in class Service<V>

    • reset

      public void reset()
      Resets the Service. May only be called while in one of the finish states, that is, SUCCEEDED, FAILED, or CANCELLED, or when READY. This method should only be called on the FX application thread. Implementation Note: Subclasses which override this method must call this super implementation.
      Overrides:
      reset in class Service<V>

    • cancel

      public boolean cancel()
      Cancels any currently running task and stops this scheduled service, such that no additional iterations will occur.
      Specified by:
      cancel in interface Worker<V>
      Overrides:
      cancel in class Service<V>
      Returns:
      whether any running task was cancelled, false if no task was cancelled. In any case, the ScheduledService will stop iterating.