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package javafx.concurrent;
import java.util.Timer;
import java.util.TimerTask;
import javafx.beans.property.BooleanProperty;
import javafx.beans.property.IntegerProperty;
import javafx.beans.property.ObjectProperty;
import javafx.beans.property.ReadOnlyIntegerProperty;
import javafx.beans.property.ReadOnlyIntegerWrapper;
import javafx.beans.property.ReadOnlyObjectProperty;
import javafx.beans.property.ReadOnlyObjectWrapper;
import javafx.beans.property.SimpleBooleanProperty;
import javafx.beans.property.SimpleIntegerProperty;
import javafx.beans.property.SimpleObjectProperty;
import javafx.util.Callback;
import javafx.util.Duration;
/**
* The ScheduledService is a {@link Service} which 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<>(Duration.seconds(1)) {
* 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;
* }
* }
* }
* }
*
*
* 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 {@link #start()}, and re-executing immediately upon successful completion.
*
* For this purposes of this class, any Duration that answers true to {@link javafx.util.Duration#isUnknown()}
* will treat that duration as if it were Duration.ZERO. Likewise, any Duration which answers true
* to {@link javafx.util.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 an backoff strategy
* callback must be prepared to handle these different potential values.
*
* The ScheduledService introduces a new property called {@link #lastValue}. The lastValue is the value that
* was last successfully computed. Because a Service clears its {@code 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.
*
* Implementer Note: The {@link #ready()}, {@link #scheduled()}, {@link #running()}, {@link #succeeded()},
* {@link #cancelled()}, and {@link #failed()} methods are implemented in this class. Subclasses which also
* override these methods must take care to invoke the super implementation.
*
* @param The computed value of the ScheduledService
* @since JavaFX 8.0
*/
public abstract class ScheduledService extends Service {
/**
* 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 {@code service} is null, then Duration.ZERO is returned. If the period is 0 then
* the result of this method will simply be {@code Math.exp(currentFailureCount)}. In all other cases,
* the returned value is the same as {@code period + (period * Math.exp(currentFailureCount))).
*/
public static final Callback, Duration> EXPONENTIAL_BACKOFF_STRATEGY
= new Callback, Duration>() {
@Override public Duration call(ScheduledService service) {
if (service == null) return Duration.ZERO;
final double period = service.getPeriod() == null ? 0 : service.getPeriod().toMillis();
final double x = service.getCurrentFailureCount();
return Duration.millis(period == 0 ? Math.exp(x) : period + (period * Math.exp(x)));
}
};
/**
* 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 {@code service} is null, then Duration.ZERO is returned. If the period is 0 then
* the result of this method will simply be {@code Math.log1p(currentFailureCount)}. In all other cases,
* the returned value is the same as {@code period + (period * Math.log1p(currentFailureCount))).
*/
public static final Callback, Duration> LOGARITHMIC_BACKOFF_STRATEGY
= new Callback, Duration>() {
@Override public Duration call(ScheduledService service) {
if (service == null) return Duration.ZERO;
final double period = service.getPeriod() == null ? 0 : service.getPeriod().toMillis();
final double x = service.getCurrentFailureCount();
return Duration.millis(period == 0 ? Math.log1p(x) : period + (period * Math.log1p(x)));
}
};
/**
* 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 {@code service} is null, then Duration.ZERO is returned. If the period is 0 then
* the result of this method will simply be {@code currentFailureCount}. In all other cases,
* the returned value is the same as {@code period + (period * currentFailureCount).
*/
public static final Callback, Duration> LINEAR_BACKOFF_STRATEGY
= new Callback, Duration>() {
@Override public Duration call(ScheduledService service) {
if (service == null) return Duration.ZERO;
final double period = service.getPeriod() == null ? 0 : service.getPeriod().toMillis();
final double x = service.getCurrentFailureCount();
return Duration.millis(period == 0 ? x : period + (period * x));
}
};
/**
* This Timer is used to schedule the delays for each ScheduledService. A single timer
* ought to be able to easily service thousands of ScheduledService objects.
*/
private static final Timer DELAY_TIMER = new Timer("ScheduledService Delay Timer", true);
/**
* 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 {@link #start()} or {@link #restart()}.
*/
private ObjectProperty delay = new SimpleObjectProperty<>(this, "delay", Duration.ZERO);
public final Duration getDelay() { return delay.get(); }
public final void setDelay(Duration value) { delay.set(value); }
public final ObjectProperty delayProperty() { return delay; }
/**
* 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.
*/
private ObjectProperty period = new SimpleObjectProperty<>(this, "period", Duration.ZERO);
public final Duration getPeriod() { return period.get(); }
public final void setPeriod(Duration value) { period.set(value); }
public final ObjectProperty periodProperty() { return period; }
/**
* 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.
*/
private ObjectProperty,Duration>> backoffStrategy =
new SimpleObjectProperty<>(this, "backoffStrategy", LOGARITHMIC_BACKOFF_STRATEGY);
public final Callback,Duration> getBackoffStrategy() { return backoffStrategy.get(); }
public final void setBackoffStrategy(Callback, Duration> value) { backoffStrategy.set(value); }
public final ObjectProperty,Duration>> backoffStrategyProperty() { return backoffStrategy; }
/**
* Indicates whether the ScheduledService should automatically restart in the case of a failure in the Task.
*/
private BooleanProperty restartOnFailure = new SimpleBooleanProperty(this, "restartOnFailure", true);
public final boolean getRestartOnFailure() { return restartOnFailure.get(); }
public final void setRestartOnFailure(boolean value) { restartOnFailure.set(value); }
public final BooleanProperty restartOnFailureProperty() { return restartOnFailure; }
/**
* 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.
*/
private IntegerProperty maximumFailureCount = new SimpleIntegerProperty(this, "maximumFailureCount", Integer.MAX_VALUE);
public final int getMaximumFailureCount() { return maximumFailureCount.get(); }
public final void setMaximumFailureCount(int value) { maximumFailureCount.set(value); }
public final IntegerProperty maximumFailureCountProperty() { return maximumFailureCount; }
/**
* The current number of times the ScheduledService has failed. This is reset whenever the
* ScheduledService is manually restarted.
*/
private ReadOnlyIntegerWrapper currentFailureCount = new ReadOnlyIntegerWrapper(this, "currentFailureCount", 0);
public final int getCurrentFailureCount() { return currentFailureCount.get(); }
public final ReadOnlyIntegerProperty currentFailureCountProperty() { return currentFailureCount.getReadOnlyProperty(); }
private void setCurrentFailureCount(int value) {
currentFailureCount.set(value);
}
/**
* 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 {@code maximumCumulativePeriod}.
*/
private ReadOnlyObjectWrapper cumulativePeriod = new ReadOnlyObjectWrapper<>(this, "cumulativePeriod", Duration.ZERO);
public final Duration getCumulativePeriod() { return cumulativePeriod.get(); }
public final ReadOnlyObjectProperty cumulativePeriodProperty() { return cumulativePeriod.getReadOnlyProperty(); }
void setCumulativePeriod(Duration value) { // package private for testing
// Make sure any null value is turned into ZERO
Duration newValue = value == null || value.toMillis() < 0 ? Duration.ZERO : value;
// Cap the newValue based on the maximumCumulativePeriod.
Duration maxPeriod = maximumCumulativePeriod.get();
if (maxPeriod != null && !maxPeriod.isUnknown() && !newValue.isUnknown()) {
if (maxPeriod.toMillis() < 0) {
newValue = Duration.ZERO;
} else if (!maxPeriod.isIndefinite() && newValue.greaterThan(maxPeriod)) {
newValue = maxPeriod;
}
}
cumulativePeriod.set(newValue);
}
/**
* 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.
*/
private ObjectProperty maximumCumulativePeriod = new SimpleObjectProperty<>(this, "maximumCumulativePeriod", Duration.INDEFINITE);
public final Duration getMaximumCumulativePeriod() { return maximumCumulativePeriod.get(); }
public final void setMaximumCumulativePeriod(Duration value) { maximumCumulativePeriod.set(value); }
public final ObjectProperty maximumCumulativePeriodProperty() { return maximumCumulativePeriod; }
/**
* 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.
*/
private ReadOnlyObjectWrapper lastValue = new ReadOnlyObjectWrapper<>(this, "lastValue", null);
public final V getLastValue() { return lastValue.get(); }
public final ReadOnlyObjectProperty lastValueProperty() { return lastValue.getReadOnlyProperty(); }
/**
* The timestamp of the last time the task was run. This is used to compute the amount
* of delay between successive iterations by taking the cumulativePeriod into account.
*/
private long lastRunTime = 0L;
/**
* Whether or not this iteration is a "fresh start", such as the initial call to start,
* or a call to restart, or a call to reset followed by a call to start.
*/
private boolean freshStart = true;
/**
* This is a TimerTask scheduled with the DELAY_TIMER. All it does is kick off the execution
* of the actual background Task.
*/
private TimerTask delayTask = null;
// This method is invoked by Service to actually execute the task. In the normal implementation
// in Service, this method will simply delegate to the Executor. In ScheduledService, however,
// we instead will delay the correct amount of time before we finally invoke executeTaskNow,
// which is where we end up delegating to the executor.
@Override protected void executeTask(final Task task) {
assert task != null;
checkThread();
if (freshStart) {
// The delayTask should have concluded and been made null by this point.
// If not, then somehow we were paused waiting for another iteration and
// somebody caused the system to run again. However resetting things should
// have cleared the delayTask.
assert delayTask == null;
// The cumulativePeriod needs to be initialized
setCumulativePeriod(getPeriod());
// Pause for the "delay" amount of time and then execute
final long d = (long) normalize(getDelay());
if (d == 0) {
// If the delay is zero or null, then just start immediately
executeTaskNow(task);
} else {
schedule(createTimerTask(task), d);
}
} else {
// We are executing as a result of an iteration, not a fresh start.
// If the runPeriod (time between the last run and now) exceeds the cumulativePeriod, then
// we need to execute immediately. Otherwise, we will pause until the cumulativePeriod has
// been reached, and then run.
double cumulative = normalize(getCumulativePeriod()); // Can never be null.
double runPeriod = clock() - lastRunTime;
if (runPeriod < cumulative) {
// Pause and then execute
assert delayTask == null;
schedule(createTimerTask(task), (long) (cumulative - runPeriod));
} else {
// Execute immediately
executeTaskNow(task);
}
}
}
/**
* @inheritDoc
*
* Implementation Note: Subclasses which override this method must call this super implementation.
*/
@Override protected void succeeded() {
super.succeeded();
lastValue.set(getValue());
// Reset the cumulative time
Duration d = getPeriod();
setCumulativePeriod(d);
// Call the super implementation of reset, which will not cause us
// to think this is a new fresh start.
ScheduledService.this.superReset();
assert freshStart == false;
// Fire it up!
ScheduledService.this.start();
}
/**
* @inheritDoc
*
* Implementation Note: Subclasses which override this method must call this super implementation.
*/
@Override protected void cancelled() {
super.cancelled();
// Stop the delayTask if it exists
if (delayTask != null) {
delayTask.cancel();
delayTask = null;
}
}
/**
* @inheritDoc
*
* Implementation Note: Subclasses which override this method must call this super implementation.
*/
@Override protected void failed() {
super.failed();
assert delayTask == null;
// Restart as necessary
setCurrentFailureCount(getCurrentFailureCount() + 1);
if (getRestartOnFailure() && getMaximumFailureCount() > getCurrentFailureCount()) {
// We've not yet maxed out the number of failures we can
// encounter, so we're going to iterate
Callback,Duration> func = getBackoffStrategy();
if (func != null) {
Duration d = func.call(this);
setCumulativePeriod(d);
}
ScheduledService.this.superReset();
assert freshStart == false;
ScheduledService.this.start();
} else {
// We've maxed out, so do nothing and things will just stop.
}
}
/**
* @inheritDoc
*
* Implementation Note: Subclasses which override this method must call this super implementation.
*/
@Override public void reset() {
super.reset();
setCumulativePeriod(getPeriod());
lastValue.set(null);
setCurrentFailureCount(0);
lastRunTime = 0L;
freshStart = true;
}
/**
* This method exists only for testing purposes. The normal implementation
* will delegate to a java.util.Timer, however during testing we want to simply
* inspect the value for the delay and execute immediately.
* @param task not null
* @param delay >= 0
*/
void schedule(TimerTask task, long delay) {
DELAY_TIMER.schedule(task, delay);
}
/**
* This method only exists for the sake of testing.
* @return freshStart
*/
boolean isFreshStart() { return freshStart; }
/**
* Gets the time of the current clock. At runtime this is simply getting the results
* of System.currentTimeMillis, however during testing this is hammered so as to return
* a time that works well during testing.
* @return The clock time
*/
long clock() {
return System.currentTimeMillis();
}
/**
* Called by this class when we need to avoid calling this class' implementation of
* reset which has the side effect of resetting the "freshStart", currentFailureCount,
* and other state.
*/
private void superReset() {
super.reset();
}
/**
* Creates the TimerTask used for delaying execution. The delay can either be due to
* the initial delay (if this is a freshStart), or it can be the computed delay in order
* to execute the task on its fixed schedule.
*
* @param task must not be null.
* @return the delay TimerTask.
*/
private TimerTask createTimerTask(final Task task) {
assert task != null;
return new TimerTask() {
@Override public void run() {
Runnable r = new Runnable() {
@Override public void run() {
executeTaskNow(task);
delayTask = null;
}
};
// We must make sure that executeTaskNow is called from the FX thread.
// This must happen on th FX thread because the super implementation of
// executeTask is going to call getExecutor so it can use any user supplied
// executor, and this property can only be read on the FX thread.
if (isFxApplicationThread()) {
r.run();
} else {
runLater(r);
}
}
};
}
/**
* Called when it is time to actually execute the task (any delay has by now been
* accounted for). Essentially this ends up simply calling the super implementation
* of executeTask and doing some bookkeeping.
*
* @param task must not be null
*/
private void executeTaskNow(Task task) {
assert task != null;
lastRunTime = clock();
freshStart = false;
super.executeTask(task);
}
/**
* Normalize our handling of Durations according to the class documentation.
* @param d can be null
* @return a double representing the millis.
*/
private static double normalize(Duration d) {
if (d == null || d.isUnknown()) return 0;
if (d.isIndefinite()) return Double.MAX_VALUE;
return d.toMillis();
}
}