Allow CronTrigger to resume from specified timestamp

Includes differentiation between lenient and fixed execution.
Includes default time zone resolution from scheduler-wide Clock.

Closes gh-19475
Closes gh-31948

spring-context/src/main/java/org/springframework/scheduling/annotation/Scheduled.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2023 the original author or authors.
+ * Copyright 2002-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -117,9 +117,9 @@ public @interface Scheduled {
 
 	/**
 	 * A time zone for which the cron expression will be resolved. By default, this
-	 * attribute is the empty String (i.e. the server's local time zone will be used).
+	 * attribute is the empty String (i.e. the scheduler's time zone will be used).
 	 * @return a zone id accepted by {@link java.util.TimeZone#getTimeZone(String)},
-	 * or an empty String to indicate the server's default time zone
+	 * or an empty String to indicate the scheduler's default time zone
 	 * @since 4.0
 	 * @see org.springframework.scheduling.support.CronTrigger#CronTrigger(String, java.util.TimeZone)
 	 * @see java.util.TimeZone

spring-context/src/main/java/org/springframework/scheduling/annotation/ScheduledAnnotationBeanPostProcessor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2023 the original author or authors.
+ * Copyright 2002-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -26,7 +26,6 @@ import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
-import java.util.TimeZone;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.concurrent.ScheduledExecutorService;
@@ -429,14 +428,14 @@ public class ScheduledAnnotationBeanPostProcessor
 					Assert.isTrue(initialDelay.isNegative(), "'initialDelay' not supported for cron triggers");
 					processedSchedule = true;
 					if (!Scheduled.CRON_DISABLED.equals(cron)) {
-						TimeZone timeZone;
+						CronTrigger trigger;
 						if (StringUtils.hasText(zone)) {
-							timeZone = StringUtils.parseTimeZoneString(zone);
+							trigger = new CronTrigger(cron, StringUtils.parseTimeZoneString(zone));
 						}
 						else {
-							timeZone = TimeZone.getDefault();
+							trigger = new CronTrigger(cron);
 						}
-						tasks.add(this.registrar.scheduleCronTask(new CronTask(runnable, new CronTrigger(cron, timeZone))));
+						tasks.add(this.registrar.scheduleCronTask(new CronTask(runnable, trigger)));
 					}
 				}
 			}

spring-context/src/main/java/org/springframework/scheduling/support/CronTrigger.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2023 the original author or authors.
+ * Copyright 2002-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -39,30 +39,45 @@ public class CronTrigger implements Trigger {
 
 	private final CronExpression expression;
 
+	@Nullable
 	private final ZoneId zoneId;
 
 
 	/**
 	 * Build a {@code CronTrigger} from the pattern provided in the default time zone.
+	 * <p>This is equivalent to the {@link CronTrigger#forLenientExecution} factory
+	 * method. Original trigger firings may be skipped if the previous task is still
+	 * running; if this is not desirable, consider {@link CronTrigger#forFixedExecution}.
 	 * @param expression a space-separated list of time fields, following cron
 	 * expression conventions
+	 * @see CronTrigger#forLenientExecution
+	 * @see CronTrigger#forFixedExecution
 	 */
 	public CronTrigger(String expression) {
-		this(expression, ZoneId.systemDefault());
+		this.expression = CronExpression.parse(expression);
+		this.zoneId = null;
 	}
 
 	/**
-	 * Build a {@code CronTrigger} from the pattern provided in the given time zone.
+	 * Build a {@code CronTrigger} from the pattern provided in the given time zone,
+	 * with the same lenient execution as {@link CronTrigger#CronTrigger(String)}.
+	 * <p>Note that such explicit time zone customization is usually not necessary,
+	 * using {@link org.springframework.scheduling.TaskScheduler#getClock()} instead.
 	 * @param expression a space-separated list of time fields, following cron
 	 * expression conventions
 	 * @param timeZone a time zone in which the trigger times will be generated
 	 */
 	public CronTrigger(String expression, TimeZone timeZone) {
-		this(expression, timeZone.toZoneId());
+		this.expression = CronExpression.parse(expression);
+		Assert.notNull(timeZone, "TimeZone must not be null");
+		this.zoneId = timeZone.toZoneId();
 	}
 
 	/**
-	 * Build a {@code CronTrigger} from the pattern provided in the given time zone.
+	 * Build a {@code CronTrigger} from the pattern provided in the given time zone,
+	 * with the same lenient execution as {@link CronTrigger#CronTrigger(String)}.
+	 * <p>Note that such explicit time zone customization is usually not necessary,
+	 * using {@link org.springframework.scheduling.TaskScheduler#getClock()} instead.
 	 * @param expression a space-separated list of time fields, following cron
 	 * expression conventions
 	 * @param zoneId a time zone in which the trigger times will be generated
@@ -70,10 +85,8 @@ public class CronTrigger implements Trigger {
 	 * @see CronExpression#parse(String)
 	 */
 	public CronTrigger(String expression, ZoneId zoneId) {
-		Assert.hasLength(expression, "Expression must not be empty");
-		Assert.notNull(zoneId, "ZoneId must not be null");
-
 		this.expression = CronExpression.parse(expression);
+		Assert.notNull(zoneId, "ZoneId must not be null");
 		this.zoneId = zoneId;
 	}
 
@@ -94,22 +107,32 @@ public class CronTrigger implements Trigger {
 	 */
 	@Override
 	public Instant nextExecution(TriggerContext triggerContext) {
-		Instant instant = triggerContext.lastCompletion();
-		if (instant != null) {
+		Instant timestamp = determineLatestTimestamp(triggerContext);
+		ZoneId zone = (this.zoneId != null ? this.zoneId : triggerContext.getClock().getZone());
+		ZonedDateTime zonedTimestamp = ZonedDateTime.ofInstant(timestamp, zone);
+		ZonedDateTime nextTimestamp = this.expression.next(zonedTimestamp);
+		return (nextTimestamp != null ? nextTimestamp.toInstant() : null);
+	}
+
+	Instant determineLatestTimestamp(TriggerContext triggerContext) {
+		Instant timestamp = triggerContext.lastCompletion();
+		if (timestamp != null) {
 			Instant scheduled = triggerContext.lastScheduledExecution();
-			if (scheduled != null && instant.isBefore(scheduled)) {
+			if (scheduled != null && timestamp.isBefore(scheduled)) {
 				// Previous task apparently executed too early...
 				// Let's simply use the last calculated execution time then,
 				// in order to prevent accidental re-fires in the same second.
-				instant = scheduled;
+				timestamp = scheduled;
 			}
 		}
 		else {
-			instant = triggerContext.getClock().instant();
+			timestamp = determineInitialTimestamp(triggerContext);
 		}
-		ZonedDateTime dateTime = ZonedDateTime.ofInstant(instant, this.zoneId);
-		ZonedDateTime next = this.expression.next(dateTime);
-		return (next != null ? next.toInstant() : null);
+		return timestamp;
+	}
+
+	Instant determineInitialTimestamp(TriggerContext triggerContext) {
+		return triggerContext.getClock().instant();
 	}
 
 
@@ -129,4 +152,99 @@ public class CronTrigger implements Trigger {
 		return this.expression.toString();
 	}
 
+
+	/**
+	 * Create a {@link CronTrigger} for lenient execution, to be rescheduled
+	 * after every task based on the completion time.
+	 * <p>This variant does not make up for missed trigger firings if the
+	 * associated task has taken too long. As a consequence, original trigger
+	 * firings may be skipped if the previous task is still running.
+	 * <p>This is equivalent to the regular {@link CronTrigger} constructor.
+	 * Note that lenient execution is scheduler-dependent: it may skip trigger
+	 * firings with long-running tasks on a thread pool while executing at
+	 * {@link #forFixedExecution}-like precision with new threads per task.
+	 * @param expression a space-separated list of time fields, following cron
+	 * expression conventions
+	 * @since 6.1.3
+	 * @see #resumeLenientExecution
+	 */
+	public static CronTrigger forLenientExecution(String expression) {
+		return new CronTrigger(expression);
+	}
+
+	/**
+	 * Create a {@link CronTrigger} for lenient execution, to be rescheduled
+	 * after every task based on the completion time.
+	 * <p>This variant does not make up for missed trigger firings if the
+	 * associated task has taken too long. As a consequence, original trigger
+	 * firings may be skipped if the previous task is still running.
+	 * @param expression a space-separated list of time fields, following cron
+	 * expression conventions
+	 * @param resumptionTimestamp the timestamp to resume from (the last-known
+	 * completion timestamp), with the new trigger calculated from there and
+	 * possibly immediately firing (but only once, every subsequent calculation
+	 * will start from the completion time of that first resumed trigger)
+	 * @since 6.1.3
+	 * @see #forLenientExecution
+	 */
+	public static CronTrigger resumeLenientExecution(String expression, Instant resumptionTimestamp) {
+		return new CronTrigger(expression) {
+			@Override
+			Instant determineInitialTimestamp(TriggerContext triggerContext) {
+				return resumptionTimestamp;
+			}
+		};
+	}
+
+	/**
+	 * Create a {@link CronTrigger} for fixed execution, to be rescheduled
+	 * after every task based on the last scheduled time.
+	 * <p>This variant makes up for missed trigger firings if the associated task
+	 * has taken too long, scheduling a task for every original trigger firing.
+	 * Such follow-up tasks may execute late but will never be skipped.
+	 * <p>Immediate versus late execution in case of long-running tasks may
+	 * be scheduler-dependent but the guarantee to never skip a task is portable.
+	 * @param expression a space-separated list of time fields, following cron
+	 * expression conventions
+	 * @since 6.1.3
+	 * @see #resumeFixedExecution
+	 */
+	public static CronTrigger forFixedExecution(String expression) {
+		return new CronTrigger(expression) {
+			@Override
+			protected Instant determineLatestTimestamp(TriggerContext triggerContext) {
+				Instant scheduled = triggerContext.lastScheduledExecution();
+				return (scheduled != null ? scheduled : super.determineInitialTimestamp(triggerContext));
+			}
+		};
+	}
+
+	/**
+	 * Create a {@link CronTrigger} for fixed execution, to be rescheduled
+	 * after every task based on the last scheduled time.
+	 * <p>This variant makes up for missed trigger firings if the associated task
+	 * has taken too long, scheduling a task for every original trigger firing.
+	 * Such follow-up tasks may execute late but will never be skipped.
+	 * @param expression a space-separated list of time fields, following cron
+	 * expression conventions
+	 * @param resumptionTimestamp the timestamp to resume from (the last-known
+	 * scheduled timestamp), with every trigger in-between immediately firing
+	 * to make up for every execution that would have happened in the meantime
+	 * @since 6.1.3
+	 * @see #forFixedExecution
+	 */
+	public static CronTrigger resumeFixedExecution(String expression, Instant resumptionTimestamp) {
+		return new CronTrigger(expression) {
+			@Override
+			protected Instant determineLatestTimestamp(TriggerContext triggerContext) {
+				Instant scheduled = triggerContext.lastScheduledExecution();
+				return (scheduled != null ? scheduled : super.determineLatestTimestamp(triggerContext));
+			}
+			@Override
+			Instant determineInitialTimestamp(TriggerContext triggerContext) {
+				return resumptionTimestamp;
+			}
+		};
+	}
+
 }

spring-context/src/test/java/org/springframework/scheduling/concurrent/CronTriggerExecutionTests.java

@@ -0,0 +1,111 @@
+/*
+ * Copyright 2002-2024 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.scheduling.concurrent;
+
+import java.time.Clock;
+import java.util.concurrent.atomic.AtomicInteger;
+
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import org.springframework.core.testfixture.EnabledForTestGroups;
+import org.springframework.scheduling.support.CronTrigger;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.springframework.core.testfixture.TestGroup.LONG_RUNNING;
+
+/**
+ * @author Juergen Hoeller
+ * @since 6.1.3
+ */
+@EnabledForTestGroups(LONG_RUNNING)
+class CronTriggerExecutionTests {
+
+	ThreadPoolTaskScheduler scheduler = new ThreadPoolTaskScheduler();
+
+	AtomicInteger count = new AtomicInteger();
+
+	Runnable quick = count::incrementAndGet;
+
+	Runnable slow = () -> {
+		count.incrementAndGet();
+		try {
+			Thread.sleep(1000);
+		}
+		catch (InterruptedException e) {
+			throw new RuntimeException(e);
+		}
+	};
+
+
+	@BeforeEach
+	void initialize() {
+		scheduler.initialize();
+	}
+
+	@AfterEach
+	void shutdown() {
+		scheduler.shutdown();
+	}
+
+
+	@Test
+	void forLenientExecutionQuick() throws Exception {
+		scheduler.schedule(quick, CronTrigger.forLenientExecution("*/1 * * * * *"));
+		Thread.sleep(2000);
+		assertThat(count.get()).isEqualTo(2);
+	}
+
+	@Test
+	void forLenientExecutionSlow() throws Exception {
+		scheduler.schedule(slow, CronTrigger.forLenientExecution("*/1 * * * * *"));
+		Thread.sleep(2000);
+		assertThat(count.get()).isEqualTo(1);
+	}
+
+	@Test
+	void forFixedExecutionQuick() throws Exception {
+		scheduler.schedule(quick, CronTrigger.forFixedExecution("*/1 * * * * *"));
+		Thread.sleep(2000);
+		assertThat(count.get()).isEqualTo(2);
+	}
+
+	@Test
+	void forFixedExecutionSlow() throws Exception {
+		scheduler.schedule(slow, CronTrigger.forFixedExecution("*/1 * * * * *"));
+		Thread.sleep(2000);
+		assertThat(count.get()).isEqualTo(2);
+	}
+
+	@Test
+	void resumeLenientExecution() throws Exception {
+		scheduler.schedule(quick, CronTrigger.resumeLenientExecution("*/1 * * * * *",
+				Clock.systemDefaultZone().instant().minusSeconds(2)));
+		Thread.sleep(1000);
+		assertThat(count.get()).isEqualTo(2);
+	}
+
+	@Test
+	void resumeFixedExecution() throws Exception {
+		scheduler.schedule(quick, CronTrigger.resumeFixedExecution("*/1 * * * * *",
+				Clock.systemDefaultZone().instant().minusSeconds(2)));
+		Thread.sleep(1000);
+		assertThat(count.get()).isEqualTo(3);
+	}
+
+}