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/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);
+	}
+
+}