Added basic components.

pom.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+
+    <groupId>nl.andrewlalis</groupId>
+    <artifactId>simply-scheduled</artifactId>
+    <version>1.0-SNAPSHOT</version>
+
+    <properties>
+        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+        <maven.compiler.source>11</maven.compiler.source>
+        <maven.compiler.target>11</maven.compiler.target>
+        <java.version>11</java.version>
+    </properties>
+
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-surefire-plugin</artifactId>
+                <version>2.22.2</version>
+            </plugin>
+        </plugins>
+    </build>
+
+    <dependencyManagement>
+        <dependencies>
+            <dependency>
+                <groupId>org.junit</groupId>
+                <artifactId>junit-bom</artifactId>
+                <version>5.7.1</version>
+                <type>pom</type>
+                <scope>import</scope>
+            </dependency>
+        </dependencies>
+    </dependencyManagement>
+
+    <dependencies>
+        <!-- https://mvnrepository.com/artifact/org.junit.jupiter/junit-jupiter-api -->
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter</artifactId>
+            <version>5.7.1</version>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+</project>

src/main/java/module-info.java

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+module simply_scheduled {
+	exports nl.andrewlalis.simply_scheduled;
+	exports nl.andrewlalis.simply_scheduled.schedule;
+	opens nl.andrewlalis.simply_scheduled;
+}

src/main/java/nl/andrewlalis/simply_scheduled/BasicScheduler.java

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+package nl.andrewlalis.simply_scheduled;
+
+import nl.andrewlalis.simply_scheduled.schedule.Schedule;
+
+import java.time.Clock;
+import java.time.Instant;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.ScheduledThreadPoolExecutor;
+import java.util.concurrent.TimeUnit;
+
+public class BasicScheduler implements Scheduler {
+	private ScheduledExecutorService executorService;
+	private final Clock clock;
+
+	public BasicScheduler(Clock clock) {
+		this.clock = clock;
+		this.executorService = new ScheduledThreadPoolExecutor(1);
+	}
+
+	public BasicScheduler() {
+		this(Clock.systemDefaultZone());
+	}
+
+	@Override
+	public void addTask(Runnable task, Schedule schedule) {
+		Instant nextExecution = schedule.getNextExecutionTime(this.clock.instant());
+		long diff = nextExecution.toEpochMilli() - System.currentTimeMillis();
+		if (diff < 1) return; // Exit immediately, if the next scheduled execution is in the past.
+		this.executorService.schedule(task, diff, TimeUnit.MILLISECONDS);
+	}
+
+	@Override
+	public void start() {
+	}
+
+	@Override
+	public void stop(boolean force) {
+		if (this.executorService != null) {
+			if (force) {
+				this.executorService.shutdownNow();
+			} else {
+				this.executorService.shutdown();
+			}
+		}
+	}
+}

src/main/java/nl/andrewlalis/simply_scheduled/Scheduler.java

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+package nl.andrewlalis.simply_scheduled;
+
+import nl.andrewlalis.simply_scheduled.schedule.Schedule;
+
+public interface Scheduler {
+	void addTask(Runnable task, Schedule schedule);
+	void start();
+	void stop(boolean force);
+}

src/main/java/nl/andrewlalis/simply_scheduled/schedule/DailySchedule.java

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+package nl.andrewlalis.simply_scheduled.schedule;
+
+import java.time.*;
+
+public class DailySchedule implements Schedule {
+	private final ZoneId zoneId;
+	private final LocalTime time;
+
+	public DailySchedule(LocalTime time) {
+		this(time, ZoneId.systemDefault());
+	}
+
+	public DailySchedule(LocalTime time, ZoneId zoneId) {
+		this.time = time;
+		this.zoneId = zoneId;
+	}
+
+	@Override
+	public Instant getNextExecutionTime(Instant referenceInstant) {
+		ZonedDateTime currentTime = referenceInstant.atZone(this.zoneId);
+		LocalDate currentDay = LocalDate.from(referenceInstant);
+		ZonedDateTime sameDayExecution = currentDay.atTime(this.time).atZone(this.zoneId);
+		if (sameDayExecution.isBefore(currentTime)) {
+			return sameDayExecution.toInstant();
+		}
+		return sameDayExecution.plusDays(1).toInstant();
+	}
+}

src/main/java/nl/andrewlalis/simply_scheduled/schedule/HourlySchedule.java

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+package nl.andrewlalis.simply_scheduled.schedule;
+
+import java.time.Instant;
+import java.time.ZoneId;
+import java.time.ZonedDateTime;
+import java.time.temporal.ChronoUnit;
+
+public class HourlySchedule implements Schedule {
+	private final ZoneId zoneId;
+	private final int minute;
+
+	public HourlySchedule(int minute) {
+		this(minute, ZoneId.systemDefault());
+	}
+
+	public HourlySchedule(int minute, ZoneId zoneId) {
+		if (minute < 0 || minute > 59) {
+			throw new IllegalArgumentException("Minute must be in the range [0, 59].");
+		}
+		this.zoneId = zoneId;
+		this.minute = minute;
+	}
+
+	@Override
+	public Instant getNextExecutionTime(Instant referenceInstant) {
+		ZonedDateTime currentTime = referenceInstant.atZone(this.zoneId);
+		int currentMinute = currentTime.getMinute();
+		if (currentMinute < this.minute) {
+			return currentTime.truncatedTo(ChronoUnit.MINUTES).plusMinutes(this.minute - currentMinute).toInstant();
+		}
+		return currentTime.plusHours(1).plusMinutes(this.minute).truncatedTo(ChronoUnit.MINUTES).toInstant();
+	}
+}

src/main/java/nl/andrewlalis/simply_scheduled/schedule/Schedule.java

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+package nl.andrewlalis.simply_scheduled.schedule;
+
+import java.time.Instant;
+
+public interface Schedule {
+	Instant getNextExecutionTime(Instant referenceInstant);
+}

src/test/java/nl/andrewlalis/simply_scheduled/SchedulerTest.java

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+package nl.andrewlalis.simply_scheduled;
+
+import nl.andrewlalis.simply_scheduled.schedule.HourlySchedule;
+import org.junit.jupiter.api.Test;
+
+import java.time.Clock;
+import java.time.Instant;
+import java.time.LocalTime;
+import java.time.ZoneOffset;
+import java.util.concurrent.atomic.AtomicBoolean;
+
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class SchedulerTest {
+
+	@Test
+	void testSchedule() {
+		Clock clock = Clock.fixed(Instant.now(), ZoneOffset.UTC);
+		Scheduler scheduler = new BasicScheduler(clock);
+		LocalTime time = LocalTime.now();
+		int secondsLeft = 60 - time.getSecond() + 1;
+		AtomicBoolean flag = new AtomicBoolean(false);
+		scheduler.addTask(() -> flag.set(true), new HourlySchedule(time.getMinute() + 1));
+		scheduler.start();
+		System.out.printf("Waiting %d seconds for task to run...", secondsLeft);
+		try {
+			Thread.sleep(secondsLeft * 1000);
+		} catch (InterruptedException e) {
+			e.printStackTrace();
+		}
+		assertTrue(flag.get());
+	}
+}