Axon Framework

import org.axonframework.eventsourcing.annotation.EventSourcedEntity;
import org.axonframework.eventsourcing.annotation.EventSourcingHandler;
import org.axonframework.eventsourcing.annotation.EventTag;
import org.axonframework.eventsourcing.annotation.reflection.EntityCreator;
import org.axonframework.messaging.commandhandling.annotation.CommandHandler;
import org.axonframework.messaging.eventhandling.gateway.EventAppender;
import org.axonframework.modelling.annotation.TargetEntityId;

record CreateCourseCommand(String courseId, String title, int capacity) {}
record EnrollStudentCommand(@TargetEntityId String courseId, String studentId) {}  // which course to load
record CourseCreatedEvent(@EventTag String courseId, String title, int capacity) {}
record StudentEnrolledEvent(@EventTag String courseId, String studentId) {}

@EventSourcedEntity(tagKey = "courseId")
public class CourseEntity {
    private String courseId;
    private int capacity;
    private int enrolledCount;

    @CommandHandler
    public static String handle(CreateCourseCommand cmd, EventAppender events) {
        if (cmd.capacity() <= 0) throw new IllegalArgumentException("Capacity must be positive");
        events.append(new CourseCreatedEvent(cmd.courseId(), cmd.title(), cmd.capacity()));
        return cmd.courseId();
    }

    @CommandHandler  // runs on the state rebuilt from this course's events
    public void handle(EnrollStudentCommand cmd, EventAppender events) {
        if (enrolledCount >= capacity) throw new IllegalStateException("Course is full");
        events.append(new StudentEnrolledEvent(courseId, cmd.studentId()));
    }

    @EventSourcingHandler  // only assigns state: it runs again on every replay
    private void on(CourseCreatedEvent e) { courseId = e.courseId(); capacity = e.capacity(); }

    @EventSourcingHandler
    private void on(StudentEnrolledEvent e) { enrolledCount++; }

    @EntityCreator
    protected CourseEntity() {}
}