The Spring Boot Journey

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public class RoadmapArchitectureLab {
    // Layer 1: Language Runtime (Java 21 records & domain immutability)
    enum TaskStatus { OPEN, COMPLETED }
    record Task(long id, String title, TaskStatus status) {}

    // Layer 4: Storage Contract (Plain Java interface for persistence boundary)
    interface TaskStore {
        Task findById(long id);
        Task save(Task task);
    }

    // Layer 3: Application Assembly (Explicit constructor dependency wiring)
    static final class TaskApp {
        private final TaskStore store;
        TaskApp(TaskStore store) { this.store = store; }

        Task completeTask(long id) {
            Task current = store.findById(id);
            System.out.println("[Layer 3: Assembly] TaskApp delegating to TaskStore");
            return store.save(new Task(current.id(), current.title(), TaskStatus.COMPLETED));
        }
    }

    // Layer 2: Network / Protocol simulation (Raw request/response mapping)
    static String handleRequest(String method, String path, TaskApp app) {
        System.out.println("[Layer 2: HTTP] Dispatching " + method + " " + path);
        if ("POST".equals(method) && path.startsWith("/tasks/")) {
            long id = Long.parseLong(path.substring("/tasks/".length()).replace("/complete", ""));
            Task completed = app.completeTask(id);
            return "HTTP/1.1 200 OK\r\nContent-Type: application/json\r\n\r\n" +
                          "{\"id\":" + completed.id() + ",\"status\":\"" + completed.status() + "\"}";
        }
        return "HTTP/1.1 404 Not Found\r\n\r\n";
    }

    public static void main(String[] args) {
        TaskStore inMemoryStore = new TaskStore() {
            public Task findById(long id) {
                System.out.println("[Layer 4: Storage] TaskStore found task: " + id + " (Read Module 0)");
                return new Task(id, "Read Module 0", TaskStatus.OPEN);
            }
            public Task save(Task task) { return task; }
        };

        TaskApp app = new TaskApp(inMemoryStore);
        String response = handleRequest("POST", "/tasks/42/complete", app);
        System.out.println("[Outcome]\n" + response.trim());
    }
}