Got it, let's skip the endless basic CRUD and Hello World repos—here are targeted project examples that check all your boxes: complex business logic, modern architecture, TDD, Spring Boot, ORM integration, design patterns, and clean code.

1. Enterprise Order Management System #

This project focuses on end-to-end order handling with real-world complexity:

• Core Business Logic: Multi-level approval workflows, inventory deduction with race condition safeguards, layered discount calculations (user tier + promotional rules), and order split/merge logic for multi-warehouse fulfillment.
• Architecture: Hexagonal (Ports & Adapters) architecture, strictly separating domain, application, and infrastructure layers to keep business logic framework-agnostic.
• TDD Practices: 100% unit test coverage for domain services, using Mockito to isolate dependencies and AssertJ for readable assertions. Includes BDD-style tests (via Cucumber) to validate end-to-end order flows.
• Tech Stack: Spring Boot + Spring Data JPA (with Hibernate under the hood), Spring Security for role-based access control.
• Design Patterns:
  • Strategy pattern for different discount calculation algorithms
  • Observer pattern to trigger inventory/logistics notifications on order status changes
  • Repository pattern to abstract data access from domain logic
  • Factory pattern for creating specialized order processors (e.g., B2B vs. B2C orders)
• Clean Code: Follows SOLID principles, uses DDD terminology (e.g., OrderAggregate, InventoryDomainService), and avoids framework annotations in the domain layer.

2. Healthcare Appointment Management System #

A system built to handle the nuanced rules of medical scheduling:

• Core Business Logic: Appointment conflict detection (doctor availability + room resources), patient medical record association, insurance claim validation rules, and cascading effects of appointment cancellations/modifications.
• Architecture: Event-Driven Architecture (EDA) with Spring Cloud Stream for domain event propagation, plus CQRS to separate read (appointment lookup) and write (booking) operations.
• TDD Practices: Starts with domain logic tests (e.g., "a doctor cannot have two overlapping appointments") before implementing code. Integration tests validate cross-service workflows, and contract tests ensure consistency between modules.
• Tech Stack: Spring Boot + Hibernate (for complex entity associations), Spring Data Redis for caching high-demand doctor schedules.
• Design Patterns:
  • Specification pattern for validating appointment request constraints
  • Mediator pattern to coordinate interactions between scheduling, patient, and billing modules
  • Value objects (e.g., DateRange, PatientIdentifier) to enforce domain invariants
• Clean Code: Modular package structure, no business logic in controllers/repositories, and static code analysis (SonarQube) enforced to maintain quality.

3. E-Commerce Inventory & Supply Chain System #

Focused on the complex rules of inventory management and supplier coordination:

• Core Business Logic: Inventory reservation/release, automated restocking triggers (based on safety stock + sales forecasts), supplier performance monitoring, and cost calculation for cross-warehouse stock transfers.
• Architecture: Modular monolith (or scalable microservices) with clear boundaries between inventory, supplier, and forecasting modules.
• TDD Practices: Test-driven development for all critical flows—including integration tests for inventory reservation race conditions, and mock tests for external supplier APIs.
• Tech Stack: Spring Boot + Spring Data JPA, Flyway for database version control, and Spring Cloud (if using microservices) for service discovery.
• Design Patterns:
  • State pattern to manage inventory states (available, reserved, locked, out-of-stock)
  • Template method pattern for standardized supplier validation workflows
  • Flyweight pattern to reuse inventory calculation rule engines across modules
• Clean Code: Uses DDD aggregate roots to encapsulate business rules, avoids anemic domain models, and maintains consistent code style with tools like SpotBugs.

Pro Tips for Learning from These Projects #

• Deep Dive into Domain Logic: Focus on how complex business rules are encapsulated in domain services, not just how endpoints are exposed.
• Analyze Test Cases: Look at how TDD is used to define edge cases (e.g., "what happens if a discount exceeds the order total?") and validate business invariants.
• Map Architecture Layers: Trace how requests flow from controllers to domain services, and how infrastructure (like databases) is abstracted away from core logic.
• Study Code Organization: Notice how packages are structured to enforce separation of concerns, making the codebase easy to extend and maintain.

内容的提问来源于stack exchange，提问作者oujia