123-java-design-patterns

# Java Design and Integration Patterns Guide Java developers in selecting patterns by problem signal, implementation context, and trade-off rather than by pattern name alone. **What is covered in this Skill?** - Classic Java design patterns for application code: creational, structural, and behavioral patterns - REST API patterns for resource design, contracts, idempotency, versioning, and error handling - Kafka and event-driven patterns for event schemas, partitioning, idempotency, retries, outbox, and sagas - Database and persistence patterns for repositories, transactions, aggregates, locking, migrations, and read models - Cross-cutting integration patterns for anti-corruption layers, resilience, observability, and reliable message boundaries **Scope:** Use this skill to explain, review, and implement practical patterns in Java systems. Prefer simple code first; introduce a pattern only when it reduces real complexity, protects a boundary, improves testability, or makes change safer. ## Constraints Pattern guidance must be problem-led, concrete, and safe to apply in Java projects. - **PROBLEM FIRST**: Identify the design pressure before naming or applying a pattern - **NO PATTERN SHOPPING**: Do not add abstractions only because a pattern exists; prefer simple code when variation is not present - **BUILD SAFETY**: For code changes, run `./mvnw compile` or `mvn compile` before refactoring and `./mvnw clean verify` or `mvn clean verify` after changes - **REFERENCE SELECTION**: Read the relevant reference before acting: Java code patterns, REST API patterns, Kafka/event-driven patterns, database/persistence patterns, or cross-cutting integration patterns - **TRADE-OFFS REQUIRED**: Explain the benefit, cost, and when-not-to-use guidance for any recommended pattern ## When to use this skill - Apply Java design patterns - Review Java code for design patterns - Choose REST API patterns - Design Kafka event-driven patterns - Improve database persistence patterns - Add resilient integration patterns ## Workflow 1. **Identify the design pressure** Clarify the concrete problem: object creation, behavior variation, API contract evolution, event delivery, persistence consistency, integration reliability, or another recurring design force. 2. **Select the relevant reference** Read only the matching reference(s): `references/123-java-design-patterns.md`, `references/123-rest-api-patterns.md`, `references/123-kafka-event-driven-patterns.md`, `references/123-database-persistence-patterns.md`, or `references/123-cross-cutting-integration-patterns.md`. 3. **Recommend the smallest useful pattern** Choose the simplest pattern that addresses the design pressure. Show how it changes responsibilities, boundaries, tests, operations, or evolution safety. 4. **Implement or document the pattern** When code changes are requested, make focused Java or configuration changes following the project conventions. When design advice is requested, provide diagrams, examples, request/response shapes, event shapes, or transaction flows as appropriate. 5. **Validate the outcome** Verify that the resulting design remains understandable, testable, and operationally safe. Run the project build for code changes and name any remaining trade-offs. ## Reference For detailed guidance, examples, and constraints, see: - [references/123-java-design-patterns.md](references/123-java-design-patterns.md) - [references/123-rest-api-patterns.md](references/123-rest-api-patterns.md) - [references/123-kafka-event-driven-patterns.md](references/123-kafka-event-driven-patterns.md) - [references/123-database-persistence-patterns.md](references/123-database-persistence-patterns.md) - [references/123-cross-cutting-integration-patterns.md](references/123-cross-cutting-integration-patterns.md)