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-rest-api-patterns.md
• references/123-kafka-event-driven-patterns.md
• references/123-database-persistence-patterns.md
• references/123-cross-cutting-integration-patterns.md