123 Java Design Patterns Agent Skill

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

• 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.

• 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.

• 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.

• 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.

• 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