java-expert

Keyword: java | Platforms: gemini,claude,codex

High-Performance Modern Java Expert Skill - Specialized in Java 21+ and cloud-native architecture.

Core Mandates

• Java 21+ First: Mandate var (LVTI) where readable, Records for DTOs, and Pattern Matching for instanceof and switch.
• Concurrency: Prefer Virtual Threads (@RunOnVirtualThread in Quarkus or Executors.newVirtualThreadPerTaskExecutor()) over traditional Thread Pools for I/O-bound tasks.
• Immutability: Use record, final fields, and unmodifiable collections (List.of, Map.of).
• Functional Style: Leverage Streams, Optional, and Functional Interfaces to reduce boilerplate and side effects.
• Visibility: Default to package-private or private. Only public what must be exposed (API/SPI).

Architectural Patterns

Hexagonal / Clean Architecture

• Domain: Pure Java, zero external dependencies (no Quarkus/Spring in domain classes).
• Ports (Interfaces): Define input (Use Cases) and output (Repositories, Gateways) interfaces.
• Adapters: Infrastructure implementations (Quarkus/Hibernate/REST) reside outside the domain.

Performance & Memory

• Garbage Collection: Favor G1GC for typical workloads or ZGC for ultra-low latency (<1ms).
• Memory Management: Minimize object allocation in hot loops. Use StringBuilder instead of + in loops.
• Profiling: Use JFR (Java Flight Recorder) for production-safe profiling and bottleneck analysis.

Testing Strategy

• JUnit 5 & AssertJ: The industry standard for fluent, readable assertions.
• Mockito: Deep stubbing and spying for complex dependencies.
• ArchUnit: Enforce architectural rules (e.g., "Domain must not depend on Infrastructure").
• Testcontainers: Real-world integration testing for databases (PostgreSQL, MySQL) and brokers (Kafka, Redis).

Bytecode & Classloading

• Jandex: Understand Jandex indexing for Quarkus's fast annotation scanning.
• Bytecode Manipulation: Familiarity with ASM or ByteBuddy for runtime/build-time enhancement.
• ClassLoader Isolation: Debugging issues related to "parent-first" vs "child-first" delegation.

Expert Tips

• Avoid System.out.println(); always use a logger (SLF4J/Logback).
• Prefer Interface-based design for testability.
• Use Sealed Classes to define closed hierarchy types for better exhaustiveness checking in switch.

References

• Java 21 Documentation
• Effective Java (Joshua Bloch)
• Java Flight Recorder (JFR) Guide
• JUnit 5 User Guide

Skill Interoperability

The java-expert ☕ skill provides the foundational language expertise (JDK 21+, Virtual Threads) required by:

• vertx-expert 🌀: Uses modern Java for high-performance reactive programming.
• quarkus-expert ⚡: Leverages the JVM features for the Quarkus framework.