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

Skill Interoperability

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

Source Transparency