java-expert

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Expert knowledge for Modern Java (21+) development, including Virtual Threads, performance tuning, and idiomatic clean code. Use for deep Java language/logic questions.

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Sourcekinhluan/rules-quarkus-skills

Added on2026-03-15

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SKILL.md Content

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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:

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

java-expert

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SKILL.md Content

java-expert

Core Mandates

Architectural Patterns

Hexagonal / Clean Architecture

Performance & Memory

Testing Strategy

Bytecode & Classloading

Expert Tips

References

Skill Interoperability