java-expert

java-expert

Keyword: java | Platforms: gemini,claude,codex

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

Core Mandates

• Google Java Style: Strictly adhere to the Google Java Style Guide for formatting, naming, and structure.
• 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).

Virtual Threads (Project Loom)

When to Use Virtual Threads

Is the task I/O-bound (DB queries, HTTP calls, file reads)?
  YES → Virtual Threads (scales to millions of concurrent tasks)
  NO → Is it CPU-bound (crypto, compression, ML inference)?
    YES → Platform Threads with ForkJoinPool (work-stealing)
    NO → Is it reactive streaming with backpressure?
      YES → Mutiny / Vert.x (Reactive Streams)
      NO → Virtual Threads (default choice for most workloads)

Patterns

// PATTERN 1: Simple virtual thread executor
try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
    List<Future<String>> futures = urls.stream()
        .map(url -> executor.submit(() -> fetchUrl(url)))
        .toList();

    List<String> results = futures.stream()
        .map(f -> { try { return f.get(); } catch (Exception e) { throw new RuntimeException(e); } })
        .toList();
}

// PATTERN 2: Structured Concurrency (Preview in 21, stable in 25)
try (var scope = new StructuredTaskScope.ShutdownOnFailure()) {
    var userTask  = scope.fork(() -> userService.findById(id));
    var orderTask = scope.fork(() -> orderService.findByUser(id));
    scope.join().throwIfFailed();
    return new UserProfile(userTask.get(), orderTask.get());
}

// PATTERN 3: Quarkus integration
@Path("/users")
public class UserResource {
    @GET
    @RunOnVirtualThread  // Offloads blocking call to virtual thread
    public List<User> list() {
        return User.listAll();  // Blocking Panache call - safe on VT
    }
}