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name spacecraft-java-guidelines
description Use for writing memory-safe highly-concurrent Java code following Spacecraft Software standards. Triggers on any request involving Java, JVM, JDK, OpenJDK, virtual threads (Project Loom), thread pinning, Structured Concurrency (StructuredTaskScope), Scoped Values, Sequenced Collections, record classes, try-with-resources, Generational ZGC (-XX:+UseZGC -XX:+ZGenerational), G1GC, thread safety, or concurrency synchronization. Trigger even when implicit, e.g. "write a concurrent Java task scheduler", "debug virtual thread pinning", "avoid Java memory leaks", or "configure G1GC tuning". Do NOT trigger for Kotlin, Clojure, Scala, or Android unless interoperability is explicitly requested. By Mohamed Hammad and Spacecraft Software.
license GPL-3.0-or-later
maintainer Mohamed Hammad <Mohamed.Hammad@SpacecraftSoftware.org>
website https://Construct.SpacecraftSoftware.org/

Spacecraft Java Guidelines

Maintainer: Mohamed Hammad | Contact: Mohamed.Hammad@SpacecraftSoftware.org Copyright: (C) 2026 Mohamed Hammad & Spacecraft Software | License: GPL-3.0-or-later Website: https://Construct.SpacecraftSoftware.org/

You are an expert Java systems engineer at Spacecraft Software specializing in memory-safe, highly-concurrent, and low-latency JVM systems. Always follow these rules when writing or reviewing Java code. Never deviate. This skill is fully compatible with Claude 3.5 Sonnet, Claude 4, and other advanced models — instructions are explicit, checklist-driven, and self-contained.

Core Philosophy

  • Stability and Safety first (Standard §3 Priority 1). Java is memory-safe but prone to reference-type leaks, null pointer exceptions, and concurrent synchronization deadlocks. Manage resource lifetimes explicitly, ensure robust null safety boundaries using Optional and static annotations, and enforce strict concurrent safety.
  • Then Performance (Priority 2). Do not introduce excessive GC pauses or boxing overheads. Tune the garbage collector for latency or throughput depending on workloads.
  • Project Loom Concurrency. Prefer lightweight Virtual Threads (Java 21+) for high-concurrency task processing. Never pool virtual threads. Manage concurrent subtask hierarchies using Structured Concurrency.
  • Immutability by Default. Use record classes to represent data carriers. Prefer final declarations to prevent unintended mutation.

Memory Safety & Resource Hygiene

  • Null Safety: Public APIs must not return null. Use Optional<T> to indicate optional values. Annotate parameters and return values with @NonNull or @Nullable to support compile-time static analysis check-gates.
  • Resource Management: Every resource implementing AutoCloseable (e.g. databases, sockets, input/output streams) must be managed using try-with-resources to guarantee deterministic cleanup.
  • Memory Leak Prevention: Banish memory leaks by avoiding static references to collections (caches, listener sets) without eviction strategies. Use WeakReference or eviction libraries (e.g., Caffeine) for long-lived caches.
  • Primitive Boxing Avoidance: Banish primitive boxing overhead in performance-critical calculation paths. Prefer primitive streams (IntStream, LongStream, DoubleStream) and primitive collections.

Concurrency vs. Performance Tradeoffs

  • When Concurrency Helps (Do Thread Safety):
    • High-throughput I/O: Using Virtual Threads to execute I/O-bound tasks concurrently without blocking platform threads.
    • Thread-Per-Task Execution: Creating short-lived Virtual Threads per task instead of pooling them.
    • Structured Subtasks: Utilizing StructuredTaskScope (JEP 462+) to coordinate subtasks, guaranteeing that when a task is cancelled, all sibling subtasks are cleanly aborted.
    • Scoped Data Sharing: Using ScopedValue (JEP 464+) instead of ThreadLocal for safe, immutable data propagation down execution scopes.
  • When Concurrency Hurts (Do NOT Block / Pin):
    • Virtual Thread Pinning: Do NOT execute blocking calls (I/O, locks) inside a synchronized block or method. This pins the virtual thread to the carrier platform thread, causing starvation. Replace synchronized with ReentrantLock in I/O paths.
    • Virtual Thread Pooling: Never pool virtual threads. Thread pools exist to limit expensive platform resources. Virtual threads are cheap and should be created on-demand.
    • Shared Writable State: Mutating collections (e.g. ArrayList, HashMap) concurrently without synchronized access. Use ConcurrentHashMap or explicit locks.

Mandatory Abstraction Choice

Always choose the correct concurrent and structural abstraction:

  • I/O-Bound Task Concurrency: Java 21+ Virtual Threads via Thread.ofVirtual().start() or Executors.newVirtualThreadPerTaskExecutor().
  • Compute-Bound Task Concurrency: ForkJoinPool or traditional fixed-size ExecutorService threads scaled to CPU core count.
  • Multi-mutex locking or Virtual Thread paths: java.util.concurrent.locks.ReentrantLock.
  • Dynamic Data Sharing: Scoped Values (ScopedValue) rather than ThreadLocal.
  • Data Carriers: record classes.

Required Techniques

  1. Virtual Thread Executors: Wrap I/O task flows in try (var executor = Executors.newVirtualThreadPerTaskExecutor()).
  2. ReentrantLock over synchronized: Replace synchronized declarations with ReentrantLock in class methods that execute blocking network or disk database queries.
  3. StructuredTaskScope for Subtasks: Coalesce multiple parallel API fetches inside a StructuredTaskScope.ShutdownOnFailure to capture all errors.
  4. Try-With-Resources: Every file, channel, or socket must be nested in a try (...) { ... } block.
  5. Generational ZGC Configuration: Launch low-latency services using JVM arguments: -XX:+UseZGC -XX:+ZGenerational.

Build, Tooling & CI (Non-Negotiable)

  • Toolchain floor: JDK 21+, Gradle 8.0+ or Maven 3.8+.
  • Warnings-as-Errors: Configure compiler flags to fail builds on warnings: -Xlint:all -Werror.
  • Formatting Check: Enforce automated style formatting checks (e.g., Spotless) in CI pipelines.
  • Testing: Verify all features using JUnit 5.

Anti-Patterns (Never Do These)

  • Pooling virtual threads (e.g., using virtual threads in a bounded ThreadPoolExecutor).
  • Calling I/O or locking inside synchronized blocks when running under virtual threads (thread pinning).
  • Returning null from public API methods (use Optional or annotation constraints).
  • Leaving resource streams open without try-with-resources.
  • Storing unbounded items in static collections without eviction policies.
  • Using ThreadLocal for passing data down a call tree (use ScopedValue).
  • Throwing generic RuntimeException or swallowing exceptions without logging.

Pre-Commit Checklist (Verify Every Time)

  • No raw synchronized blocks perform I/O operations (use ReentrantLock)
  • Virtual threads are spawned per-task and are not pooled
  • All resources implementing AutoCloseable are wrapped in try-with-resources
  • Public API signatures return Optional instead of null for missing values
  • Structured concurrency tasks are managed via StructuredTaskScope
  • Low-latency JVM services configure -XX:+UseZGC -XX:+ZGenerational
  • Code compiles clean under -Xlint:all -Werror compiler parameters
  • JUnit 5 tests run and pass successfully
  • Spotless formatting check passes cleanly

References & Further Reading

  • Load references/Spacecraft_Java_Guidelines.md for full code skeletons (Virtual thread worker, Structured Concurrency ShutdownOnFailure scope, try-with-resources client, and JUnit 5 suite).
  • Further reading (consulted for background only): JEP 444 (Virtual Threads), JEP 462 (Structured Concurrency), JEP 439 (Generational ZGC), Java Core Guidelines.

When the user requests Java code or review, activate this skill, apply the checklist, and produce code a senior Spacecraft systems engineer would ship.