125-java-concurrency

# Java rules for Concurrency objects Identify and apply Java concurrency best practices to improve thread safety, scalability, and maintainability by using modern `java.util.concurrent` utilities and virtual threads. **What is covered in this Skill?** - Thread safety fundamentals: `ConcurrentHashMap`, `AtomicInteger`, `ReentrantLock`, `ReadWriteLock`, Java Memory Model - `ExecutorService` thread pool configuration: sizing, keep-alive, bounded queues, rejection policies, graceful shutdown - Producer-Consumer and Publish-Subscribe patterns with `BlockingQueue` - `CompletableFuture` for non-blocking async composition (`thenApply`/`thenCompose`/`exceptionally`/`orTimeout`) - Immutability and safe publication (`volatile`, static initializers) - Lock contention and false-sharing performance optimization - Virtual threads (`Executors.newVirtualThreadPerTaskExecutor()`) for I/O-bound scalability - Structured Concurrency (`StructuredTaskScope`) for related subtasks in Java 27 preview - `ScopedValue` over `ThreadLocal` for immutable cross-task data - Cooperative cancellation and `InterruptedException` discipline - Backpressure with bounded queues and `CallerRunsPolicy` - Deadlock avoidance via global lock ordering and `tryLock` with timeouts - ForkJoin/parallel-stream discipline for CPU-bound work - Virtual-thread pinning detection (JFR `VirtualThreadPinned`) - Thread naming and `UncaughtExceptionHandler` observability - Fit-for-purpose primitives: `LongAdder`, `CopyOnWriteArrayList`, `StampedLock`, `Semaphore`, `CountDownLatch`, `Phaser` **Scope:** The reference is organized by examples (good/bad code patterns) for each core area. Apply recommendations based on applicable examples. ## Constraints Before applying any concurrency changes, ensure the project compiles. If compilation fails, stop immediately — compilation failure is a blocking condition. After applying improvements, run full verification. - **MANDATORY**: Run `./mvnw compile` or `mvn compile` before applying any change - **SAFETY**: If compilation fails, stop immediately — compilation failure is a blocking condition that prevents any further processing - **VERIFY**: Run `./mvnw clean verify` or `mvn clean verify` after applying improvements - **BEFORE APPLYING**: Read the reference for detailed good/bad examples, constraints, and safeguards for each concurrency pattern ## When to use this skill - Review Java code for concurrency ## Workflow 1. **Compile project before concurrency changes** Run `./mvnw compile` or `mvn compile` and stop immediately if compilation fails. 2. **Read concurrency reference and analyze hotspots** Read `references/125-java-concurrency.md` and identify thread-safety, coordination, and throughput issues to address. 3. **Apply concurrency improvements** Implement suitable concurrency patterns, structured task scopes where they fit related subtasks, cancellation discipline, and fit-for-purpose primitives. 4. **Verify with full build** Run `./mvnw clean verify` or `mvn clean verify` after applying improvements. ## Reference For detailed guidance, examples, and constraints, see [references/125-java-concurrency.md](references/125-java-concurrency.md).