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
- Review Java code for thread safety
- Fix race conditions in Java concurrency code
- Choose ExecutorService or virtual threads in Java
- Improve synchronization and shared mutable state handling
- Apply structured concurrency for related Java subtasks
Workflow
- Compile project before concurrency changes
Run ./mvnw compile or mvn compile and stop immediately if compilation fails.
- Read concurrency reference and analyze hotspots
Read references/125-java-concurrency.md and identify thread-safety, coordination, and throughput issues to address.
- Apply concurrency improvements
Implement suitable concurrency patterns, structured task scopes where they fit related subtasks, cancellation discipline, and fit-for-purpose primitives.
- 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.