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.

125-java-concurrency

Java rules for Concurrency objects