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.