125-java-concurrency

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Use when you need to apply Java concurrency best practices — including thread safety fundamentals, ExecutorService thread pool management, concurrent design patterns like Producer-Consumer, asynchronous programming with CompletableFuture, immutability and safe publication, deadlock avoidance, virtual threads, scoped values, backpressure, cancellation discipline, and observability for concurrent systems. This should trigger for requests such as Review Java code for concurrency. Part of cursor-rules-java project

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Added on2026-05-19

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SKILL.md Content

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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
```
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

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, 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.