# concurrency-review

Reviews Java multi-threaded code for race conditions, deadlocks, and thread safety. Covers classic locks alongside Virtual Threads and CompletableFuture.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 345 installs · ★ 760
- Author: decebals
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/decebals/claude-code-java/tree/HEAD/skills/concurrency-review/SKILL.md
- Updated: 2026-09-06
- HTML: https://affpapa.org/en/nevibecode/skills/concurrency-review

## What it does

The skill evaluates Java code for shared state hazards, race conditions, and deadlocks. It inspects synchronization blocks, `volatile` fields, explicit `Lock` objects, and `CompletableFuture` chains, ensuring asynchronous operations do not drop unhandled errors.

Support for Java 21 and 25 highlights modern patterns. The review targets thread pinning inside `synchronized` blocks under Java 21-23, structured concurrency with `StructuredTaskScope`, and migrations from `ThreadLocal` to `ScopedValue`. If an application handles fewer than 10,000 concurrent tasks, virtual threads offer no measurable advantage.

It also flags Spring `@Async` misconfigurations. The skill catches same-class self-invocations that bypass proxies, non-public asynchronous methods, and missing security context propagation. Using the default `SimpleAsyncTaskExecutor` creates a fresh thread per task and invites `OutOfMemoryError` crashes under traffic.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add decebals/claude-code-java --skill concurrency-review -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/concurrency-review/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/concurrency-review/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add decebals/claude-code-java --skill concurrency-review -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/concurrency-review/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add decebals/claude-code-java --skill concurrency-review -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/concurrency-review/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install decebals/claude-code-java concurrency-review
```

2. **Manually: put the files in place**
- File: `.github/skills/concurrency-review/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add decebals/claude-code-java --skill concurrency-review -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/decebals/claude-code-java --path skills/concurrency-review
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/concurrency-review/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add decebals/claude-code-java --skill concurrency-review -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/decebals/claude-code-java/tree/HEAD/skills/concurrency-review/SKILL.md

## When to use it
- detect race conditions and deadlocks in Java methods
- inspect Spring `@Async` proxy bypasses and thread pool settings
- prevent silent failures in `CompletableFuture` error pipelines
- audit code for Virtual Threads, `ScopedValue`, and task scopes

## Example prompts
- Check thread safety in OrderProcessingService.java
- Perform a concurrency review on our CompletableFuture pipeline
- Inspect this multi-threaded code for virtual thread pinning and leaks

## Source text · SKILL.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 56ec21887644103e.

BEGIN THIRD-PARTY FILE 56ec21887644103e
````text
---
name: concurrency-review
description: Review Java concurrency code for thread safety, race conditions, deadlocks, and modern patterns (Virtual Threads, CompletableFuture, @Async). Use when user asks "check thread safety", "concurrency review", "async code review", or when reviewing multi-threaded code.
license: MIT
---

# Concurrency Review Skill

Review Java concurrent code for correctness, safety, and modern best practices.

## Why This Matters

> Nearly 60% of multithreaded applications encounter issues due to improper management of shared resources. - ACM Study

Concurrency bugs are:
- **Hard to reproduce** - timing-dependent
- **Hard to test** - may only appear under load
- **Hard to debug** - non-deterministic behavior

This skill helps catch issues **before** they reach production.

## When to Use
- Reviewing code with `synchronized`, `volatile`, `Lock`
- Checking `@Async`, `CompletableFuture`, `ExecutorService`
- Validating thread safety of shared state
- Reviewing Virtual Threads / Structured Concurrency code
- Any code accessed by multiple threads

---

## Modern Java (21/25): Virtual Threads

### When to Use Virtual Threads

```java
// ✅ Perfect for I/O-bound tasks (HTTP, DB, file I/O)
try (ExecutorService executor = Executors.newVirtualThreadPerTaskExecutor()) {
    for (Request request : requests) {
        executor.submit(() -> callExternalApi(request));
    }
}

// ❌ Not beneficial for CPU-bound tasks
// Use platform threads / ForkJoinPool instead
```

**Rule of thumb**: If your app never has 10,000+ concurrent tasks, virtual threads may not provide significant benefit.

### Java 25: Synchronized Pinning Fixed

In Java 21-23, virtual threads became "pinned" when entering `synchronized` blocks with blocking operations. **Java 25 fixes this** (JEP 491).

```java
// In Java 21-23: ⚠️ Could cause pinning
synchronized (lock) {
    blockingIoCall();  // Virtual thread pinned to carrier
}

// In Java 25: ✅ No longer an issue
// But consider ReentrantLock for explicit control anyway
```

### ScopedValue Over ThreadLocal

```java
// ❌ ThreadLocal problematic with virtual threads
private static final ThreadLocal<User> currentUser = new ThreadLocal<>();

// ✅ ScopedValue (Java 21+ preview, improved in 25)
private static final ScopedValue<User> CURRENT_USER = ScopedValue.newInstance();

ScopedValue.where(CURRENT_USER, user).run(() -> {
    // CURRENT_USER.get() available here and in child virtual threads
    processRequest();
});
```

### Structured Concurrency (Java 25 Preview)

```java
// ✅ Structured concurrency - tasks tied to scope lifecycle
try (StructuredTaskScope.ShutdownOnFailure scope = new StructuredTaskScope.ShutdownOnFailure()) {
````
END THIRD-PARTY FILE 56ec21887644103e

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