concurrency-review

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

// ✅ 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).

// 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

// ❌ 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();
});