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();
});

Structured Concurrency (Java 25 Preview)

// ✅ Structured concurrency - tasks tied to scope lifecycle
try (StructuredTaskScope.ShutdownOnFailure scope = new StructuredTaskScope.ShutdownOnFailure()) {
    Subtask<User> userTask = scope.fork(() -> fetchUser(id));
    Subtask<Orders> ordersTask = scope.fork(() -> fetchOrders(id));

    scope.join();            // Wait for all
    scope.throwIfFailed();   // Propagate exceptions

    return new Profile(userTask.get(), ordersTask.get());
}
// All subtasks automatically cancelled if scope exits

Spring @Async Pitfalls

1. Forgetting @EnableAsync

// ❌ @Async silently ignored
@Service
public class EmailService {
    @Async
    public void sendEmail(String to) { }
}

// ✅ Enable async processing
@Configuration
@EnableAsync
public class AsyncConfig { }

2. Calling Async from Same Class

@Service
public class OrderService {

    // ❌ Bypasses proxy - runs synchronously!
    public void processOrder(Order order) {
        sendConfirmation(order);  // Direct call, not async
    }

    @Async
    public void sendConfirmation(Order order) { }
}

// ✅ Inject self or use separate service
@Service
public class OrderService {
    @Autowired
    private EmailService emailService;  // Separate bean

    public void processOrder(Order order) {
        emailService.sendConfirmation(order);  // Proxy call, async works
    }
}

3. @Async on Non-Public Methods

// ❌ Non-public methods - proxy can't intercept
@Async
private void processInBackground() { }

@Async
protected void processInBackground() { }

// ✅ Must be public
@Async
public void processInBackground() { }

4. Default Executor Creates Thread Per Task

// ❌ Default SimpleAsyncTaskExecutor - creates new thread each time!
// Can cause OutOfMemoryError under load

// ✅ Configure proper thread pool
@Configuration
@EnableAsync
public class AsyncConfig {

    @Bean
    public Executor taskExecutor() {
        ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
        executor.setCorePoolSize(10);
        executor.setMaxPoolSize(50);
        executor.setQueueCapacity(100);
        executor.setThreadNamePrefix("async-");
        executor.setRejectedExecutionHandler(new CallerRunsPolicy());
        executor.initialize();
        return executor;
    }
}

5. SecurityContext Not Propagating

// ❌ SecurityContextHolder is ThreadLocal-bound
@Async
public void auditAction() {
    // SecurityContextHolder.getContext() is NULL here!
    String user = SecurityContextHolder.getContext().getAuthentication().getName();
}

// ✅ Use DelegatingSecurityContextAsyncTaskExecutor
@Bean
public Executor taskExecutor() {
    ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
    // ... configure ...
    return new DelegatingSecurityContextAsyncTaskExecutor(executor);
}

CompletableFuture Patterns

Error Handling

// ❌ Exception silently swallowed
CompletableFuture.supplyAsync(() -> riskyOperation());
// If riskyOperation throws, nobody knows

// ✅ Always handle exceptions
CompletableFuture.supplyAsync(() -> riskyOperation())
    .exceptionally(ex -> {
        log.error("Operation failed", ex);
        return fallbackValue;
    });

// ✅ Or use handle() for both success and failure
CompletableFuture.supplyAsync(() -> riskyOperation())
    .handle((result, ex) -> {
        if (ex != null) {
            log.error("Failed", ex);
            return fallbackValue;
        }
        return result;
    });

Timeout Handling (Java 9+)

// ✅ Fail after timeout
CompletableFuture.supplyAsync(() -> slowOperation())
    .orTimeout(5, TimeUnit.SECONDS);  // Throws TimeoutException

// ✅ Return default after timeout
CompletableFuture.supplyAsync(() -> slowOperation())
    .completeOnTimeout(defaultValue, 5, TimeUnit.SECONDS);

Combining Futures

// ✅ Wait for all
CompletableFuture.allOf(future1, future2, future3)
    .thenRun(() -> log.info("All completed"));

// ✅ Wait for first
CompletableFuture.anyOf(future1, future2, future3)
    .thenAccept(result -> log.info("First result: {}", result));

// ✅ Combine results
future1.thenCombine(future2, (r1, r2) -> merge(r1, r2));

Use Appropriate Executor

// ❌ CPU-bound task in ForkJoinPool.commonPool (default)
CompletableFuture.supplyAsync(() -> cpuIntensiveWork());

// ✅ Custom executor for blocking/I/O operations
ExecutorService ioExecutor = Executors.newFixedThreadPool(20);
CompletableFuture.supplyAsync(() -> blockingIoCall(), ioExecutor);

// ✅ In Java 21+, virtual threads for I/O
ExecutorService virtualExecutor = Executors.newVirtualThreadPerTaskExecutor();
CompletableFuture.supplyAsync(() -> blockingIoCall(), virtualExecutor);

Classic Concurrency Issues

Race Conditions: Check-Then-Act

// ❌ Race condition
if (!map.containsKey(key)) {
    map.put(key, computeValue());  // Another thread may have added it
}

// ✅ Atomic operation
map.computeIfAbsent(key, k -> computeValue());

// ❌ Race condition with counter
if (count < MAX) {
    count++;  // Read-check-write is not atomic
}

// ✅ Atomic counter
AtomicInteger count = new AtomicInteger();
count.updateAndGet(c -> c < MAX ? c + 1 : c);

Visibility: Missing volatile

// ❌ Other threads may never see the update
private boolean running = true;

public void stop() {
    running = false;  // May not be visible to other threads
}

public void run() {
    while (running) { }  // May loop forever
}

// ✅ Volatile ensures visibility
private volatile boolean running = true;

Non-Atomic long/double

// ❌ 64-bit read/write is non-atomic on 32-bit JVMs
private long counter;

public void increment() {
    counter++;  // Not atomic!
}

// ✅ Use AtomicLong or synchronization
private AtomicLong counter = new AtomicLong();

// ✅ Or volatile (for single-writer scenarios)
private volatile long counter;

Double-Checked Locking

// ❌ Broken without volatile
private static Singleton instance;

public static Singleton getInstance() {
    if (instance == null) {
        synchronized (Singleton.class) {
            if (instance == null) {
                instance = new Singleton();  // May be seen partially constructed
            }
        }
    }
    return instance;
}

// ✅ Correct with volatile
private static volatile Singleton instance;

// ✅ Or use holder class idiom
private static class Holder {
    static final Singleton INSTANCE = new Singleton();
}

public static Singleton getInstance() {
    return Holder.INSTANCE;
}

Deadlocks: Lock Ordering

// ❌ Potential deadlock
// Thread 1: lock(A) -> lock(B)
// Thread 2: lock(B) -> lock(A)

public void transfer(Account from, Account to, int amount) {
    synchronized (from) {
        synchronized (to) {
            // Transfer logic
        }
    }
}

// ✅ Consistent lock ordering
public void transfer(Account from, Account to, int amount) {
    Account first = from.getId() < to.getId() ? from : to;
    Account second = from.getId() < to.getId() ? to : from;

    synchronized (first) {
        synchronized (second) {
            // Transfer logic
        }
    }
}

Thread-Safe Collections

Choose the Right Collection

ConcurrentHashMap Pitfalls

// ❌ Non-atomic compound operation
if (!map.containsKey(key)) {
    map.put(key, value);
}

// ✅ Atomic
map.putIfAbsent(key, value);
map.computeIfAbsent(key, k -> createValue());

// ❌ Nested compute can deadlock
map.compute(key1, (k, v) -> {
    return map.compute(key2, ...);  // Deadlock risk!
});

Concurrency Review Checklist

🔴 High Severity (Likely Bugs)

• No check-then-act on shared state without synchronization
• No synchronized calling external/unknown code (deadlock risk)
• volatile present for double-checked locking
• Non-volatile fields not read in loops waiting for updates
• ConcurrentHashMap.compute() doesn’t call other map operations
• @Async methods are public and called from different beans

🟡 Medium Severity (Potential Issues)

• Thread pools properly sized and named
• CompletableFuture exceptions handled (exceptionally/handle)
• SecurityContext propagated to async tasks if needed
• ExecutorService properly shut down
• Lock.unlock() in finally block
• Thread-safe collections used for shared data

🟢 Modern Patterns (Java 21/25)

• Virtual threads used for I/O-bound concurrent tasks
• ScopedValue considered over ThreadLocal
• Structured concurrency for related subtasks
• Timeouts on CompletableFuture operations

📝 Documentation

• Thread safety documented on shared classes
• Locking order documented for nested locks
• Each volatile usage justified

Analysis Commands

# Find synchronized blocks
grep -rn "synchronized" --include="*.java"

# Find @Async methods
grep -rn "@Async" --include="*.java"

# Find volatile fields
grep -rn "volatile" --include="*.java"

# Find thread pool creation
grep -rn "Executors\.\|ThreadPoolExecutor\|ExecutorService" --include="*.java"

# Find CompletableFuture without error handling
grep -rn "CompletableFuture\." --include="*.java" | grep -v "exceptionally\|handle\|whenComplete"

# Find ThreadLocal (consider ScopedValue in Java 21+)
grep -rn "ThreadLocal" --include="*.java"