Java Concurrency

Java Concurrency

Priority: P1 (HIGH)

Modern concurrent programming emphasizing Virtual Threads (Loom) and safety.

Implementation Guidelines

• Virtual Threads (Java 21): Use for high-throughput I/O. Executors.newVirtualThreadPerTaskExecutor().
• Structured Concurrency: Use StructuredTaskScope to treat related tasks as a single unit (Scope, Fork, Join).
• Immutability: Share immutable data between threads to avoid race conditions.
• CompletableFuture: Use for composing asynchronous pipelines (if not using Virtual Threads).
• Atomic Variables: Use AtomicInteger, LongAdder for simple counters.
• Locks: Prefer ReentrantLock / ReadWriteLock over synchronized blocks for fine-grained control.
• Thread Safety: Document @ThreadSafe or @NotThreadSafe.

Anti-Patterns

• new Thread(): Never manually create threads. Use Executors.
• Thread Pooling Virtual Threads: Virtual threads are cheap; do not pool them.
• Blocking inside synchronized: Pins the carrier thread (Loom pitfall). Use ReentrantLock.
• Shared Mutable State: The root of all concurrency bugs.
• Thread.stop/suspend: Deprecated and dangerous.

Code

For full StructuredTaskScope and VirtualThread examples: references/structured-concurrency.md

// Virtual Threads (Loom)
try (var scope = new StructuredTaskScope.ShutdownOnFailure()) {
    var user = scope.fork(() -> api.fetchUser());
    scope.join().throwIfFailed();
}

Related Topics

language | best-practices