Question 1

What is volatile in ServiceNow?

Accepted Answer

Reads/writes go through main memory across threads. Doesn't make `x++` atomic. Use Atomic* for that.

Question 2

What is ExecutorService in ServiceNow?

Accepted Answer

Submit Callable/Runnable, get Future. Always shut down (`shutdown()` / `awaitTermination`) — non-daemon pool threads keep the JVM alive.

Question 3

What is CompletableFuture in ServiceNow?

Accepted Answer

`supplyAsync`, `thenApply`, `thenCompose`, `allOf`. Replaces nested callbacks for async work.

Question 4

What is Virtual Thread in ServiceNow?

Accepted Answer

Project Loom: millions of threads cheap to create. Great for blocking IO. Don't pool them — start one per task.

Question 5

What is happens-before in ServiceNow?

Accepted Answer

Defines when one thread's write is visible to another's read. Provided by synchronized, volatile, locks, thread start/join.

Question 6

What is ReentrantLock in ServiceNow?

Accepted Answer

tryLock, fairness, interruptible. Always `lock.lock(); try { } finally { lock.unlock(); }`.

Question 7

What is Semaphore in ServiceNow?

Accepted Answer

Acquire N permits; bounds concurrent access to a resource (e.g. connection pool gate).

Question 8

What is CountDownLatch in ServiceNow?

Accepted Answer

Threads await(); main countDown() N times. Reusable alternative: CyclicBarrier.

Question 9

What is ForkJoinPool in ServiceNow?

Accepted Answer

Underlies parallel streams and CompletableFuture's commonPool. Don't submit blocking tasks.

Question 10

What is ThreadLocal in ServiceNow?

Accepted Answer

Useful for SimpleDateFormat or contextual data. Always `remove()` in pooled threads to avoid leaks.

Question 11

What is Atomic* in ServiceNow?

Accepted Answer

AtomicInteger, AtomicReference, AtomicLongFieldUpdater. CAS-based, faster than synchronized for single-variable updates.

Question 12

What is synchronized vs Lock in ServiceNow?

Accepted Answer

Lock supports tryLock, fairness, multiple condition variables; synchronized is simpler and recently faster after JVM improvements.

Question 13

What is ScheduledExecutorService in ServiceNow?

Accepted Answer

scheduleAtFixedRate / scheduleWithFixedDelay. Tasks throwing uncaught exceptions stop being rescheduled — wrap them.

Question 14

What is StampedLock in ServiceNow?

Accepted Answer

tryOptimisticRead returns a stamp; validate before using read data. Up to 3× faster than ReentrantReadWriteLock for read-heavy paths.

Question 15

What is Structured Concurrency in ServiceNow?

Accepted Answer

StructuredTaskScope.ShutdownOnFailure forks subtasks and joins/cancels together — eliminates orphan tasks.

🧵 CONCURRENCY

Term Glossary