Java / Java Concurrency and Multi Threading in Java 17 and Java 21 Interview questions
What is the difference between ExecutorService shutdown() and shutdownNow()?
| shutdown() | shutdownNow() |
| Graceful: stops accepting new tasks but lets queued and running tasks finish. | Aggressive: attempts to stop actively executing tasks and returns tasks that were never started. |
| Does not interrupt running tasks. | Interrupts running tasks via Thread.interrupt(). |
| Returns void. | Returns a List<Runnable> of tasks that were awaiting execution. |
Neither call blocks; both return immediately after signaling the pool. To actually wait for termination, you follow up with awaitTermination(timeout, unit), checking its boolean result to know whether the pool finished in time.
A common pattern is to call shutdown() first for a graceful stop, then fall back to shutdownNow() if awaitTermination times out, ensuring the application doesn't hang indefinitely waiting for stuck tasks.
Which method attempts to interrupt actively running tasks?
Does calling shutdown() block until all tasks finish?
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