The worker that cannot be stopped
A hard Java puzzle in the exception handling mistakes family. One line below contains a real bug. Try to spot it first, then read why it fails and how to fix it.
Highlighted: line 7. Play this one as a puzzle without the answer.
What is wrong
When a thread is interrupted during sleep, the interrupted flag is cleared and InterruptedException is thrown. Ignoring it means the loop condition sees a clear flag and keeps running, so shutdown requests are lost and the pool never terminates.
The fix
How to avoid it next time
On InterruptedException either rethrow it or restore the flag with Thread.currentThread().interrupt() and stop work.
Want the background? Read Interruption and InterruptedException in the Java concepts library.
About exception handling mistakes
Swallowed exceptions, lost causes and returns in finally blocks hide failures until the data is already wrong. The program keeps running and nobody knows why the result is missing.
In code review: An empty catch block, a catch that only prints, or an exception rethrown without its cause are all red flags. Log with the exception object and rethrow or handle meaningfully.
More puzzles in this family
Frequently asked questions
What is the bug in "The worker that cannot be stopped"?
When a thread is interrupted during sleep, the interrupted flag is cleared and InterruptedException is thrown. Ignoring it means the loop condition sees a clear flag and keeps running, so shutdown requests are lost and the pool never terminates. The faulty line is line 7: } catch (InterruptedException e) {
How do I fix it?
Replace it with: } catch (InterruptedException e) { Thread.currentThread().interrupt(); return; }. On InterruptedException either rethrow it or restore the flag with Thread.currentThread().interrupt() and stop work.
How do I catch exception handling mistakes in code review?
An empty catch block, a catch that only prints, or an exception rethrown without its cause are all red flags. Log with the exception object and rethrow or handle meaningfully.
Is there a new puzzle every day?
Yes. The daily Spot the Bug puzzle picks one of these snippets for each UTC day, with the difficulty set by the weekday. Your streak is stored only in your browser.