A volatile counter that loses clicks
A easy Java puzzle in the threading and race conditions 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 5. Play this one as a puzzle without the answer.
What is wrong
volatile guarantees visibility but not atomicity. hits++ is read, add, write, and two threads can both read the same value, so increments are lost. The total under load is lower than the real number of calls.
The fix
How to avoid it next time
Use AtomicInteger or LongAdder for counters, not volatile.
Want the background? Read volatile and atomic classes in the Java concepts library.
About threading and race conditions
Without synchronization, threads can see stale values and interleave read-modify-write sequences. A volatile field gives visibility but not atomicity, and check-then-act sequences are never atomic by themselves.
In code review: For every shared variable ask who writes it, who reads it, and what protects it. Prefer atomic classes, concurrent collections and immutable objects.
More puzzles in this family
Frequently asked questions
What is the bug in "A volatile counter that loses clicks"?
volatile guarantees visibility but not atomicity. hits++ is read, add, write, and two threads can both read the same value, so increments are lost. The total under load is lower than the real number of calls. The faulty line is line 5: hits++;
How do I fix it?
Replace it with: private final java.util.concurrent.atomic.AtomicInteger hits = new java.util.concurrent.atomic.AtomicInteger(); public void record() { hits.incrementAndGet(); }. Use AtomicInteger or LongAdder for counters, not volatile.
How do I catch threading and race conditions in code review?
For every shared variable ask who writes it, who reads it, and what protects it. Prefer atomic classes, concurrent collections and immutable objects.
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