Double-checked locking without volatile
A hard 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 2. Play this one as a puzzle without the answer.
What is wrong
Without volatile, another thread can see a non-null reference to an object whose fields are not yet visible, because the write of the reference may be reordered with the constructor. The consumer then uses a half-constructed Registry.
The fix
How to avoid it next time
Declare the field volatile, or use the initialization-on-demand holder idiom or an enum singleton.
Want the background? Read Singleton pattern 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 "Double-checked locking without volatile"?
Without volatile, another thread can see a non-null reference to an object whose fields are not yet visible, because the write of the reference may be reordered with the constructor. The consumer then uses a half-constructed Registry. The faulty line is line 2: private static Registry instance;
How do I fix it?
Replace it with: private static volatile Registry instance;. Declare the field volatile, or use the initialization-on-demand holder idiom or an enum singleton.
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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