The date formatter shared by all threads
A medium Java puzzle in the shared and static mutable state 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
SimpleDateFormat keeps internal state while formatting, so one shared instance used by several threads produces garbled or wrong dates and sometimes throws exceptions. It looks harmless because single-threaded tests pass.
The fix
How to avoid it next time
Use java.time.DateTimeFormatter, which is immutable and thread-safe, instead of SimpleDateFormat.
Want the background? Read static and final members in the Java concepts library.
About shared and static mutable state
Static fields and singleton beans are shared by every request and thread. Mutable state kept there leaks data between users, makes tests order-dependent and creates race conditions that appear only under load.
In code review: Look for non-final static fields and instance fields in singleton beans that are written during a request. Prefer local variables, immutable objects and explicit synchronization.
More puzzles in this family
Frequently asked questions
What is the bug in "The date formatter shared by all threads"?
SimpleDateFormat keeps internal state while formatting, so one shared instance used by several threads produces garbled or wrong dates and sometimes throws exceptions. It looks harmless because single-threaded tests pass. The faulty line is line 5: private static final SimpleDateFormat FORMAT = new SimpleDateFormat("yyyy-MM-dd HH:mm");
How do I fix it?
Replace it with: private static final DateTimeFormatter FORMAT = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm");. Use java.time.DateTimeFormatter, which is immutable and thread-safe, instead of SimpleDateFormat.
How do I catch shared and static mutable state in code review?
Look for non-final static fields and instance fields in singleton beans that are written during a request. Prefer local variables, immutable objects and explicit synchronization.
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