73-simple-date-format

Example 73 — SimpleDateFormat Shared State

Demonstrates SimpleDateFormatDetector: an AuditLogger uses a single static SimpleDateFormat instance. SimpleDateFormat is not thread-safe — concurrent calls to format() or parse() corrupt its internal Calendar state, producing wrong dates or throwing NumberFormatException.

The Problem

AuditLogger declares private static final SimpleDateFormat SDF. The Calendar field inside SimpleDateFormat is updated during every format() and parse() call without synchronisation. When two threads call formatTimestamp() simultaneously, one thread's Calendar mutation overwrites the other's, producing an incorrect formatted string or a parse failure.

How to Reproduce

1. Remove @Disabled from testFormatTimestamp_concurrent_detectsSharedSdf.
2. Run: mvn test or ./gradlew test
3. The test fails with a SimpleDateFormatDetector report listing concurrent format/parse accesses on the shared SimpleDateFormat instance.

Fix: use DateTimeFormatter from java.time (inherently thread-safe), or create a new SimpleDateFormat per call, or use a ThreadLocal<SimpleDateFormat>.