19-shared-timezone

Shared TimeZone Example

This example demonstrates the SharedTimeZoneDetector (Phase 12, async-test-lib 0.10.0).

The Problem

SchedulingService.getOffsetHours() calls setRawOffset() on a shared TimeZone instance. TimeZone is a mutable class — setRawOffset() modifies the object in place and is not thread-safe. When multiple threads call getOffsetHours() concurrently:

1. Thread A writes offset +5h
2. Thread B writes offset −3h
3. Thread A reads back −3h (B's value)

The resulting offset arithmetic is silently wrong, with no exception thrown.

Why Sequential Tests Miss This Bug

@Test
void part1_getOffset_singleThread() {
    SchedulingService svc = new SchedulingService();
    assertEquals(5, svc.getOffsetHours(5 * 3_600_000)); // ✅ Passes
}

Sequential execution means the write and read are adjacent — no other write can interleave.

How @AsyncTest Exposes the Bug

@AsyncTest(threads = 4, invocations = 3, timeoutMs = 5000, includes = DetectorType.SHARED_TIMEZONE)
void part2_detectSharedTimeZone() {
    var d = AsyncTestContext.sharedTimeZoneDetector();
    d.recordMutation(sharedTz, "setRawOffset", Thread.currentThread());
    sharedTz.setRawOffset(5 * 3_600_000); // mutating shared instance — flagged!
}

The detector reports:

SHARED TIMEZONE MUTATION DETECTED:
  - TimeZone instance mutated from 4 threads via 'setRawOffset'
    unsynchronized concurrent mutations corrupt date/time arithmetic
    Fix: create a new TimeZone per operation, or switch to the immutable java.time.ZoneId API.

Running the Example

cd examples/19-shared-timezone
mvn clean test
# ✅ Tests pass — @Test gives false confidence

# Upgrade to 0.10.0 and enable @AsyncTest (see comments in the test file)

The Fix

Option 1 — new instance per call:

int getOffsetHoursFixed(int offsetMillis) {
    TimeZone tz = TimeZone.getTimeZone("UTC"); // ✅ Fresh instance
    tz.setRawOffset(offsetMillis);
    return tz.getRawOffset() / 3_600_000;
}

Option 2 — use the immutable java.time API:

ZoneOffset offset = ZoneOffset.ofTotalSeconds(offsetMillis / 1000); // ✅ Immutable

Severity

Failure mode	Symptom
Wrong offset reads	Scheduled jobs fire at wrong times — silent correctness bug
Non-reproducible test failures	Results depend on thread interleaving order — flaky CI