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JAVA DOUBLE CHECKED LOCKING

Pattern​

CachedToken current = tokenCache.get(appId);
if (current != null && !current.isExpired()) {
    return current.token;  // fast path — no lock needed
}
synchronized (lock) {
    // re-check: another thread may have refreshed while we waited for the lock
    current = tokenCache.get(appId);
    if (current != null && !current.isExpired()) {
        return current.token;
    }
    return refreshToken(appConfig);
}

Why Two Checks?​

  • First check avoids locking when value is already cached (majority of calls)
  • Second check inside synchronized prevents duplicate refresh from racing threads

Visibility Requirement​

The shared variable must be safely published. Either:

  • Use volatile on the reference, or
  • Use a thread-safe collection (e.g. ConcurrentHashMap) so readers see the latest write

Without proper visibility guarantees, the first (unsynchronized) check may read a partially constructed object due to instruction reordering (see cpu_cache_and_memory_visibility).

References​