96-weak-hashmap-shared

Example 96 — Weak HashMap Shared

Demonstrates WeakHashMapSharedDetector catching unsynchronized access to a WeakHashMap.

The Problem

WeakCacheService uses a WeakHashMap<Object, String> as a shared cache visible to all threads. WeakHashMap is explicitly not thread-safe: concurrent put() calls can corrupt the internal hash table, and GC-triggered entry expiry runs concurrently with all mutations.

How to Reproduce

1. Remove @Disabled from test_concurrent_detectsSharedWeakHashMap in WeakCacheServiceTest.
2. Run the test:

   mvn test
   gradle test
   

3. WeakHashMapSharedDetector will report multiple threads accessing the same WeakHashMap instance without synchronization.

The Fix

Replace WeakHashMap with a ConcurrentHashMap backed by WeakReference values, or wrap access in Collections.synchronizedMap(new WeakHashMap<>()) with external locking during iteration.