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Java / Java 17 Garbage Collection Interview Questions

What is the difference between soft, weak, and phantom references?

Beyond ordinary strong references, Java's java.lang.ref package offers three progressively weaker reference types that change how eagerly the GC reclaims the referent.

TypeClearedTypical use
SoftReferenceOnly right before an OutOfMemoryErrorMemory-sensitive caches
WeakReferenceAt the next GC cycle once unreachableWeakHashMap keys, canonicalizing maps
PhantomReferenceNever returns the referent; queued after finalizationPost-mortem cleanup via ReferenceQueue / Cleaner

SoftReference<byte[]> cache = new SoftReference<>(loadLargeData());
WeakReference<Session> sessionRef = new WeakReference<>(session);
ReferenceQueue<Resource> queue = new ReferenceQueue<>();
PhantomReference<Resource> cleanupRef = new PhantomReference<>(resource, queue);

Phantom references are notable because get() always returns null for them - they exist purely so code can be notified, via a ReferenceQueue, exactly when an object has become phantom-reachable, which is the mechanism the modern java.lang.ref.Cleaner API uses in place of the deprecated finalize().

A SoftReference's referent is cleared:
Calling get() on a PhantomReference:

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