java.lang.ref

https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/ref/package-summary.html

https://dzone.com/articles/weak-soft-and-phantom-references-in-java-and-why-they-matter

1. SoftReference: Cleared when GC is response to memory demand. Often used to implement memory-sensitive cache.
2. WeakReference: Don’t prevent to be finalized. Often used to implement canonicalizing mapping. (Mapping only reachable object instances)
3. PhantomReference: Are enqueued after determining to be reclaimed. Not automatically cleared by GC. Object referenced via phantom reference won’t be cleared by GC automatically until phantom reference cleared.