Java references

In java we have 4 types of references

• Strong
• Weak
• Soft
• Phantom

Soft vs Weak vs Phantom References
Type	Purpose	Use	When GCed
Strong	An ordinary reference. Keeps objects alive as long as they are referenced.	normal reference.	Any object not pointed to can be reclaimed.
Soft	Keeps objects alive provided there’s enough memory.	to keep objects alive even after clients have removed their references (memory-sensitive caches), in case clients start asking for them again by key.	After a first gc pass, the JVM decides it still needs to reclaim more space.
Weak	Keeps objects alive only while they’re in use (reachable) by clients.	Containers that automatically delete objects no longer in use.	After gc determines the object is only weakly reachable
Phantom	Lets you clean up after finalization but before the space is reclaimed (replaces or augments the use of finalize())	Special clean up processing	After finalization.

Articles to read

Baeldung :  weak, soft, phantom

Kdgregory : java references

Sun/Oracle : understanding references
Dzone : finalization and phantom