phantom references : Java Glossary

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phantom references
A phantom reference lets you do final touch up closing on an object, even after it has been declared dead — no longer referenced by any live object. Java has four orders of strength in holding onto Objects. In descending order from strongest to weakest they are:
  1. The JVM (Java Virtual Machine) holds onto regular Objects until they are no longer reachable by either clients or any container. In other words Objects are garbage collected when there are no more live references to them. Dead references don’t count.
  2. Soft references can be deleted from a container if the clients are no longer referencing them and memory is tight.
  3. Weak references are automatically deleted from a container as soon clients stop referencing them.
  4. Phantom references point to objects that are already dead and have been finalised.
Soft vs Weak vs Phantom References
Type Purpose Use When GCed Implementing Class
Strong Reference An ordinary reference. Keeps objects alive as long as they are referenced. normal reference. Any object not pointed to can be reclaimed. default
Soft Reference 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. java.lang.ref.SoftReference
Weak Reference 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 java.lang.ref.WeakReference
java.util.WeakHashMap
Phantom Reference 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. java.lang.ref.PhantomReference

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