Question

There are four type of references in Java: Strong, Soft, Weak, and phantom. Kindly elaborate where and how Java exactly use phantom reference.

EDIT :

I did read the javaDoc, as assylias posted below. the section says an object is phantom rechable if it has been finalized and some phantom refernce refers to it. I dint quite get that statement.

What is the use of phantom and in which cases shall we use it?

Answer 1

How each type of reference works is described in the java.lang.ref package javadoc:

Going from strongest to weakest, the different levels of reachability reflect the life cycle of an object. They are operationally defined as follows:

• An object is strongly reachable if it can be reached by some thread without traversing any reference objects. A newly-created object is strongly reachable by the thread that created it.
• An object is softly reachable if it is not strongly reachable but can be reached by traversing a soft reference.
• An object is weakly reachable if it is neither strongly nor softly reachable but can be reached by traversing a weak reference. When the weak references to a weakly-reachable object are cleared, the object becomes eligible for finalization.
• An object is phantom reachable if it is neither strongly, softly, nor weakly reachable, it has been finalized, and some phantom reference refers to it.
• Finally, an object is unreachable, and therefore eligible for reclamation, when it is not reachable in any of the above ways.

There are a few posts on Stack OVerflow that give examples of how and when you might need a PhantomReference:

• Have you ever used Phantom reference in any project?
• When to use phantom references in Java?