A phantom reference is a kind of reference in
Java
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, where the memory can be reclaimed. The phantom reference is one of the strengths or levels of 'non
strong' reference defined in the Java programming language; the others being
weak and
soft. Phantom reference are the weakest level of reference in Java; in order from strongest to weakest, they are: strong, soft, weak, ''phantom.''
An object is phantomly referenced after it has been
finalized.
In Java 8 and earlier versions, the reference needs to be cleared before the memory for a finalized referent can be reclaimed. A change in Java 9 will allow memory from a finalized referent to be reclaimable immediately.
Use
Phantom references are of limited use, primarily narrow technical uses.
First, it can be used instead of a
finalize method, guaranteeing that the object is not resurrected during finalization. This allows the object to be garbage collected in a single cycle, rather than needing to wait for a second GC cycle to ensure that it has not been resurrected. A second use is to detect exactly when an object has been removed from memory (by using in combination with a
ReferenceQueue object), ensuring that its memory is available, for example deferring allocation of a large amount of memory (e.g., a large image) until previous memory is freed.
See also
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Ephemeron
An ephemeron is a data structure that solves two related problems in garbage collected systems. On the one hand, an ephemeron provides a notification when some object is about to be collected. On the other hand, an ephemeron allows data to be ...
*
Weak reference
In computer programming, a weak reference is a reference that does not protect the referenced object from collection by a garbage collector, unlike a strong reference. An object referenced ''only'' by weak references – meaning "every chain of ref ...
*
Soft reference A soft reference is a reference that is garbage-collected less aggressively. The soft reference is one of the strengths or levels of 'non strong' reference defined in the Java programming language, the others being weak and phantom. In order from ...
*
Circular reference
A circular reference is a series of references where the last object references the first, resulting in a closed loop.
In language
A circular reference is not to be confused with the logical fallacy of a circular argument. Although a circular ...
References
Programming constructs
Memory management
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