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A 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.
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.
- "java.lang.ref (Java Platform SE 8 )". Java™ Platform, Standard Edition 8 API Specification. Oracle. Retrieved 6 August 2016.
- oracle.com, kim barrett at (28 December 2015). "hg: jdk9/hs-rt/jdk: 8071507: (ref) Clear phantom reference as soft and weak references do".
- Nicholas, Ethan (May 4, 2006). "Understanding Weak References". java.net. Retrieved October 1, 2010. External link in
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