Phantom reference

A '''phantom reference''' is a kind of reference in Java, where the memory can be reclaimed. The class is <code>java.lang.ref.PhantomReference</code>. 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.<ref>{{cite web|title=java.lang.ref (Java Platform SE 8 )|url=https://docs.oracle.com/javase/8/docs/api/java/lang/ref/package-summary.html|website=Java™ Platform, Standard Edition 8 API Specification|publisher=Oracle|accessdate=6 August 2016}}</ref> 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<ref>{{cite web|last1=oracle.com|first1=kim barrett at|title=hg: jdk9/hs-rt/jdk: 8071507: (ref) Clear phantom reference as soft and weak references do|url=https://mail.openjdk.java.net/pipermail/jdk9-hs-rt-changes/2015-December/001914.html|date=28 December 2015}}</ref> will allow memory from a finalized referent to be reclaimable immediately.

==Use== Phantom references are of limited use, primarily narrow technical uses.<ref>{{cite web |last=Nicholas |first=Ethan |url=http://weblogs.java.net/blog/2006/05/04/understanding-weak-references |title=Understanding Weak References |website=www.java.net|date=May 4, 2006 |accessdate=October 1, 2010 |url-status=dead |archiveurl=https://web.archive.org/web/20100819115659/http://weblogs.java.net/blog/2006/05/04/understanding-weak-references |archivedate=August 19, 2010 }}</ref> First, it can be used instead of a <code>finalize</code> 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 <code>java.lang.ref.ReferenceQueue</code> 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.

==Example== <syntaxhighlight lang="java"> import java.lang.ref.PhantomReference; import java.lang.ref.ReferenceQueue;

class Resource { private final String name;

public Resource(String name) { this.name = name; }

public String getName() { return name; } }

public class PhantomReferenceExample { public static void main(String[] args) throws InterruptedException { Resource resource = new Resource("My resource"); ReferenceQueue<Resource> queue = new ReferenceQueue<>(); PhantomReference<Resource> phantomRef = new PhantomReference<>(resource, queue);

resource = null;

System.gc();

Reference<? extends Resource> ref = queue.poll(); if (ref != null) { System.out.printf("Object is ready to be collected: %s%n", ((PhantomReference<?>)ref).get()); } } } </syntaxhighlight>

== See also == {{Portal|Computer programming}} * Ephemeron * Weak reference * Soft reference * Circular reference

==References== {{Reflist}}

Category:Programming constructs Category:Memory management