Weak reference: Difference between revisions

Revision as of 15:06, 24 November 2019

Revision as of 14:50, 26 November 2019

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public class ReferenceTest {

public static void main(String[] args) throws InterruptedException {

WeakReference r = new WeakReference("I'm here");

StrongReference sr = new StrongReference("I'm here");

System.out.println("beforeBefore gc: r=" + r.get() + ", static=" + sr.get());

System.gc();

Thread.sleep(100);

// onlyOnly r.get() becomes null.

WeakReference r = new WeakReference("I'm here");

System.out.println("afterAfter gc: r=" + r.get() + ", static=" + sr.get());

StrongReference sr = new StrongReference("I'm here");

}

System.out.println("before gc: r=" + r.get() + ", static=" + sr.get());

System.gc();

Thread.sleep(100);

// only r.get() becomes null

System.out.println("after gc: r=" + r.get() + ", static=" + sr.get());

}

}

</source>

Another use of weak references is in writing a [[cache (computing)|cache]]. Using, for example, a weak [[hash map]], one can store in the cache the various referred objects via a weak reference. When the garbage collector runs &mdash; when for example the application's memory usage gets sufficiently high &mdash; those cached objects which are no longer directly referenced by other objects are removed from the cache.

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===[[Vala (programming language)|Vala]]===

<source lang="vala">

class Node {

public weak Node prev; // a weak reference is used to avoid circular references between nodes of a doubly-linked list

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</source>

===Python3Python===

<source lang=python3"pycon">

>>> import weakref

>>> import gc