Java : WeakReference with Examples
March 9, 2024 
August 14, 2022
WeakReference (Java SE 18 & JDK 18) API Examples.
You will find code examples on most WeakReference methods.
Summary
Weak reference objects, which do not prevent their referents from being made finalizable, finalized, and then reclaimed. Weak references are most often used to implement canonicalizing mappings.
var sb = new StringBuilder("abcd");
final var ref = new WeakReference<>(sb);
System.out.println(ref.get()); // abcd
System.gc();
System.out.println(ref.get()); // abcd
sb = null;
System.out.println(ref.get()); // abcd
System.gc();
System.out.println(ref.get()); // null
Constructors
WeakReference (T referent)
Creates a new weak reference that refers to the given object.
var sb = new StringBuilder("abcd");
final var ref = new WeakReference<>(sb);
System.out.println(ref.get()); // abcd
sb = null;
System.out.println(ref.get()); // abcd
System.gc();
System.out.println(ref.get()); // null
WeakReference (T referent, ReferenceQueue<? super T> q)
Creates a new weak reference that refers to the given object and is registered with the given queue.
final var queue = new ReferenceQueue<StringBuilder>();
var sb = new StringBuilder("abcd");
final var ref = new WeakReference<>(sb, queue);
System.out.println(ref.get()); // abcd
System.out.println(queue.poll()); // null
sb = null;
System.out.println(ref.get()); // abcd
System.out.println(queue.poll()); // null
System.gc();
System.out.println(ref.get()); // null
System.out.println(queue.poll()); // java.lang.ref.WeakReference@6b2a628e
Methods declared in Reference
clear, clone, enqueue, get, isEnqueued, reachabilityFence, refersTo
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