Java Tutorial/Reflection/WeakReference
A weak reference is used to determine when an object is no longer being referenced.
<source lang="java">
import java.lang.ref.Reference; import java.lang.ref.ReferenceQueue; import java.lang.ref.WeakReference; public class Main {
public static void main(String[] argv) throws Exception { ReferenceQueue rq = new ReferenceQueue(); WeakReference<String> wr = new WeakReference<String>("string", rq); while (true) { Reference r = rq.remove(); if (r == wr) { System.out.println("no longer referenced"); } } }
}</source>
Testing WeakReference
<source lang="java">
import java.lang.ref.PhantomReference; import java.lang.ref.Reference; import java.lang.ref.ReferenceQueue; import java.lang.ref.SoftReference; import java.lang.ref.WeakReference; import java.util.HashMap; public class Main {
public static void main(String[] args) { ReferenceQueue referenceQueue = new ReferenceQueue(); Object object = new Object() { public String toString() { return "Referenced Object"; } }; Object data = new Object() { public String toString() { return "Data"; } }; HashMap map = new HashMap(); Reference reference = null; System.out.println("Testing WeakReference."); reference = new WeakReference(object, referenceQueue); map.put(reference, data); System.out.println(reference.get()); System.out.println(map.get(reference)); System.out.println(reference.isEnqueued()); System.gc(); System.out.println(reference.get()); System.out.println(map.get(reference)); System.out.println(reference.isEnqueued()); object = null; data = null; System.gc(); System.out.println(reference.get()); System.out.println(map.get(reference)); System.out.println(reference.isEnqueued()); }
}</source>