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  1. import java.util.function.BiFunction;
  2.  
  3. import java.lang.ref.PhantomReference;
  4. import java.lang.ref.WeakReference;
  5. import java.lang.ref.ReferenceQueue;
  6. import java.lang.ref.Reference;
  7.  
  8. class ReferencesTest {
  9.     private static SomeObject resurrectedObject = null;
  10.     private static int nextAttemptToKillCounter = 0;
  11.  
  12.     private static void referenceTest(
  13.             BiFunction<SomeObject, ReferenceQueue<? super SomeObject>,
  14.                 Reference<? extends SomeObject>> constr, int delay)
  15.             throws InterruptedException {
  16.         ReferenceQueue<SomeObject> rq = new ReferenceQueue<>();
  17.         SomeObject strongRef = new SomeObject();
  18.         Reference<? extends SomeObject> ref = constr.apply(strongRef, rq);
  19.  
  20.         // delete the strong reference
  21.         strongRef = null;
  22.         // try to trigger the GC
  23.         System.gc();
  24.         while (true) {
  25.             // try to poll something from the RQ
  26.             Reference<? extends SomeObject> pulledRef = rq.poll();
  27.             System.out.printf("poll(): %s\n", pulledRef);
  28.             if (pulledRef != null) {
  29.                 System.out.printf("ref.get(): %s\n",
  30.                     ref.get());
  31.                 System.out.printf("resurrectedObject = %s\n",
  32.                     resurrectedObject);
  33.                 break;
  34.             } 
  35.  
  36.             if (resurrectedObject != null
  37.                     && nextAttemptToKillCounter == 0) {
  38.                 System.out.println(
  39.                     "*exterminate the resurrected object*");
  40.                 resurrectedObject = null;
  41.                 // try to trigger the GC
  42.                 System.gc();
  43.             }
  44.             if (nextAttemptToKillCounter > 0) {
  45.                 System.out.printf(
  46.                     "*next attempt to kill in %d steps*\n",
  47.                     nextAttemptToKillCounter);
  48.                 nextAttemptToKillCounter--;
  49.             }
  50.  
  51.             Thread.sleep(delay);
  52.         }
  53.     }
  54.  
  55.     public static void main(String[] args) throws InterruptedException {
  56.         System.out.println("**phantom reference test**");
  57.         referenceTest(PhantomReference::new, 250);
  58.  
  59.         System.out.println("\n**weak reference test**");
  60.         referenceTest(WeakReference::new, 250);
  61.     }
  62.  
  63.     private static class SomeObject {
  64.         @Override
  65.         public void finalize() throws Throwable {
  66.             System.out.println("*at the finalize method*");
  67.             resurrectedObject = this;
  68.             nextAttemptToKillCounter = 10;
  69.             System.out.println("*resurrection*");
  70.         }
  71.     }
  72. }
Success #stdin #stdout 0.1s 35236KB
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Standard input is empty
stdout
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**phantom reference test**
poll(): null
*at the finalize method*
*resurrection*
poll(): null
*next attempt to kill in 10 steps*
poll(): null
*next attempt to kill in 9 steps*
poll(): null
*next attempt to kill in 8 steps*
poll(): null
*next attempt to kill in 7 steps*
poll(): null
*next attempt to kill in 6 steps*
poll(): null
*next attempt to kill in 5 steps*
poll(): null
*next attempt to kill in 4 steps*
poll(): null
*next attempt to kill in 3 steps*
poll(): null
*next attempt to kill in 2 steps*
poll(): null
*next attempt to kill in 1 steps*
poll(): null
*exterminate the resurrected object*
poll(): java.lang.ref.PhantomReference@e9e54c2
ref.get(): null
resurrectedObject = null

**weak reference test**
poll(): null
*at the finalize method*
*resurrection*
poll(): java.lang.ref.WeakReference@eed1f14
ref.get(): null
resurrectedObject = ReferencesTest$SomeObject@7229724f
https://ideone.com/27AlHW
language:
Java (HotSpot 12)
created:
4 years ago
visibility:
public

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