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  1. import java.lang.ref.PhantomReference;
  2. import java.lang.ref.ReferenceQueue;
  3. import java.util.concurrent.TimeUnit;
  4.  
  5. class Ideone {
  6.   public static void main(String[] args) throws InterruptedException {
  7.     Beta b1 = new Beta();
  8.     Beta b2 = new Beta();
  9.     Alpha a1 = new Alpha();
  10.     Alpha a2 = new Alpha();
  11.     a1.b1 = b1;
  12.     a1.b2 = b1;
  13.     a2.b2 = b2;
  14.     PhantomReference<Alpha> a1Phantom =
  15.         new PhantomReference<>(a1, new ReferenceQueue<>());
  16.     PhantomReference<Alpha> a2Phantom =
  17.         new PhantomReference<>(a2, new ReferenceQueue<>());
  18.     PhantomReference<Beta> b1Phantom =
  19.         new PhantomReference<>(b1, new ReferenceQueue<>());
  20.     PhantomReference<Beta> b2Phantom =
  21.         new PhantomReference<>(b2, new ReferenceQueue<>());
  22.     a1 = null;
  23.     b1 = null;
  24.     b2 = null;
  25.     System.gc();
  26.     Thread.sleep(TimeUnit.SECONDS.toMillis(1));
  27.     System.out.printf(
  28.         "a1 has %sbeen collected%n",
  29.         a1Phantom.isEnqueued() ? "" : "not ");
  30.     System.out.printf(
  31.         "a2 has %sbeen collected%n",
  32.         a2Phantom.isEnqueued() ? "" : "not ");
  33.     System.out.printf(
  34.         "b1 has %sbeen collected%n",
  35.         b1Phantom.isEnqueued() ? "" : "not ");
  36.     System.out.printf(
  37.         "b2 has %sbeen collected%n",
  38.         b2Phantom.isEnqueued() ? "" : "not ");
  39.   }
  40. }
  41.  
  42. class Beta {
  43. }
  44.  
  45. class Alpha {
  46.   static Beta b1;
  47.   Beta b2;
  48. }
Success #stdin #stdout 0.12s 51528KB
comments (?)
stdin 
Standard input is empty
stdout 
a1 has been collected
a2 has not been collected
b1 has not been collected
b2 has not been collected
https://ideone.com/AFm5tL
language:
Java (HotSpot 12)
created:
3 years ago
visibility:
secret

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