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  1. import java.lang.ref.PhantomReference;
  2. import java.lang.ref.ReferenceQueue;
  3.  
  4. /**
  5.  * @author Peter Chng
  6.  */
  7. class Ideone {
  8.  
  9.   // Just so we have a non-primitive, non-interned object that will be GC'd.
  10.   public static class SampleObject<T> {
  11.     private final T value;
  12.  
  13.     public SampleObject(final T value) {
  14.       this.value = value;
  15.     }
  16.  
  17.     @Override
  18.     public String toString() {
  19.       return String.valueOf(this.value);
  20.     }
  21.   }
  22.  
  23.   public static class PhantomReferenceMetadata<T, M> extends PhantomReference<T> {
  24.     // Some metadata stored about the object that will be used during some cleanup actions.
  25.     private final M metadata;
  26.  
  27.     public PhantomReferenceMetadata(final T referent, final ReferenceQueue<? super T> q,
  28.         final M metadata) {
  29.       super(referent, q);
  30.       this.metadata = metadata;
  31.     }
  32.  
  33.     public M getMetadata() {
  34.       return this.metadata;
  35.     }
  36.   }
  37.  
  38.   public static void main(final String[] args) {
  39.     // The object whose GC lifecycle we want to track.
  40.     SampleObject<String> helloObject = new SampleObject<>("Hello");
  41.  
  42.     // Reference queue that the phantom references will be registered to.
  43.     // They will be enqueued here when the appropriate reachability changes are detected by the JVM.
  44.     final ReferenceQueue<SampleObject<String>> refQueue = new ReferenceQueue<>();
  45.  
  46.     // In this case, the metadata we associate with the object is some name.
  47.     final PhantomReferenceMetadata<SampleObject<String>, String> helloPhantomReference = new PhantomReferenceMetadata<>(
  48.         helloObject, refQueue, "helloObject");
  49.  
  50.     new Thread(() -> {
  51.       System.out.println("Starting ReferenceQueue consumer thread.");
  52.       final int numToDequeue = 1;
  53.       int numDequed = 0;
  54.       while (numDequed < numToDequeue) {
  55.         // Unfortunately, need to downcast to the appropriate type.
  56.         try {
  57.           @SuppressWarnings("unchecked")
  58.           final PhantomReferenceMetadata<SampleObject<String>, String> reference = (PhantomReferenceMetadata<SampleObject<String>, String>) refQueue
  59.               .remove();
  60.  
  61.           // At this point, we know the object referred to by the PhantomReference has been finalized.
  62.           // So, we can do any other clean-up that might be allowed, such as cleaning up some temporary files
  63.           // associated with the object.
  64.           // The metadata stored in PhantomReferenceMetadata could be used to determine which temporary files
  65.           // should be cleaned up.
  66.           // You probably shouldn't rely on this as the ONLY method to clean up those temporary files, however.
  67.           System.out.println(reference.getMetadata() + " has been finalized.");
  68.         } catch (final InterruptedException e) {
  69.           // Just for the purpose of this example.
  70.           break;
  71.         }
  72.         ++numDequed;
  73.       }
  74.       System.out.println("Finished ReferenceQueue consumer thread.");
  75.     }).start();
  76.  
  77.     // Lose the strong reference to the object.
  78.     helloObject = null;
  79.  
  80.     // Attempt to trigger a GC.
  81.     System.gc();
  82.   }
  83. }
  84.  
Success #stdin #stdout 0.23s 33836KB
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Standard input is empty
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Starting ReferenceQueue consumer thread.
helloObject has been finalized.
Finished ReferenceQueue consumer thread.
https://ideone.com/I8f4U3
language:
Java (HotSpot 12)
created:
6 years ago
visibility:
public

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