Java Tutorial/Reflection/PhantomReference

A phantom reference is used to determine when an object is just about to be reclaimed.

import java.lang.ref.PhantomReference;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
public class Main {
  public static void main(String[] argv) throws Exception {
    ReferenceQueue rq = new ReferenceQueue();
    PhantomReference<String> pr = new PhantomReference<String>("object", rq);
    while (true) {
      Reference r = rq.remove();
      if (r == pr) {
        // about to be reclaimed.
        r.clear();
      }
    }
  }
}

Testing PhantomReference

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 PhantomReference.");
    reference = new PhantomReference(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());
  }
}