PhantomReferenceSample

import java.lang.ref.PhantomReference;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.util.ArrayList;
import java.util.List;


public final class PhantomReferenceSample {

    public static void main(String[] args) {
        List<Object> objects = new ArrayList<>();
        ReferenceQueue<Object> queue = new ReferenceQueue<>();

        List<DataReference> dataReferences = new ArrayList<>();

        for (int i = 0; i < 100; i++) {
            Object data = "junk" + i;
            objects.add(data);
            DataReference pr = new DataReference(i, data, queue);
            dataReferences.add(pr);
        }

        objects.clear();
        System.gc();

        while (true) {
            try {
                Reference remove = queue.remove(1000l);
                if(remove != null){
                    DataReference df = (DataReference) remove;
                    System.out.println("clean " + df.getIndex());
                } else {
                   break;
                }

            } catch (Exception e) {
                e.printStackTrace();
            }
        }

    }
}

class DataReference extends PhantomReference<Object> {
    private int index;

    public DataReference(int index, Object referent, ReferenceQueue<Object> q) {
        super(referent, q);
        this.index = index;
    }

    public int getIndex() {
        return index;
    }
}