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Java PhantomReference Tutorial with Examples

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1- PhantomReference

In this article, we will discuss PhantomReference class. Before starting I recommend you to learn about WeakReference and SoftReference class. All three classes have the same basic characteristics and need not be mentioned again.
PhantomReference​ Constructors

PhantomReference​(T innerObject, ReferenceQueue<? super T> queue)
PhantomReference has only one constructor. To create a PhantomReference object, you must provide two parameters:
  1. innerObject: The object will be wrapped inside the PhantomReference object.
  2. queue: A queue used to store this PhantomReference object when its innerObject is removed from memory by the GC.

ReferenceQueue<AnyObject> queue = new ReferenceQueue<>();

AnyObject innerObject = new AnyObject("Obj1");

PhantomReference phantomRef = new PhantomReference(innerObject, queue);
All methods of PhantomReference are inherited from the parent class.

// Methods inherited from parent.
public T get()  
public void clear()  
public boolean isEnqueued()  
public boolean enqueue()
The phantomReference.get() method always returns null, the purpose of which is to prevent access or attempt to revive an object that has almost been removed.
You may be wondering about the characteristics of PhantomReference and your question now is what is PhantomReference used for?

PhantomReference phantomRef = new PhantomReference(innerObject, queue);
Basically, PhantomReference gives you the ability to determine exactly when its innerObject object is removed from memory. phantomRef.isEnqueued() method returns true which means that innerObject object has been removed from memory. When innerObject object is removed from memory, phantomRef object will be placed in the queue.
For example: If you need to allocate memory to handle large video files, then using PhantomReference is a good choice. First, use PhantomReference to allocate memory to process the first video, then you need to check to make sure that memory has been freed before continuing to allocate memory to process the next video file. This lessens the risk of getting an OutOfMemoryError.
VideoProcessor class simulates the processing of a large video file:
VideoProcessor.java

package org.o7planning.phantomreference.ex;

public class VideoProcessor {
    private String video;

    public VideoProcessor(String video)  {
        this.video = video;
        System.out.println("\nNew VideoProcessor: " + this.video);
    }

    public void process() {
        System.out.println("   >>> Processing video: " + this.video);
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) { }
        System.out.println("   >>> Completed processing video: " + this.video);
    }
    
    // !IMPORTANT: Do not override finalize() method.
    //  (Java9+: If you override this method, PhantomReference will not work!!)
    //    @Override
    //    protected void finalize() throws Throwable {
    //        System.out.println("VideoProcessor is being removed from memory\n");
    //        super.finalize();
    //    }
}
PhantomReferenceEx1.java

package org.o7planning.phantomreference.ex;

import java.lang.ref.PhantomReference;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;

public class PhantomReferenceEx1 {

    public static void main(String[] args) {

        String[] videos = new String[] { "video1.mp4", "video2.mp4", "video3.mp4" };

        ReferenceQueue<VideoProcessor> queue = new ReferenceQueue<VideoProcessor>();

        for (String video : videos) {
            VideoProcessor videoProcessor = new VideoProcessor(video);

            PhantomReference<VideoProcessor> phantomRef = new PhantomReference<>(videoProcessor, queue);
            videoProcessor.process();

            videoProcessor = null;
            // Call GC:
            System.gc();

            while (true) {
                boolean isEnqueued = phantomRef.isEnqueued();
                System.out.println(" phantomRef.isEnqueued: " + isEnqueued);
            
                if (!isEnqueued) {
                    try {
                        Thread.sleep(1000);
                    } catch (InterruptedException e) {
                    }
                    continue;
                }
                break;
            }
        }
        System.out.println("\nObjects in the queue:");
        Reference<? extends VideoProcessor> ref= null;
        
        while((ref = queue.poll())!= null)  {
            System.out.println(ref);
        }
    }
}
Output:


New VideoProcessor: video1.mp4
   >>> Processing video: video1.mp4
   >>> Completed processing video: video1.mp4
 phantomRef.isEnqueued: false
 phantomRef.isEnqueued: true

New VideoProcessor: video2.mp4
   >>> Processing video: video2.mp4
   >>> Completed processing video: video2.mp4
 phantomRef.isEnqueued: false
 phantomRef.isEnqueued: true

New VideoProcessor: video3.mp4
   >>> Processing video: video3.mp4
   >>> Completed processing video: video3.mp4
 phantomRef.isEnqueued: false
 phantomRef.isEnqueued: true

Objects in the queue:
java.lang.ref.PhantomReference@5e265ba4
java.lang.ref.PhantomReference@156643d4
java.lang.ref.PhantomReference@123a439b

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