Phantom reference virtual reference code demonstration

package com.xxl.job.admin.mytest;

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

public class ReferenceDemo {
    
    @Override
    protected void finalize() {
    
        System.out.println(" Trigger  gc  Notify garbage collection ...");
    }

    public static void main(String[] args) {
    
        ReferenceQueue<ReferenceDemo> referenceQueue = new ReferenceQueue<>();
        PhantomReference<ReferenceDemo> reference = new PhantomReference<>(new ReferenceDemo(), referenceQueue);


        List<byte[]> list = new ArrayList<>();

        new Thread(()->{
    
            while(true) {
    
                list.add(new byte[1*1024*1024]);
                try {
    
                    TimeUnit.SECONDS.sleep(1);
                    System.out.println(reference.get());
                } catch (InterruptedException e) {
    
                    e.printStackTrace();
                }
            }
        }).start();
        new Thread(()->{
    
            while(true) {
    
                Reference<? extends ReferenceDemo> poll = referenceQueue.poll();
                //  When the virtual reference object is killed , Will be put into the virtual reference queue 
                if (poll != null) {
    
                    System.out.println("------ A virtual object entered the queue ");
                }
            }
        }).start();


        try {
    
            TimeUnit.SECONDS.sleep(5);
        } catch (InterruptedException e) {
    
            throw new RuntimeException(e);
        }
    }
}

Pay attention to setting the heap memory size in advance before running the code -Xms10m -Xmx10m

Running results ：

null
null
null
null
 Trigger  gc  Notify garbage collection ...
null
null
Exception in thread "Thread-0" java.lang.OutOfMemoryError: Java heap space
	at com.xxl.job.admin.mytest.ReferenceDemo.lambda$main$0(ReferenceDemo.java:25)
	at com.xxl.job.admin.mytest.ReferenceDemo$$Lambda$1/517380410.run(Unknown Source)
	at java.lang.Thread.run(Thread.java:750)
------ A virtual object entered the queue 

Process finished with exit code 130 (interrupted by signal 2: SIGINT)

The virtual reference is just to be able to notify before this object is recycled JVM A sound , Then throw this object into the queue associated with the virtual reference , Waiting to be arranged . Trigger gc Notify garbage collection ... The output indicates that the virtual reference object is recycled ,------ A virtual object entered the queue The output result indicates that the virtual reference object is placed in the virtual reference queue .