Java : PhantomReference with Examples

Last Modified : July 1, 2023 Published : August 25, 2022

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PhantomReference (Java SE 18 & JDK 18) API Examples.
You will find code examples on most PhantomReference methods.

Summary

class Foo {
    @SuppressWarnings("removal")
    @Override
    protected void finalize() {
        System.out.println("Finalize!");
    }
}

class FooRef extends PhantomReference<Foo> {
    FooRef(Foo referent, ReferenceQueue<? super Foo> q) {
        super(referent, q);
    }

    void clean() {
        System.out.println("Clean!");
    }
}

final var queue = new ReferenceQueue<Foo>();
final var executorService = Executors.newSingleThreadExecutor();

try {
    executorService.submit(() -> {
        try {
            var foo = new Foo();
            final var ref = new FooRef(foo, queue);

            // The get method of a phantom reference always returns null. 
            System.out.println("ref.get : " + ref.get());
            System.out.println("ref.referesTo : " + ref.refersTo(foo));

            System.out.println("queue.poll : " + queue.poll());

            foo = null;

            System.out.println("-- gc --");
            System.gc();

            TimeUnit.SECONDS.sleep(3);
            System.out.println("-- sleep --");

            System.out.println("-- gc --");
            System.gc();

            TimeUnit.SECONDS.sleep(3);
            System.out.println("-- sleep --");

        } catch (InterruptedException e) {
            System.out.println(e);
        }
    });

    final var ref = queue.remove();
    System.out.println("queue.remove : " + ref.getClass().getSimpleName());

    if (ref instanceof FooRef fooRef) {
        fooRef.clean();
    }

} finally {
    executorService.shutdown();
}

final var ret = executorService.awaitTermination(5, TimeUnit.SECONDS);
System.out.println("term : " + ret);

// Result
// ↓
//ref.get : null
//ref.referesTo : true
//queue.poll : null
//-- gc --
//Finalize!
//-- sleep --
//-- gc --
//queue.remove : FooRef
//Clean!
//-- sleep --
//term : true

Constructors

var sb = new StringBuilder("abcd");

final var queue = new ReferenceQueue<StringBuilder>();
final var ref = new PhantomReference<>(sb, queue);

System.out.println(ref.get()); // null
System.out.println(ref.refersTo(sb)); // true

System.out.println(queue.poll()); // null

sb = null;
System.gc();

System.out.println(queue.remove()); // java.lang.ref.PhantomReference@6cb107fd

Methods

final var sb = new StringBuilder("abcd");

final var queue = new ReferenceQueue<StringBuilder>();
final var ref = new PhantomReference<>(sb, queue);

System.out.println(ref.get()); // null
System.out.println(ref.refersTo(sb)); // true

Methods declared in Reference

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