Sample usage of SoftReference and ReferenceQueue.

SoftRefSample.java

import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;

public class SoftRefSample {
    static class Hoge {
        final String s;
        Hoge(String s) {
            this.s = s;
        }
    }

    static class NamedReference<T> extends SoftReference<T> {
        final String name;
        NamedReference(T ref, String name, ReferenceQueue<T> queue) {
            super(ref, queue);
            this.name = name;
        }
    }

    static class ReferenceQueueChecker<T> implements Runnable {
        final ReferenceQueue<T> queue;
        ReferenceQueueChecker(ReferenceQueue<T> queue) {
            this.queue = queue;
        }

        public void run() {
            for (Reference ref = queue.poll(); ref != null; ref = queue.poll()) {
                System.out.println("q:" + ref.get());
                if (ref instanceof NamedReference) {
                    NamedReference namedRef = (NamedReference) ref;
                    System.out.println("n:" + namedRef.name);
                }
                ref.clear();
            }
        }
    }

    public static void main(String[] args) throws Exception {
        ScheduledExecutorService scheduler = Executors.newSingleThreadScheduledExecutor();
        ReferenceQueue<Hoge> queue = new ReferenceQueue<Hoge>();
        scheduler.scheduleWithFixedDelay(
            new ReferenceQueueChecker<Hoge>(queue), 0L, 10L, TimeUnit.MILLISECONDS);
        NamedReference<Hoge> ref = new NamedReference<Hoge>(new Hoge("fuga"), "mage", queue);
        System.out.println("r:" + ref.get());
        oom();
        System.out.println("r:" + ref.get());
        scheduler.shutdown();
        scheduler.awaitTermination(10, TimeUnit.SECONDS);
        System.out.println("ends.");
    }

    static void oom() {
        String s = "aa";
        try {
            for (;;) {
                s += s;
            }
        } catch (OutOfMemoryError e) {
            System.out.println(e);
        }
    }
}

r:SoftRefSample$Hoge@7000a32b
java.lang.OutOfMemoryError: Java heap space
q:null
r:null
n:mage
ends.