java - Why won't my objects die?

Question

I'm trying to implement a mechanism that deletes cached files when the objects that hold them die, and decided to use PhantomReferences to get notified on garbage collection of an object. The problem is I keep experiencing weird behavior of the ReferenceQueue. When I change something in my code it suddenly doesn't fetch objects anymore. So I tried to make this example for testing, and ran into the same problem:

public class DeathNotificationObject {
    private static ReferenceQueue<DeathNotificationObject> 
            refQueue = new ReferenceQueue<DeathNotificationObject>();

    static {
        Thread deathThread = new Thread("Death notification") {
            @Override
            public void run() {
                try {
                    while (true) {
                        refQueue.remove();
                        System.out.println("I'm dying!");
                    }
                } catch (Throwable t) {
                    t.printStackTrace();
                }
            }
        };
        deathThread.setDaemon(true);
        deathThread.start();
    }

    public DeathNotificationObject() {
        System.out.println("I'm born.");
        new PhantomReference<DeathNotificationObject>(this, refQueue);
    }

    public static void main(String[] args) {
        for (int i = 0 ; i < 10 ; i++) {
            new DeathNotificationObject();                  
        }
        try {
            System.gc();    
            Thread.sleep(3000); 
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

The output is:

I'm born.
I'm born.
I'm born.
I'm born.
I'm born.
I'm born.
I'm born.
I'm born.
I'm born.
I'm born.

Needless to say, changing the sleep time, calling gc multiple times etc. didn't work.

UPDATE

As suggested, I called Reference.enqueue() of my reference, which solved the problem.

The weird thing, is that I have some code that works perfectly (just tested it), although it never calls enqueue. Is it possible that putting the Reference into a Map somehow magically enqueued the reference?

public class ElementCachedImage {
    private static Map<PhantomReference<ElementCachedImage>, File> 
            refMap = new HashMap<PhantomReference<ElementCachedImage>, File>();
    private static ReferenceQueue<ElementCachedImage> 
            refQue = new ReferenceQueue<ElementCachedImage>();

    static {
        Thread cleanUpThread = new Thread("Image Temporary Files cleanup") {
            @Override
            public void run() {
                try {
                    while (true) {
                        Reference<? extends ElementCachedImage> phanRef = 
                                refQue.remove();
                        File f = refMap.remove(phanRef);
                        Calendar c = Calendar.getInstance();
                        c.setTimeInMillis(f.lastModified());
                        _log.debug("Deleting unused file: " + f + " created at " + c.getTime());
                        f.delete();
                    }
                } catch (Throwable t) {
                    _log.error(t);
                }
            }
        };
        cleanUpThread.setDaemon(true);
        cleanUpThread.start();
    }

    ImageWrapper img = null;

    private static Logger _log = Logger.getLogger(ElementCachedImage.class);

    public boolean copyToFile(File dest) {
        try {
            FileUtils.copyFile(img.getFile(), dest);
        } catch (IOException e) {
            _log.error(e);
            return false;
        }
        return true;
    }

    public ElementCachedImage(BufferedImage bi) {
        if (bi == null) throw new NullPointerException();
        img = new ImageWrapper(bi);
        PhantomReference<ElementCachedImage> pref = 
                new PhantomReference<ElementCachedImage>(this, refQue);
        refMap.put(pref, img.getFile());

        new Thread("Save image to file") {
            @Override
            public void run() {
                synchronized(ElementCachedImage.this) {
                    if (img != null) {
                        img.saveToFile();
                        img.getFile().deleteOnExit();
                    }
                }
            }
        }.start();
    }
}

Some filtered output:

2013-08-05 22:35:01,932 DEBUG Save image to file: <>\AppData\Local\Temp\tmp7..0.PNG

2013-08-05 22:35:03,379 DEBUG Deleting unused file: <>\AppData\Local\Temp\tmp7..0.PNG created at Mon Aug 05 22:35:02 IDT 2013

score 6 · Accepted Answer

答えは、あなたの例ではPhantomReferenceそれ自体に到達できないため、参照されたオブジェクト自体がガベージコレクションされる前にガベージコレクションされるということです。そのため、オブジェクトがGCされた時点ではそれ以上のものはなくReference、GCは何かをどこかでキューに入れる必要があることを知りません。

もちろん、これはある種の直接対決です:-)

これは、参照を到達可能なコレクションに入れると、例が機能する理由も説明します (新しいコードを深く調べる必要はありません)。

参考までに（しゃれを意図して）、最初の例の修正版を次に示します（私のマシンで動作します:-)すべての参照を保持するセットを追加しました。

import java.lang.ref.PhantomReference;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.util.HashSet;
import java.util.Set;

public class DeathNotificationObject {
    private static ReferenceQueue<DeathNotificationObject> refQueue = new ReferenceQueue<DeathNotificationObject>();
    private static Set<Reference<DeathNotificationObject>> refs = new HashSet<>();

    static {
        Thread deathThread = new Thread("Death notification") {
            @Override
            public void run() {
                try {
                    while (true) {
                        Reference<? extends DeathNotificationObject> ref = refQueue.remove();
                        refs.remove(ref);
                        System.out.println("I'm dying!");
                    }
                } catch (Throwable t) {
                    t.printStackTrace();
                }
            }
        };
        deathThread.setDaemon(true);
        deathThread.start();
    }

    public DeathNotificationObject() {
        System.out.println("I'm born.");
        PhantomReference<DeathNotificationObject> ref = new PhantomReference<DeathNotificationObject>(this, refQueue);
        refs.add(ref);
    }

    public static void main(String[] args) {
        for (int i = 0 ; i < 10 ; i++) {
            new DeathNotificationObject();                  
        }
        try {
            System.gc();    
            Thread.sleep(3000); 
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

アップデート

あなたの例では手動で呼び出すenqueueことはできますが、実際のコードではできません。それは明白な間違った結果をもたらします。enqueueコンストラクターを呼び出して、別のを使用して示しmainます。

public DeathNotificationObject() {
    System.out.println("I'm born.");
    PhantomReference<DeathNotificationObject> ref = new PhantomReference<DeathNotificationObject>(this, refQueue);
    ref.enqueue();
}

public static void main(String[] args) throws InterruptedException {

    for (int i = 0 ; i < 5 ; i++) {
        DeathNotificationObject item = new DeathNotificationObject();

        System.out.println("working with item "+item);
        Thread.sleep(1000);
        System.out.println("stopped working with item "+item);
        // simulate release item
        item = null;
    }

    try {
        System.gc();    
        Thread.sleep(3000); 
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}

出力は次のようになります。

I'm born.
I'm dying!
working with item DeathNotificationObject@6908b095
stopped working with item DeathNotificationObject@6908b095

つまり、参照キューでやりたいことはすべて、アイテムがまだ生きているときに実行されます。

java - Why won't my objects die?

1 に答える 1

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