Question

我尝试使用ReferenceQueue释放垃圾收集对象使用的资源。问题是我的引用队列总是为空，即使有证据表明其中一个引用的对象是垃圾回收的。这是一个非常简单，自包含的JUnit测试，它说明了我要做的事情（跟踪对象的删除）：

@Test
public void weakReferenceTest() {
    ReferenceQueue<Object> refQueue = new ReferenceQueue<Object>();
    Object myObject1 = new Object();
    Object myObject2 = new Object();
    WeakReference<Object> ref1 = new WeakReference<Object>(myObject1, refQueue);
    WeakReference<Object> ref2 = new WeakReference<Object>(myObject2, refQueue);
    myObject1 = null;

    // simulate the application running and calling GC at some point
    System.gc();

    myObject1 = ref1.get();
    myObject2 = ref2.get();
    if (myObject1 != null) {
        System.out.println("Weak Reference to MyObject1 is still valid.");
        fail();
    } else {
        System.out.println("Weak Reference to MyObject1 has disappeared.");
    }
    if (myObject2 != null) {
        System.out.println("Weak Reference to MyObject2 is still valid.");
    } else {
        System.out.println("Weak Reference to MyObject2 has disappeared.");
        fail();
    }
    Reference<? extends Object> removedRef = refQueue.poll();
    boolean trackedRemoval = false;
    while (removedRef != null) {
        if (removedRef == ref1) {
            System.out.println("Reference Queue reported deletion of MyObject1.");
            trackedRemoval = true;
        } else if (removedRef == ref2) {
            System.out.println("Reference Queue reported deletion of MyObject2.");
            fail();
        }
        removedRef = refQueue.poll();
    }
    if (trackedRemoval == false) {
        fail();
    }
}

对我来说，这总是打印出来：

Weak Reference to MyObject1 has disappeared.
Weak Reference to MyObject2 is still valid.

...这很好，但由于trackedRemoval最后为false，测试总是失败 - ReferenceQueue始终为空。

我使用ReferenceQueue和/或WeakReference错了吗？我也尝试了PhantomReference s，但没有区别。

有趣的是，如果您将单元测试转换为常规public static void main(String[] args)方法，那么它就像魅力一样！

任何人都可以解释这个特定的行为吗？我现在已经有很长一段时间一直在寻找答案了。

Answer 1

这对我来说似乎是一种竞争条件。当GC确定myObject1引用的对象是GCable时，它将对其进行GC并从WeakReference中清除它。然后，它会将WeakReference添加到“待处理参考列表”中。有一个引用处理程序线程，它将从该列表中删除并添加到相应的ReferenceQueue。以上是支持javadoc

的实现细节

在同一时间或稍晚时间，它会将这些列入其中新引用的弱引用，通过引用注册队列。

代码到达时

Reference<? extends Object> removedRef = refQueue.poll();

参考处理程序线程不得将WeakReference添加到ReferenceQueue，因此poll会返回null。

Answer 2

好吧，这可能不是一个明确的答案，但至少从问题中得到单元测试才能正常运行。我尝试使用以下类，而不是常规的java.lang.ref.ReferenceQueue：

public class MyReferenceQueue<T> {

    private Set<WeakReference<T>> WeakReferences = new HashSet<WeakReference<T>>();

    public void enqueue(final WeakReference<T> reference) {
        if (reference == null) {
            throw new IllegalArgumentException("Cannot add reference NULL!");
        }
        this.WeakReferences.add(reference);
    }

    public WeakReference<T> poll() {
        Iterator<WeakReference<T>> iterator = this.WeakReferences.iterator();
        while (iterator.hasNext()) {
            WeakReference<T> ref = iterator.next();
            T object = ref.get();
            if (object == null) {
                iterator.remove();
                return ref;
            }
        }
        return null;
    }
}

这种方法的缺点是：

需要明确enqueue WeakReference来跟踪
仅适用于WeakReference s，不适用于PhantomReference s（因为他们的get方法始终按定义返回null）

有了这个，单元测试按预期成功。此外，它不依赖于GC来执行任何特定操作（例如将WeakReference添加到某个队列中），除了它使WeakReference无效（它始终可靠地执行）。也许它会做一些我不知道的超级丑陋的东西，但是现在它完成了工作。

修改

这是使用我的自定义实现的更新的JUnit-Test。

public class WeakReferenceTest { @Test public void weakReferenceTest() { MyReferenceQueue<Object> refQueue = new MyReferenceQueue<Object>(); Object myObject1 = new Object(); Object myObject2 = new Object(); // note that the second constructor argument (the reference queue) is gone... WeakReference<Object> ref1 = new WeakReference<Object>(myObject1); WeakReference<Object> ref2 = new WeakReference<Object>(myObject2); // ... instead we enqueue the references manually now refQueue.enqueue(ref1); refQueue.enqueue(ref2); // the rest of the test remains the same myObject1 = null; // simulate the application running and calling GC at some point System.gc(); myObject1 = ref1.get(); myObject2 = ref2.get(); if (myObject1 != null) { System.out.println("Weak Reference to MyObject1 is still valid."); fail(); } else { System.out.println("Weak Reference to MyObject1 has disappeared."); } if (myObject2 != null) { System.out.println("Weak Reference to MyObject2 is still valid."); } else { System.out.println("Weak Reference to MyObject2 has disappeared."); fail(); } Reference<? extends Object> removedRef = refQueue.poll(); boolean trackedRemoval = false; while (removedRef != null) { if (removedRef == ref1) { System.out.println("Reference Queue reported deletion of MyObject1."); trackedRemoval = true; } else if (removedRef == ref2) { System.out.println("Reference Queue reported deletion of MyObject2."); fail(); } removedRef = refQueue.poll(); } if (trackedRemoval == false) { fail(); } } }

此测试按预期成功。

为什么ReferenceQueue总是空的？

2 个答案: