java - 为什么ReferenceQueue总是空的？

Question

我正在尝试使用 ReferenceQueue 来释放垃圾收集对象使用的资源。问题是我的引用队列始终为空，即使有证据表明引用的对象之一已被垃圾收集。这是一个非常简单、独立的 JUnit 测试，它说明了我正在尝试执行的操作(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:

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

...这很好，但测试总是失败，因为 trackedRemoval 最后为 false - ReferenceQueue 始终为空.

我使用的ReferenceQueue和/或WeakReference是错误的吗？我也尝试了 PhantomReference，但没有什么区别。

有趣的是，如果将单元测试转换为常规的 public static void main(String[] args) 方法，那么它的工作方式就像一个魅力!

谁能解释一下这种具体行为吗？我长期以来一直在寻找这个问题的答案。

Answer 1

对我来说这似乎是一种竞争条件。当GC确定myObject1引用的对象是GCable时，它会对其进行GC并将其从WeakReference中清除。然后，它将将该 WeakReference 添加到“待处理引用列表”中。有一个引用处理程序线程将从该列表中删除并添加到适当的ReferenceQueue。以上是支持javadoc的实现细节

At the same time or at some later time it will enqueue those newly-cleared weak references that are registered with reference queues.

当您的代码达到

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

引用处理程序线程不得将您的 WeakReference 添加到 ReferenceQueue 中，因此 poll 返回 null .