Java:WeakHashMap的条目逐出时，谁更改大小? [英] Java: who change size when WeakHashMap's entries evict?

问题描述

最近，我尝试弄清楚 WeakHashMap 的工作方式.如代码所示，在 System.gc()和 size 被强引用之后， map.isEmpty()将返回true，那么谁来更改大小?

Recently, I try to figure out how WeakHashMap works. As code shows, map.isEmpty() will return true after System.gc() and size is strong referenced, so who change size?

    WeakHashMap<UniqueKey, BigImage> map = new WeakHashMap<>();

    UniqueKey key = new UniqueKey("abc");
    map.put(key, new BigImage());
    BigImage value = map.get(key);
    assertNotNull(value);

    key = null;
    System.gc();

    Thread.sleep(1000);
    assertTrue(map.isEmpty());

推荐答案

在内部查看实现:

public int size() {
    if (size == 0)
        return 0;
    expungeStaleEntries();
    return size;
}

专门用于 expungeStaleEntries 方法，该方法使用:

specifically for expungeStaleEntries method that uses :

/**
 * Reference queue for cleared WeakEntries
 */
private final ReferenceQueue<Object> queue = new ReferenceQueue<>();

在返回给您之前，此方法还会更新 size .

This method also updates size, before it is returned to you.

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