Java7 WeakHashMap isEmpty() 似乎不对

Question

我正在尝试使用 Java7 的 WeakHashMap，我发现它的 isEmpty() 方法给我错误的结果。

import java.util.Map;
import java.util.WeakHashMap;

public class Test
{

    public static void main(final String[] args)
    {
        final Map<String, Boolean> map = new WeakHashMap<>();

        String b = new String("B");
        map.put(b, true);
        b = null;

        System.gc();
        System.out.println(map.isEmpty());
        System.out.println(map.keySet().isEmpty());
        System.out.println(map);
    }

}

实际结果：

错误

正确

{}

也就是说，

map.isEmpty() 和 map.keySet().isEmpty() 不一致。有人可以帮我理解吗？非常感谢。

Answer 1

你应该阅读 WeakHashMap:

的 javadoc

The behavior of the WeakHashMap class depends in part upon the actions of the garbage collector, so several familiar (though not required) Map invariants do not hold for this class. Because the garbage collector may discard keys at any time, a WeakHashMap may behave as though an unknown thread is silently removing entries. In particular, even if you synchronize on a WeakHashMap instance and invoke none of its mutator methods, it is possible for the size method to return smaller values over time, for the isEmpty method to return false and then true, for the containsKey method to return true and later false for a given key, for the get method to return a value for a given key but later return null, for the put method to return null and the remove method to return false for a key that previously appeared to be in the map, and for successive examinations of the key set, the value collection, and the entry set to yield successively smaller numbers of elements.

总而言之，您看到的效果是完全有效的。

Answer 2

WeakHashMap::isEmpty 说：

...This result is a snapshot, and may not reflect unprocessed entries that will be removed before next attempted access because they are no longer referenced.

所以你会期望 isEmpty() returns GC 和访问后的正确值。此代码演示了这一点：

public class Scratch1 {
    public static void main(final String[] args) {
        final Map<String, Boolean> map = new WeakHashMap<>();

        String b = new String("B");
        map.put(b, true);
        b = null;

        System.gc();

        // map not internally accessed at this point
        System.out.println(map.isEmpty());

        // let's access the Map's internals (and hopefully coerce
        // it into removing no-longer-referenced keys)
        System.out.println(map.keySet()
                              .isEmpty());

        // map HAS now been accessed
        System.out.println(map.isEmpty());
    }

}

产量：

false
true
true

Java7 WeakHashMap isEmpty() 似乎不对

Java7 WeakHashMap isEmpty() seems wrong

java

weakhashmap