Java的WeakHashMap和缓存：为什么它引用了键而不是值？ [英] Java's WeakHashMap and caching: Why is it referencing the keys, not the values?

问题描述

Java的 WeakHashMap 经常被认为对缓存很有用。虽然它的弱引用是根据地图的键而不是它的值定义的，但看起来很奇怪。我的意思是，这是我想要缓存的值，除了缓存之外没有其他人强烈引用它们，我想要收集垃圾，不是吗？

Java's WeakHashMap is often cited as being useful for caching. It seems odd though that its weak references are defined in terms of the map's keys, not its values. I mean, it's the values I want to cache, and which I want to get garbage collected once no-one else besides the cache is strongly referencing them, no?

In哪种方式有助于保持对密钥的弱引用？如果你执行 ExpensiveObject o = weakHashMap.get（some_key），那么我希望缓存保持为'o'，直到调用者不能保持强引用了，我根本不关心字符串对象some_key。

In which way does it help to hold weak references to the keys? If you do a ExpensiveObject o = weakHashMap.get("some_key"), then I want the cache to hold on to 'o' until the caller doesn't hold the strong reference anymore, and I don't care at all about the string object "some_key".

我错过了什么吗？

推荐答案

WeakHashMap 不是用作缓存，至少是大多数人对它的看法。如你所说，它使用弱键，而不是弱值，因此它不是为大多数人想要使用它而设计的（事实上，我已经 see 人们错误地使用它。

WeakHashMap isn't useful as a cache, at least the way most people think of it. As you say, it uses weak keys, not weak values, so it's not designed for what most people want to use it for (and, in fact, I've seen people use it for, incorrectly).

WeakHashMap主要用于保存关于生命周期无法控制的对象的元数据。例如，如果您有一堆对象通过您的类，并且您希望跟踪有关它们的额外数据，而不需要在它们超出范围时得到通知，并且不参考它们使它们保持活动状态。

WeakHashMap is mostly useful to keep metadata about objects whose lifecycle you don't control. For example, if you have a bunch of objects passing through your class, and you want to keep track of extra data about them without needing to be notified when they go out of scope, and without your reference to them keeping them alive.

一个简单的例子（以及我之前使用的一个例子）可能是这样的：

A simple example (and one I've used before) might be something like:

WeakHashMap<Thread, SomeMetaData>

您可以跟踪系统中的各种线程正在做什么;当线程死亡时，该条目将从您的地图中静默删除，如果您是最后一次引用该线程，则不会使该线程被垃圾收集。然后，您可以迭代该映射中的条目，以找出有关系统中活动线程的元数据。

where you might keep track of what various threads in your system are doing; when the thread dies, the entry will be removed silently from your map, and you won't keep the Thread from being garbage collected if you're the last reference to it. You can then iterate over the entries in that map to find out what metadata you have about active threads in your system.

请参阅 WeakHashMap不在缓存中！以获取更多信息。

See WeakHashMap in not a cache! for more information.

对于您所使用的缓存类型，请使用专用缓存系统（例如 EHCache ）或者查看 google-collections' MapMaker类;类似

For the type of cache you're after, either use a dedicated cache system (e.g. EHCache) or look at google-collections' MapMaker class; something like

new MapMaker().weakValues().makeMap();

会做你想做的事，或者如果你想得到想象，你可以添加定时到期：

will do what you're after, or if you want to get fancy you can add timed expiration:

new MapMaker().weakValues().expiration(5, TimeUnit.MINUTES).makeMap();

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