blocks|key|1733528|text|例如，如果您想要跟踪某个类创建的所有对象。为了仍然允许对这些对象进行垃圾回收，您需要保留对这些对象的弱引用的列表/映射，而不是对象本身。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1733529|现在如果有人能向我解释幻影的引用，我会很高兴...|1733530|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

If you for example want to keep track of all objects created of a certain class. To still allow these objects to be garbage collected, you keep a list/map of weak references to the objects instead of the objects themselves.

Now if someone could explain phantom-references to me, I'd be happy...

blocks|key|1518593|text|我对WeakReferences的一个实际应用是，如果你有一个很少使用的非常大的对象。当不需要它时，您不希望将其保留在内存中；但是，如果另一个线程需要相同的对象，您也不希望它们中的两个在内存中。您可以在某个地方保留对对象的弱引用，并在使用它的方法中保留硬引用；当这两个方法都完成时，对象将被收集。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1518594|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

One real world use I had for WeakReferences is if you have a single, very large object that's rarely used. You don't want to keep it in memory when it's not needed; but, if another thread needs the same object, you don't want two of them in memory either. You can keep a weak reference to the object somewhere, and hard references in the methods that use it; when the methods both finish, the object will be collected.

blocks|key|1518677|text|您可以使用弱caching实现无资源缓存，以创建可扩展的对象。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1518678|但请注意，不希望有可变对象。我用它来缓存查询结果(执行大约需要400ms)到一个很少更新的文本搜索引擎。|1518679|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

you can use weakhashmap to implement a resource-free caching for expansive object creation. 

but note that it is not desireable to have mutable objects.
i used it to cache query results (which take about 400 ms to execute) to a text-search engine, which is rarely updated.

blocks|key|1518744|text|如上所述，只要强引用存在，弱引用就会一直存在。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1518745|一个示例用法是在侦听器中使用WeakReference，这样一旦对其目标对象的主要引用消失，侦听器就不再活动。请注意，这并不意味着将WeakReference从listeners列表中删除，仍然需要清理，但可以在计划的时间执行。这还可以防止侦听的对象持有强引用，并最终成为内存膨胀的来源。示例:引用生命周期比窗口更长的模型的Swing+GUI组件。|1518746|在使用如上所述的侦听器时，我们很快意识到，从用户的角度来看，对象是“立即”收集的。|1518747|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|H|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|I|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|J|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|G|$]]

As stated above, weak reference are held for as long as a strong reference exists.

An example usage would be to use WeakReference inside listeners, so that the listeners are no longer active once the main reference to their target object is gone.
Note that this does not mean the WeakReference is removed from the listeners list, cleaning up is still required but can be performed, for example, at scheduled times.
This has also the effect of preventing the object listened to from holding strong references and eventually be a source of memory bloat.
Example: Swing GUI components refering a model having a longer lifecycle than the window.

While playing with listeners as described above we rapidly realised that objects get collected "immediately" from a user's point of view.

blocks|key|1518859|text|尤其是WeakReference和WeakHashMap的一种常见用法是向对象添加属性。有时，你想给对象添加一些功能或数据，但是子类化和/或组合不是一个选项，在这种情况下，显而易见的事情是创建一个哈希图，将你想要扩展的对象链接到你想要添加的属性。然后，当您需要该属性时，您可以在地图中查找它。但是，如果要添加属性的对象往往会被销毁并大量创建，则最终会导致地图中的许多旧对象占用大量内存。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1518860|如果您使用WeakHashMap，那么一旦程序的其余部分不再使用这些对象，这些对象就会离开您的地图，这是所需的行为。|1518861|我必须这样做，以便向java.awt.Component添加一些数据，以绕过JRE1.4.2和1.5之间的变化，我可以通过对每个我感兴趣的组件(JButton，JFrame，JPanel...)进行子类化来修复它。但用更少的代码，这就容易多了。|1518862|entityMap^0|3|D|H|B|0|5|B|0|A|I|20|7|28|6|2F|6|0^^$0|@$1|2|3|4|5|6|7|L|8|@$9|M|A|N|B|C]|$9|O|A|P|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|Q|8|@$9|R|A|S|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|T|8|@$9|U|A|V|B|C]|$9|W|A|X|B|C]|$9|Y|A|Z|B|C]|$9|10|A|11|B|C]]|D|@]|E|$]]|$1|J|3|-4|5|6|7|12|8|@]|D|@]|E|$]]]|K|$]]

One Common use of <code>WeakReference</code>s and <code>WeakHashMap</code>s in particular is for adding properties to objects. Occasionally you want to add some functionality or data to an object but subclassing and/or composition are not an option in that case the obvious thing to do would be to create a hashmap linking the object you want to extend to the property you want to add. then whenever you need the property you can just look it up in the map. However, if the objects you are adding properties to tend to get destroyed and created a lot, you can end up with a lot of old objects in your map taking up a lot of memory.

If you use a <code>WeakHashMap</code> instead the objects will leave your map as soon as they are no longer used by the rest of your program, which is the desired behavior.

I had to do this to add some data to <code>java.awt.Component</code> to get around a change in the JRE between 1.4.2 and 1.5, I could have fixed it by subclassing every component I was interested int (<code>JButton</code>, <code>JFrame</code>, <code>JPanel</code>....) but this was much easier with much less code.

The use of weak references is something that I've never seen an implementation of so I'm trying to figure out what the use case for them is and how the implementation would work. When have you needed to use a <code>WeakHashMap</code> or <code>WeakReference</code> and how was it used?

When would you use a WeakHashMap or a WeakReference?

弱引用的使用是我从未见过的实现，所以我试图弄清楚它们的用例是什么，以及实现是如何工作的。您何时需要使用WeakHashMap或WeakReference？它是如何使用的？

问什么时候会使用WeakHashMap或WeakReference？
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