blocks|key|637747|text|您可能想看看Java提供的不同引用类型：strong,+soft,+weak,+and+phantom。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|637748|对于缓存，您通常会将引用包装到WeakReference或SoftReference中，一旦没有更多的强引用，就让对象被收集。|637749|entityMap|0|LINK|mutability|MUTABLE|url|http://weblogs.java.net/blog/enicholas/archive/2006/05/understanding_w.html^0|K|V|0|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@$A|O|B|P|1|Q]]|C|$]]|$1|D|3|E|5|6|7|R|8|@]|9|@]|C|$]]|$1|F|3|-4|5|6|7|S|8|@]|9|@]|C|$]]]|G|$H|$5|I|J|K|C|$L|M]]]]

You might want to take a look at different reference types Java provides: <a href="http://weblogs.java.net/blog/enicholas/archive/2006/05/understanding_w.html" rel="nofollow noreferrer">strong, soft, weak, and phantom</a>.

For cache you would normally wrap a reference into WeakReference or SoftReference and let the object be collected once there are no more strong references left.

blocks|key|743482|text|type|unstyled|depth|inlineStyleRanges|entityRanges|data|743483|我有一个对象缓存|blockquote|743484|743485|743486|考虑使用WeakHashMap而不是HashMap来获取它们。|offset|length|style|CODE|743487|以下是API的摘录：|743488|743489|一个基于哈希表的Map实现，具有弱密钥。当+WeakHashMap+中的条目不再是通常使用的时，该条目将被自动删除。更准确地说，给定键的映射不会阻止该键被垃圾收集器丢弃，也就是说，使其成为可终结键、终结键，然后回收。当一个键被丢弃时，它的条目将有效地从映射中删除，因此这个类的行为与其他Map实现略有不同。|BOLD|743490|743491|entityMap|0|LINK|mutability|MUTABLE|url|http://java.sun.com/javase/6/docs/api/java/util/WeakHashMap.html^0|0|0|0|0|4|B|I|7|4|B|0|0|0|0|8|3|M|B|3Z|3|M|B|Y|C|0|0^^$0|@$1|2|3|-4|4|5|6|10|7|@]|8|@]|9|$]]|$1|A|3|B|4|C|6|11|7|@]|8|@]|9|$]]|$1|D|3|-4|4|5|6|12|7|@]|8|@]|9|$]]|$1|E|3|-4|4|5|6|13|7|@]|8|@]|9|$]]|$1|F|3|G|4|5|6|14|7|@$H|15|I|16|J|K]|$H|17|I|18|J|K]]|8|@$H|19|I|1A|1|1B]]|9|$]]|$1|L|3|M|4|5|6|1C|7|@]|8|@]|9|$]]|$1|N|3|-4|4|5|6|1D|7|@]|8|@]|9|$]]|$1|O|3|P|4|C|6|1E|7|@$H|1F|I|1G|J|K]|$H|1H|I|1I|J|K]|$H|1J|I|1K|J|K]|$H|1L|I|1M|J|Q]|$H|1N|I|1O|J|Q]]|8|@]|9|$]]|$1|R|3|-4|4|5|6|1P|7|@]|8|@]|9|$]]|$1|S|3|-4|4|5|6|1Q|7|@]|8|@]|9|$]]]|T|$U|$4|V|W|X|9|$Y|Z]]]]

<blockquote>
 I have a cache of objects
</blockquote>

Consider using a <a href="http://java.sun.com/javase/6/docs/api/java/util/WeakHashMap.html" rel="nofollow noreferrer"><code>WeakHashMap</code></a> instead of <code>HashMap</code> to get hold of them all.

Here's an extract of the API:

<blockquote>
 A hashtable-based <code>Map</code> implementation with weak keys. An entry in a <code>WeakHashMap</code> will automatically be removed when its key is no longer in ordinary use. More precisely, the presence of a mapping for a given key will not prevent the key from being discarded by the garbage collector, that is, made finalizable, finalized, and then reclaimed. When a key has been discarded its entry is effectively removed from the map, so this class behaves somewhat differently from other <code>Map</code> implementations. 
</blockquote>

blocks|key|2152263|text|没有一种发布的方式可以公开这样的信息。让语言来做垃圾收集，JVM供应商的工作就是按照语言的指定来实现它。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2152264|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|$]]

There isn't a published way that exposes such an information. Let the language do the garbage collection, it is the job of the JVM vendor to implement it as specified by the language.

blocks|key|743555|text|不清楚你真正想做的是什么，所以我将逐字回答你的问题。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|743556|743557|有没有办法找到一个对象的所有引用？|blockquote|743558|743559|743560|没有办法做到这一点。您可以从包含引用的变量或字段开始，但Java不允许您从另一个方向开始。从理论上讲，可以使用Java调试器机制来做这类事情，但它太复杂且代价太高，不切实际。|743561|743562|我有一个对象的缓存，并希望定期扫描它，看看删除对象是否会导致它被销毁。|743563|743564|743565|(我假设您的意思是从缓存中删除对象。)再说一次，我不明白如何在Java中做到这一点。SoftReference和WeakReference类将允许您判断对象是否已被垃圾收集，但不能告诉您它是否将被垃圾收集。再说一次，这种事情在理论上可能是可以实现的，但它太复杂了，而且成本太高，不切实际。|743566|但是，如果您的真正目标是实现一个不会阻止垃圾收集对象的缓存，则应该能够使用WeakHashMap或通过构建自己的适当使用WeakReference或SoftReference实例的缓存来实现这一点。|743567|编辑：我想你也可以考虑一个显式的引用计数方案。然而，这种方案很难在Java中正确实现。|offset|length|style|BOLD|743568|与C%2B%2B不同，在Java语言中，当变量超出作用域时，不会调用析构函数；当属性的父对象被垃圾回收时，不会调用属性。要实现引用计数，您需要：|743569|743570|将所有赋值的显式代码递增/递减计数添加到引用计数的变量和字段，|unordered-list-item|743571|添加try+{+...+}+finally块以处理引用计数的变量可能超出作用域的所有可能方式，并将|CODE|743572|和add+finalize方法添加到具有引用计数类型属性的每个类。|743573|<+code+>F215|743574|这在理论上是可能的，但(国际海事组织)除了最简单的情况外，要想在所有情况下都做对太难了。而且(我重复一遍)，这在Java中比在C%2B%2B中要难得多，因为Java没有析构函数和赋值操作符的重载。|743575|entityMap^0|0|0|0|0|0|0|0|0|0|0|0|0|0|2|0|0|0|0|2|J|0|5|8|0|0|0^^$0|@$1|2|3|4|5|6|7|1F|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|1G|8|@]|9|@]|A|$]]|$1|C|3|D|5|E|7|1H|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|1I|8|@]|9|@]|A|$]]|$1|G|3|-4|5|6|7|1J|8|@]|9|@]|A|$]]|$1|H|3|I|5|6|7|1K|8|@]|9|@]|A|$]]|$1|J|3|-4|5|6|7|1L|8|@]|9|@]|A|$]]|$1|K|3|L|5|E|7|1M|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|1N|8|@]|9|@]|A|$]]|$1|N|3|-4|5|6|7|1O|8|@]|9|@]|A|$]]|$1|O|3|P|5|6|7|1P|8|@]|9|@]|A|$]]|$1|Q|3|R|5|6|7|1Q|8|@]|9|@]|A|$]]|$1|S|3|T|5|6|7|1R|8|@$U|1S|V|1T|W|X]]|9|@]|A|$]]|$1|Y|3|Z|5|6|7|1U|8|@]|9|@]|A|$]]|$1|10|3|-4|5|6|7|1V|8|@]|9|@]|A|$]]|$1|11|3|12|5|13|7|1W|8|@]|9|@]|A|$]]|$1|14|3|15|5|13|7|1X|8|@$U|1Y|V|1Z|W|16]]|9|@]|A|$]]|$1|17|3|18|5|13|7|20|8|@$U|21|V|22|W|16]]|9|@]|A|$]]|$1|19|3|1A|5|6|7|23|8|@]|9|@]|A|$]]|$1|1B|3|1C|5|6|7|24|8|@]|9|@]|A|$]]|$1|1D|3|-4|5|6|7|25|8|@]|9|@]|A|$]]]|1E|$]]

It is not clear what you are really trying to do, so I will answer your question literally.

<blockquote>
 Is there any way to find all references to an object (in Java)?
</blockquote>

There is no way to do this. You can go from a variable or field containing a reference, but Java does not allow you to go in the other direction. It is theoretically possible to do this kind of thing using the Java debugger mechanisms, but it would be too complicated and expensive to be practical.

<blockquote>
 I have a cache of objects, and would like to periodically scan it to see if removing the object will cause it to be destroyed.
</blockquote>

(I assume that you mean removing the object from the cache.) Again, I don't see how you could do this in Java. The SoftReference and WeakReference classes will allow you to tell if an object has been garbage collected, but not if it will be garbage collected. Once again, this kind of thing might theoretically implementable, but it is too complex and expensive to be practical.

However, if your real goal is to implement a cache that does not stop objects being garbage collected, you should be able to do this using WeakHashMap, or by building your own cache that appropriately use WeakReference or SoftReference instances.

EDIT: I suppose that you could also consider an explicit reference counting scheme. However, such a scheme would be difficult to implement properly in Java. 

Unlike C++, no destructor gets called in Java when a variable goes out of scope, or on an attribute when its parent object is garbage collected. To implement reference counting, you would need to:

<ul>
<li>add explicit code increment/decrement counts on all assignments to reference counted variables and fields, </li>
<li>add <code>try { ... } finally</code> blocks to deal with all possible ways that a reference counted variable can go out of scope, and </li>
<li>and add <code>finalize</code> methods to every class that has an attribute of the reference counted type.</li>
</ul>

It is theoretically possible, but (IMO) it is too difficult to get right in all but the simplest of cases. And (just repeating myself), this is much harder to do in Java than in C++ because Java doesn't have destructors and overloading of assignment operators.

blocks|key|743441|text|您可以使用引用计数方案。有一个管理对这些对象的访问的类，这样你就需要在这个类上调用get()+(可能带有一些参数)来获取一个实例-这应该会为该实例增加一个计数器。当您的客户端代码处理完一个对象时，它应该返回该对象，并递减计数器。则计数为0的所有对象将只被容器引用。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|743442|当然，您需要非常小心，以免有任何代码忘记在计数器的引用超出作用域之前递减计数器。|743443|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 could use a reference-counting scheme. Have a class that manages access to these objects, such that you need to call get() on the class (possibly with some argument) to get an instance - this should increment a counter for that instance. When your client code is done with an object, it should return it, decrementing the counter. Then all objects with a count of 0 would be referenced by no one but the container. 

Of course, you would need to be very careful that you don't have any code that forgets to decrement the counter before its reference goes out of scope.

Is there any way to find all references to an object (in Java)?

I have a cache of objects, and would like to periodically scan it to see if removing the object will cause it to be destroyed.

Java references

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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有没有办法找到一个对象的所有引用(在Java中)？我有一个对象的缓存，并希望定期扫描它，看看删除对象是否会导致它被销毁。

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