java.lang.ClassCastException的含义：someClass与someClass不兼容 [英] Meaning of java.lang.ClassCastException: someClass incompatible with someClass

问题描述

我在XPage应用程序中遇到偶然的异常：

I was experiencing occasional exceptions in XPages application:

java.lang.ClassCastException: someClass incompatible with someClass.

两个提到的类都是相同的，它是用作会话bean的类。我无法谷歌任何覆盖我的问题。对此的通常解释是设计元素的变化，而不是我的情况。

Both mentioned classes are the same, it is class used as session bean. I was not able to google anything covering my problem. Usual explanation for this was change in design elements, not my case.

从那时起，XPage应用程序变得无法使用（使用会话bean someClass的页面），直到重启http任务，或者是faces-config.xml的重新保存。

The XPage application become unusable (pages using session bean someClass) since that moment, until restart of http task, or resave of faces-config.xml.

在某些情况下，这与其他例外有关：

In some cases this is related to other exception:

com.ibm.jscript.InterpretException: Script interpreter error, line=x, col=y: 
Java method 'method(signature containg someClass)'
on java class 'someOtherClass' not found

此行为背后是什么？

推荐答案

Philippe Riand通过电子邮件解释了这一点：

Philippe Riand explaned this by email:

此类强制转换是因为同一班级由两个不同的班级装载机装载两次。因此，从Java的角度来看，它们是不同的，并且转换失败。

This class cast happens because the same class had been loaded twice by 2 different class loaders. Thus, from a Java standpoint, they are different and the cast fails.

现在，每个XPages应用程序都有自己的类加载器。但是，每次通过Domino Designer进行设计更改时，都会丢弃此类加载器。这是必需的，因为对XPages的更改会生成一个新的Java类，然后应该加载而不是之前的Java类。发生这种情况时，将丢弃类加载器并创建一个新的。然后，所有与应用程序相关的类都会在需要时重新加载，即使它们没有更改。这是J2EE服务器实现的常见行为。
也就是说，如果您的代码缓存了在设计更改发生时 not 被丢弃的范围内的对象，则可能会发生这种情况。例如，applicationScope&当设计更改发生时，sessionScope当前不会被丢弃，这可能会导致此问题。这是一个设计选择，因为丢弃范围有时会提供糟糕的开发人员体验，但有这个缺点。

Now, each XPages application is having its own classloader. But this class loader is discarded each time a design change happens to the application, through Domino Designer for example. This is required as a change to an XPages generates a new Java class that should then be loaded instead of the previous one. When this happens, the classloader is discarded and a new is created. Then all the application related classes are reloaded, as they are needed, even though they didn't change.This is a common behavior implemented by J2EE servers. That said, if your code is caching an object in a scope that is not discarded when a design change occurs, then this is likely to happen. For example, the applicationScope & sessionScope are currently not discarded when a design change happens, which might lead to this problem. This was a design choice as discarding the scopes sometimes provides a bad developer experience, but with this drawback.

最后，保存faces-config.xml可以解决此问题。保存此文件后，整个模块将从内存中丢弃，包括范围，这就解释了它的工作原理。对自定义Java类进行更改应重新加载模块并删除问题。

Finally, saving faces-config.xml works as a workaround. When this file is saved, then the entire module is discarded from memory, including the scopes, This explains why it works. Making a change to your custom Java class should reload the module and remove the issue.

因此，似乎将bean（甚至是间接的）放入sessionScope或applicationScope是原因。

So it seems putting beans (even indirectly) into sessionScope or applicationScope is the cause.

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