Java 转换/类加载器问题 [英] Java casting / classloader issues

问题描述

这是问题的简化版本:

 SomeClass c = (SomeClass) obj.getSomeClassParent()

并非总是如此，但有时会触发异常

not always but it happens sometimes to trigger exception

 org.somepackage.SomeClass can't be cast to org.somepackage.SomeClass 

这怎么可能?我想这与 JAI imageio 是本机 lib 的事实有关，但是中继怎么会发生这种情况?我可能错过了一些东西，但什么?

How is this possible ? I suppose it has something to do with the fact that JAI imageio is native lib, but relay how can this happen ? I'm probably missing something but what ?

I'm using JAI imageio version 1.1 
dcm4che 2.0.21  DICOM lib

这是原始代码

  ImageInputStream iis = ImageIO.createImageInputStream(src);
  Iterator<ImageReader> iter = ImageIO.getImageReadersByFormatName("DICOM");
  ImageReader reader = iter.next();
  DicomImageReadParam param = (DicomImageReadParam) reader.getDefaultReadParam();

和原来的异常

org.dcm4che2.imageio.plugins.dcm.DicomImageReadParam can't be cast to    
org.dcm4che2.imageio.plugins.dcm.DicomImageReadParam

异常图片 http://img215.imageshack.us/img215/3894/exception.jpg

推荐答案

我认为如果

1. 一个 SomeClass 实例是从 ClassLoader X 加载的(所以它的类是 CL X 的 SomeClass 或者我们称之为:CL(X).SomeClass代码>)
2. 但它正在不同的类加载器中进行转换.例如.当前的线程类加载器是 Y 所以 SomeClass 实际上是 CL(Y).SomeClass

1. a SomeClass instance was loaded from ClassLoader X (so its class is SomeClass of CL X or let's call it: CL(X).SomeClass)
2. but it is being cast in a different class loader. E.g. the current Threads class loader is Y so SomeClass is actually CL(Y).SomeClass

所以你有:

• 实例类 = CL(X).SomeClass
• class cast target = CL(Y).SomeClass

或者换句话说 - 不是同一个类 - 因此类抛出异常.

Or in other words - not the same class - thus the class cast exception.

可能重复:ClassCastException 当投射到同一类时- 它也有一些很好的建议.

Possible duplicate of: ClassCastException when casting to the same class - it has some good suggestions as well.

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