Java casting problem

Question

Here is the simplified version of the problem:

 SomeClass c = (SomeClass) obj.getSomeClassParent()

not always but it happens sometimes to trigger exception

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

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

Here is the original code

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

And the original exception

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

Answer 1

I think it can happen if

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

So you have:

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

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

---

Possible duplicate of: http://stackoverflow.com/questions/826319/classcastexception-when-casting-to-the-same-class - it has some good suggestions as well.

Answer 2

Strange have you tried casting it to a Object it extends, not sure if it has the functionality you require but might be worth trying to see if it still throws the exception.

Answer 3

From the image I see that it looks like a web application. I read 'catalina'. So there is a big chance, that it is a pure classloading problem.

It could happen, for example, if the ImageReader you get from the ImageIO class was loaded by a different classloader (maybe because it's deployed in a different webapp), so the DicomImageReadParam object returned by the getDefaultReadParam() method is an instance of a - technically spoken - different class.