Microsoft KB Archive/260280

= FIX: java.lang.ClassCastException on Variant Objects =

Article ID: 260280

Article Last Modified on 6/14/2006

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APPLIES TO


 * Microsoft Java Virtual Machine

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This article was previously published under Q260280



SYMPTOMS
A java.lang.ClassCastException is generated when you use a COM object obtained from a com.ms.com.Variant.toObject or Variant.toDispatch. Specifically, the Variant, generated through Variant.clone or Variant.cloneIndirect, contains a reference to a COM proxy (COM interface pointer marshaled to another thread).



CAUSE
The Microsoft virtual machine's implementation of Variant.clone and Variant.cloneIndirect invokes the Win32 VariantCopy and VariantCopyInd functions, respectively, on the caller's thread instead of the Variant's home thread. If the underlying object stored in the Variant is a COM proxy, an attempt to perform a QueryInterface to the COM interface causes a COM error, which is represented as a java.lang.ClassCastException in Java.

An example that demonstrates this problem is included in the &quot;More Information&quot; section.



RESOLUTION
A workaround for this problem is to use ComLib.executeOnContext to call Variant.clone or Variant.cloneIndirect on the Variant's home thread by passing the target Variant as the first parameter to ComLib.executeOnContext.



STATUS
Microsoft has confirmed that this is a bug in the Microsoft products that are listed at the beginning of this article.

This problem was corrected in Windows 2000 Service Pack 1.



Steps to Reproduce Behavior
This code causes the java.lang.ClassCastException that is described in the &quot;Symptoms&quot; section: import com.ms.com.*;

public class Class1 implements NoAutoMarshaling { static Object m_obj; static Variant m_variant; static Variant m_cloned_variant; public static void main (String[] args) { // COM hostable thread ComLib.declareMessagePumpThread; // force creation of Java/COM wrapper for instance // of Class1 m_obj = ComLib.makeProxyRef(new Class1); // invoke Thread1.run on new thread Thread t1 = new Thread(new Thread1); t1.start; // pump messages for COM PumpMessages; } public static class Thread1 implements Runnable { // marshals m_obj to this thread, and stores instance // in m_variant public void run { // COM hostable thread ComLib.declareMessagePumpThread; // Force virtual machine to marshal m_obj // to this thread int nPtr = ComLib.unknownToPtr(m_obj,                                     ComLib.IID_IUnknown); Object objProxy = ComLib.ptrToUnknown(IUnknown.class,                                           nPtr,                                           false); // Store proxy to m_obj in a variant. // Note that COM proxy (objProxy) is only valid // on this thread: // --performing any COM operation on this proxy // will cause a COM error m_variant = new Variant(Variant.VariantObject,                              objProxy); // Invoke Thread2.run on a new thread Thread t2 = new Thread(new Thread2); t2.start; PumpMessages; } }

public static class Thread2 implements Runnable { // Calls Variant.clone on m_variant and performs COM // QueryInterface on internal COM object public void run { // COM hostable thread ComLib.declareMessagePumpThread; try { // Clone m_variant m_cloned_variant = (Variant)m_variant.clone; System.out.println(&quot;after clone&quot;); // Get COM proxy to m_obj from cloned // variant Object obj = m_cloned_variant.getObject; System.out.println(&quot;after getObject&quot;); // Try to perform COM QueryInterface on        // COM proxy. This fails with a        // ClassCastException because the COM // proxy is being invoked on the wrong // thread boolean b = ComLib.supportsInterface(obj,                                             ComLib.IID_IDispatch); System.out.println(&quot;supportsInterface &quot;+b); }     catch (Exception e) { e.printStackTrace; }     try { System.out.print(&quot;Please press enter &quot;+                        &quot;to exit..&quot;); System.in.read; }     catch (Exception e) { }     System.exit(0); } }  // standard Win32 message pump public static void PumpMessages { com.ms.win32.MSG msg = new com.ms.win32.MSG;

while( com.ms.win32.User32.GetMessage( msg, 0, 0, 0 ) ) { com.ms.win32.User32.TranslateMessage( msg ); com.ms.win32.User32.DispatchMessage( msg ); } } }

