"java.lang.VerifyError: Cannot inherit from final class" when running GWTTestCase - google-app-engine

Question

I just wrote my very first GWTTestCase but running it always gives me a VerifyError. Here's my embryo of a great testsuite to be :
public void test() {
Invoice invoice = new Invoice();
invoice.setInvoiceDate(DateTimeFormat.getFormat(DomainResource.DATE_PATTERN).format(new Date()));
InvoiceEditorDriver driver = GWT.create(InvoiceEditorDriver.class);
InvoiceNumberEditor editor = new InvoiceNumberEditor();
driver.initialize(editor);
driver.edit(invoice);
assertFalse(driver.isDirty());
}
For completeness, I use Google AppEngine and GWT together, and I think this may relate to the security sandbox of AppEngine.
The class that is being loaded is "com.gargoylesoftware.htmlunit.SocksSocketFactory".
The full stacktrace here :
Exception in thread "htmlUnit client thread" java.lang.VerifyError: Cannot inherit from final class
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:800)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:449)
at java.net.URLClassLoader.access$100(URLClassLoader.java:71)
at java.net.URLClassLoader$1.run(URLClassLoader.java:361)
at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
at java.lang.ClassLoader.loadClass(ClassLoader.java:425)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
at java.lang.ClassLoader.loadClass(ClassLoader.java:358)
at com.gargoylesoftware.htmlunit.HttpWebConnection.createHttpClient(HttpWebConnection.java:542)
at com.gargoylesoftware.htmlunit.HttpWebConnection.getHttpClient(HttpWebConnection.java:506)
at com.gargoylesoftware.htmlunit.HttpWebConnection.getResponse(HttpWebConnection.java:150)
at com.gargoylesoftware.htmlunit.WebClient.loadWebResponseFromWebConnection(WebClient.java:1281)
at com.gargoylesoftware.htmlunit.WebClient.loadWebResponse(WebClient.java:1198)
at com.gargoylesoftware.htmlunit.WebClient.getPage(WebClient.java:307)
at com.gargoylesoftware.htmlunit.WebClient.getPage(WebClient.java:376)
at com.gargoylesoftware.htmlunit.WebClient.getPage(WebClient.java:361)
at com.google.gwt.junit.RunStyleHtmlUnit$HtmlUnitThread.run(RunStyleHtmlUnit.java:136)
I tried excluding GAE validation for given class but without success. I'm not sure if excluding validation is sufficient and if it's possilble to setup validation exclude rules for files in a jar.

Answer 1

Might be a classpath problem.
There was a similar problem on the mailing list, solved by removing a second instance of the PlainSocketFactory from the classpath.