JarClassLoader & Groovy -- side effects

It appears that when you ask JarClassLoader to load compiled Groovy classes (compiled with Ant groovyc), the resulting classes do not behave the same way as regular java classes. In particular, the Groovy classes don't know what package they belong to:

 MyClass.java 
  
 println MyClass.class.getPackage() 
 >my.package
  
 MyClass.groovy 
  
 println MyClass.class.getPackage() 
 >null

The Java class gives the correct answer whether it is loaded by ClasspathClassLoader or by JarClassLoader. However, the Groovy class only works when called by ClasspathClassLoader, not by JarClassLoader.