Annotation-Processing Using Custom ClassLoader Fails

Question

Hi,
I have the following problem concerning annotations.
I am using Jaxb2 to marshal some classes and Jaxb relies heavily on processing annotation information contained in them.
If I put the Jaxb JAR's directly in the classpath everything works fine.
However, if I use a custom URLClassLoader to load all the Jaxb-related classes the annotation processing fails thus making Jaxb believe that the objects can't be marshaled.
Note that the classes to marshal containing the annotations to read are loaded with a different ClassLoader which is higher in the hierarchy.
I tracked down the problems to the class RuntimeInlineAnnotationReader of the Jaxb API which simply calls Class.getAnnotion(..).
When searching for this bug I also found the bug report 5015623 which relates to this problem. But it is rather outdated, so I hope that there are some better solutions than just ignoring it.
I wouldn't like having the Jaxb JARs on the global classpath 'cause they are used in only one class out of 100 and thats not worth the possible conflicts with other 3rd party libs.
Best regards,
beebop

Answer 1

No problem, I will give you some details about my architecture.
First of all I encapsulated the code which uses the Jaxb-API in one class, say Foo, which implements the interface FooInterface.
Secondly I load the Foo class in another class, Bar, using my derivation of URLClassLoader, called MyClassLoader. Then I use the interface to marshal an object gen of type GenClass where GenClass was generated using xjc:
class Bar {
  void method(URL[] urls, GenClass gen) {
    ClassLoader parent = Bar.class.getClassLoader();
    ClassLoader myCL = new MyClassLoader(urls,parent);
    Class clazz = myCL.loadClass("Foo");
    FooInterface foo = (FooInterface)clazz.newInstance();
    foo.marshal(gen);
}So my class loader will be a child of the current class loader. The delegation model of class loading is reversed in MyClassLoader so that first the urls will be checked and then, if the class was not found, the parent class loader.
Note that GenClass and its ObjectFactory which acutally contain the annotations Jaxb needs will be loaded by the current class loader, not my own. All classes of the Jaxb-API will be loaded by MyClassLoader. If I try to marshal gen, the annotations of the ObjectFactory corresponding to GenClass won't be readable causing the Class.getAnnotation(...) method called in the class RuntimeInlineAnnotationReader to return null.
If I don't use my own class loader, say
FooInterface foo = new Foo();everything works fine and the annotations of the ObjectFactory are loaded correctly.
I observed the different behaviour by stepping through the classes and methods around JaxbContext.newInstance(...).
MyClassLoader is not that different from URLClassLoader, only the loadClass method is overridden to change the delegation behaviour. Thats why I suspect an error in the annotations framework and not in my class loader but I may be mistaken.
Best regards,
beebop