Groovy Generics Not Playing nice with java Proxies - java.lang.ClassCastException: $Proxy374 cannot be cast to ...

We've also been annoyed by this bug. Rather than hacking Spring, we worked around it with an extra layer of indirection. We wanted to use this:

@Transactional
interface OurInterface<T extends TypeInterface> {
  T getOne(String filter)
}

class OurImpl implements OurInterface<TypeImpl> {
  TypeImpl getOne(String filter) { ... neat groovy code ... }
}

That exhibits the bug described if we use OurImpl directly. However, fronting it with a Java implementation works. We created this java class, after renaming OurImpl to OurGroovyImpl:

class OurImpl implements OurInterface<TypeImpl> {
  private OurGroovyImpl groovy = new OurGroovyImpl();
  TypeImpl getOne(String filter) { return groovy.getOne(filter); }
}

Not ideal, but it works.

FYI Using @Transactional on interfaces is not considered best practices. http://static.springsource.org/spring/docs/3.0.x/reference/htmlsingle/

Spring recommends that you only annotate concrete classes (and methods of concrete classes) with the @Transactional annotation, as opposed to annotating interfaces. You certainly can place the @Transactional annotation on an interface (or an interface method), but this works only as you would expect it to if you are using interface-based proxies. The fact that Java annotations are not inherited from interfaces means that if you are using class-based proxies (proxy-target-class="true") or the weaving-based aspect (mode="aspectj"), then the transaction settings are not recognized by the proxying and weaving infrastructure, and the object will not be wrapped in a transactional proxy, which would be decidedly bad.