Groovy抽象控制器<Domain Class>

Question

Embarrassed, but I can't find the errors in my ways. No matter what I try M is always object (not the actual generic type). I have a simple setup( M is a grails domain):

class NewsController extends AbstractController<News> {

}

class AbstractController<M> {

    def show(Long id){
        log.info(M)
        // prints: INFO  common.AbstractController  - class java.lang.Object
        def entity = M.get(id)  //errors
        // .. other code
    }
}

Please be gentle, this feels like a brain fart/something stupid.

Answer 1

I would say Type Erasure(or its groovy equivalent).

So the M is there just for compile time checking(At least in java). Now how this works in an interpreted language is a little more interesting, but I think your are mostly likely a victim of related circumstances.

It appears that groovy has even stricter requirements throwing away the generic information from source completely. Which probably leads to some interesting behavioral stuff at runtime.

http://groovy.codehaus.org/Generics

Answer 2

Well simple as it may be, the alternative I came up with (because of type erasures , IMHO a major flaw rendering class generics useless) was:

class AbstractController{
    abstract getDomainClass();   
    def show(Long id){
       log.info(getDomainClass())
    }
    def entity = getDomainClass().get(id) 
}

class NewsController extends AbstractController {
    def getDomainClass(){
        return News
    }
 }