Question

I read somewhere that it’s impossible to get generic interface of a class in run time. so how does hibernate do this? for example in a OneToMany mapping how does hibernate find out the Many part class (using annotation)?

class A{
    ...
    @OneToMany(mapped-by="a")
    public List<B> getBs(){ 
    ...
    }
}

Answer 1

I read somewhere that it’s impossible to get generic interface of a class in run time.

It depends. You can’t ask an instance of a generic type what its type arguments are – but you can get that information from the metadata about classes, method return types etc.

So for example, Method.getGenericReturnType returns a Type rather than a Class<?>, and that lets you get at the relevant information.

Sample code:

import java.lang.reflect.*;
import java.util.*;

public class Test {

    public static void main(String[] args) throws Exception {
        Method method = Test.class.getMethod("getList");
        Type returnType = method.getGenericReturnType();
        System.out.println(returnType); // java.util.List<java.lang.Integer>
        ParameterizedType parameterized = (ParameterizedType) returnType;
        System.out.println(parameterized.getActualTypeArguments()[0]);
    }    

    public List<Integer> getList() {
        return null;
    }
}