getGenericSuperclass() in Java. How does it work?

Question

Let's consider the following piece of code:

package index;

public class Main {
    public static void main(String[] args) {
        Inner inner = new Inner();
        Type t = inner.getClass().getGenericSuperclass();
        ParameterizedType p = (ParameterizedType) t;
        Type[] a = p.getActualTypeArguments();
        try {
            Custom c = (Custom) ((Class) a[0]).newInstance();
            c.f();
        } catch (Exception e){}
    }

    private static abstract class AbstractClass<T> {
        public abstract void doSth();
    }
    private static class Inner extends AbstractClass<Custom>{
        public void doSth() {
        }
    }
    private static class Custom{
        public Custom(){

        }
        public void f(){
            System.out.println("Custom");
        }
    }
}

1. It works and I cannot understand how does it works. It shows that it is possible to get information about superclasses parameters. I don't know how it is possible to get a such information because I looked at bytecode and there is no such information:

2. Is it ok to do it?

Answer 1

There is a Signature attribute in .class file, see JVMS §4.7.9.

If you decompile Main.Inner with javap -verbose, you'll find this attribute:

Signature: #14    // Lindex/Main$AbstractClass<Lindex/Main$Custom;>;

Class.getGenericSuperclass implementation extracts this attribute with the help of a native method and then parses, validates and converts it to Java representation. So, it is absolutely possible and OK to get this information.