How can I get all subclasses of a class in Java?

So I have a class A and there are multiple classes that extend from class A. I create an instance of this class, is there a way to get all the other classes that extend A?

Something like:

A a = new A();
a.getAllSubclasses();

Only way I tought of doing this is scanning the entire project for all the classes and storing them in a list and then checking manuall if each class extends from A with instanceOf.

List<Class> classList;
classList.each{
    if(it instanceOf a)
        //do stuff
}

This seems bad especially for a huge project with hundred of classes.