public class A
{
    int age=10;


    class B
    {

    }
}

public class Main
{
    public static void main(String[] args) throws NoSuchFieldException, IllegalAccessException
    {
        A a=new A();
        A.B b=a.new B();
        Class bClass=b.getClass();
        Field field=bClass.getDeclaredField("this$0");
        A outClass=(A)field.get(b);//get(Object obj) 返回指定对象obj上此 Field 表示的字段的值
        Field age=outClass.getClass().getDeclaredField("age");
        System.out.println(outClass);
    }
}

public class Outer
{//this$0

    public class FirstInner
    {//this$1

        public class SecondInner
        {//this$2

            public class ThirdInner
            {

            }
        }
    }
}