Re: why do I get this runtime error

Lew <>
Fri, 05 Oct 2007 09:40:22 -0400
Aryeh M. Friedman wrote:

When I run this:

package scratch;


public class Main extends ClassLoader
        public Main()

        public static void main(String[] args)
                throws Throwable
                Main m=new Main();

                File f=null;

Side point: why initialize f to null? You throw away the value immediately,
so why use it at all?

                f = new File("/usr/home/plos/obj/scratch/Main.class");
                int size = (int)f.length();

Why not "long size = ..."?

                byte buff[] = new byte[size];
                FileInputStream fis = new FileInputStream(f);
                DataInputStream dis = new DataInputStream(fis);

                Class klass=m.defineClass("scratch.Main",buff,

                Main m2=(Main) klass.newInstance();

I get:
Exception in thread "main" java.lang.ClassCastException: scratch.Main
cannot be cast to scratch.Main
          at scratch.Main.main(

Looks to me like the two classes are loaded from different class loaders. The
cast uses Main from the "normal" class loader to cast a Main from the custom
class loader, thus not compatible.

I'm very inexperienced with ClassLoader idioms so I am far from sure of this

Maybe you'd be better off using a ClassLoader that isn't the class already
loaded in order to run the ClassLoader that it itself is.


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