Re: why do I get this runtime error

From:

Daniel Pitts <newsgroup.spamfilter@virtualinfinity.net>

Newsgroups:

comp.lang.java.programmer

Date:

Fri, 05 Oct 2007 08:37:10 -0700

Message-ID:

<A-adneYzbZQiwZvanZ2dnUVZ_hudnZ2d@wavecable.com>

Aryeh M. Friedman wrote:

When I run this:

package scratch;

import java.io.*;

public class Main extends ClassLoader
{
        public Main()
        {
                super(Main.class.getClassLoader());
        }

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

                File f=null;

                f = new File("/usr/home/plos/obj/scratch/Main.class");
                int size = (int)f.length();
                byte buff[] = new byte[size];
                FileInputStream fis = new FileInputStream(f);
                DataInputStream dis = new DataInputStream(fis);
                dis.readFully(buff);
                dis.close();

                Class klass=m.defineClass("scratch.Main",buff,
0,buff.length);

                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(Main.java:29)

Notes:

This is distilled from attempting to write a class loader and every
attempt ends with the same error (even cutting and pasting web and
junit examples of class loaders and calling them instead of mine).

The "same" class loaded by a different class loader is considered a
different class. Think of it as an invisible runtime namespace.
<customClassLoader>.scratch.Main cannot be cast to
<systemClassLoader>.scratch.Main.

Remember, if you're trying to dynamically load/unload/reload a class,
you can't have any reference to that class in the system class loader.
You also can't expect non-reflective communication between your
dynamically loaded class and the rest of your application, unless the
class implements/extends an interface/class which is loaded in the
system class loader. And then you can only rely on the methods in your
base class/interface.

HTH.

--
Daniel Pitts' Tech Blog: <http://virtualinfinity.net/wordpress/>