why do I get this runtime error

[Aryeh M. Friedman]

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).

 

[Lew]

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;

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);
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)

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
analysis.

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.

 

[Daniel Pitts]

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.

 

[Joshua Cranmer]

When I run this:

[ snip ]
> I get:
Exception in thread "main" java.lang.ClassCastException: scratch.Main
cannot be cast to scratch.Main
at scratch.Main.main(Main.java:29)

Fun exceptions happen when working with class loaders.

The short answer:
Multiple class loaders don't mix. One can change the default class
loader through some java -D option; I don't remember what it is however.

The long answer:
A class is uniquely determined by its instantiating class loader and its
fully-qualified name.

This is an example of a working loadClass function. I did not write this
myself, and it is based off of a very old version of Java (discussion of
custom class loaders is almost nil), so I do not know which parts are
irrelevant.


public final Class<?> loadClass(String name, boolean resolve) throws
ClassNotFoundException {
// Don't bother loading Java classes
if (name.startsWith("java.") || name.startsWith("javax."))
return super.loadClass(name, resolve);

// Did we already define it?
Class result = findLoadedClass(name);
if (result == null) {
String internalName = name.replace('.', '/') + ".class";
InputStream is = getParent().getResourceAsStream(internalName);
if (is != null) {
try {
// Load the raw bytes.
ByteArrayOutputStream bos = new ByteArrayOutputStream();
int b;
while ((b=is.read())!=-1)
bos.write((byte)b);
byte[] bytes = bos.toByteArray();
result = defineClass(name, bytes, 0, bytes.length);
} catch (IOException ex) {
throw new ClassNotFoundException(name +
" could not be loaded", ex);
}
}
}

if (result != null) {
if (resolve) {
resolveClass(result);
}
} else {
// Let someone else find it.
result = super.loadClass(name, resolve);
}
return result;
}

Even here, it is still incompatible with system-default loaded classes.

 

[Roedy Green]

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

It would help people help you if you told them the exception occurred
in this line and there was no other stack trace.

see:
http://mindprod.com/jgloss/runerrormessages.html#CLASSCASTEXCEPTION

Hint: class X is not the same as class X if they were loaded with
different class loaders.