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Message:

Two Rodents are NOT Better than One

Posted by Bill Venners on 22 Dec 1998, 8:12 PM

> My applet is in a jar file, which includes the Rodent class.
> The applet dynamically loads the Mouse class from another
> directory (which includes the Rodent class), but fails to
> cast the retrieved class to the Rodent class.
>
As Yoda said to Luke Skywalker, "That is why you fail."

At least I think that's why you are failing. Here's my guess at
the problem you are having:

The trouble is that you are loading in Rodent twice, once into
the applet class loader's name space (the one in the JAR file)
and once into the RodentClassLoader's name space, by getting
it from, as you put it, "another directory."

Even assuming both of these Rodent's are identical, they will
be parsed into the method area twice and two java.lang.Class
objects will be created to represent them.

This causes you to encounter one of the lesser known aspects of
Java's object model: A cast or instanceof succeeds only if
the is-a relationship holds in the name space of the class
whose code is asking for the cast or instanceof.

In your case, the Mouse is a subclass of the Rodent loaded by
RodentClassLoader, but isn't a subclass of the Rodent loaded
by the applet class loader, even though both Rodent's are
identical. Since your applet is the class which is asking
for the cast, the JVM checks to see if Mouse is an instance
of the Rodent loaded by the applet class loader, which it
isn't. Thus, you get a ClassCastException.

Here's how to solve the problem:

You'll need to edit the RodentClassLoader to first ask not
the primordial loader for the type, but the applet class
loader. If the applet class loader can't find the type, then
go ahead and look in that "other directory."

How do you get at the applet class loader? Before you
instantiate RodentClassLoader in your applet, do something
like:

Class myClass = getClass();
ClassLoader appletClassLoader = myClass.getClassLoader();

Then, you'll have to make RodentClassLoader accept a
reference to a class loader in its constructor so you
can pass in the reference to the applet class loader
like this:

RodentClassLoader rcl = new RodentClassLoader(
    appletClassLoader, "that/other/directory");

To ask the applet class loader for a type from the
RodentClassLoader's loadClass() method, just invoke
loadClass() on the reference to the applet class loader,
passing along the fully qualified name of the type to load.

If you want to get Java 2ish, you can specify the
applet class loader as a "parent class loader" and
simply override findLocalClass() with the code that
looks in the "other directory." Check the Java 2
class loader documentation for details on this
approach.

Let us know how it goes.

bv

• Thank you Patrik Mihailescu 23 Dec 1998, 3:23 PM (1)
  • afsa asf November 15, 2000 at 1:21 PM (0)