Casting gone Wild - ClassCastException

[Mike Curwen]

Hey there,

When would the following happen?

1. Object t = // obtain reference to the object.
2. out.print(t.getClass());
3. com.the.class td = null;
4. td = (com.the.class)t; 

if line 2 produces "class com.the.class", under what circumstances would line 4 throw a ClassCastException ?

[Ernest Friedman-Hill]

The object pointed to by "t" was loaded by a different class loader than the current context class loader (i.e., generally, by a different one than the one that loaded the code we're looking at.) That other loader also has its own copy of com.the.class . Finally, com.the.class isn't accessible to the system class loader, and the class we're looking at wasn't loaded by the system class loader, either.
[ June 22, 2005: Message edited by: Ernest Friedman-Hill ]

[Mike Curwen]

blaow!!!

DUH. Yah, that was it. How many years doing web-apps and **knowing** about that particular classloader thingy??? argh.

The "code we're looking at" is in one webapp and it's referencing Threads (t) started in another webapp.

I still think it's "kinda funny" about the getClass() result and then failing the cast. It brings to mind that good ole' Jedi Mind trick "These aren't the classes you're looking for"...

[Mike Curwen]

Yah.. and now I realize that my plans to be able to 'monitor' a thread without having to learn about JMX is right out the window.

I was going to have all my daemon threads implement a common 'DaemonThread' interface with methods like "getQueueCount" and "getStatus" and "shutDownRightNow" and such. Each of my webapps starts a number of these threads.

Then I'd have a single status application that was gonna be the 'christmas tree' with green/orange/red indicators.

But I won't be able to cast the Thread returned from code that starts:
Thread ct = Thread.currentThread();
ThreadGroup tg = ct.getThreadGroup();
// etc...

drat!

[Ernest Friedman-Hill]

Well, you still have a chance if you can get the common classes loaded by a higher-up class loader (a common ancestor to the two web app's loaders.) Under (say) Tomcat, I think you may be able to do this just by putting the jar in the "common" directory.