Reloading classes in Java?

It's doable by writing a JVM Agent which instruments the classes at load time, but it's by no means an easy feat,

Take a look at http://zeroturnaround.com/software/jrebel, a piece of software that does exactly that (unfortunally it's not opensource)

Thanks - that's what I thought. As I'm using Javassist, could I modify the class, remove the old properties and methods and rebuild them? Or does that hit the same limitation? No other classes reference this class and I only store instances of it in a hash map.

You need to remove/add field and methods for debugging purposes or it's a integral part of your design ?

It's an integral part of my design (prototype) - yes I know I'm crazy.

Sometimes craziness are good, even if it ends being a dead end, you've learned the internals of the JVM.

I suppose you're accessing the instances methods and fields through reflection. If so, you could use javassist plus a custom classloader to generate new classes at runtime.

The quirk is that instances belonging to the older class will not upgrade themselves to the new implementation. To overcome this, all instances which could be redefined should be proxies instead.

Take a look at the JDK's dynamic proxy classes, it provides an interface to programatically define the message dispatching logic of an object.

I am using reflection and it works beautifully except I can't create my new classes.

Instances of the old classes are deleted when I create the new classes as they get replaced with instances of the classes.

Another option would be to rename the old classes but I can't get the to work either.

Thanks

Can't you create your new classes because of the naming issue ? Or couldn't you integrate your new classes on the Java's class loader mechanism ?

If the problem is the naming clashing, you could create a synthetic name to your new classes (perhaps a MD5 of the new class bytecode), that will prevent the name clashing issues.

Be aware that if create to many classes, your app may leak memory, as the old classes are still referenced by the classloader. I know there's a workaround for that issue (Tomcat's hot deployment mechanism was plagued by that bug, and now seems to be working fine), but unfortunately I don't know it was fixed.

My problem is a link error due to a duplicate class. I'm creating a simple test case to see if I can get around this using a new class loader instance each time I create new versions of the class. I may have something working.

Using a different class loader seems to work. I know it leaves the old classes in the old class loser it that's fine as the apps will be restarted once the structure of the new classes is finalized. Thanks

Glad to help ;)