Jump to content

  • Log In with Google      Sign In   
  • Create Account


Reloading classes in Java?


Old topic!
Guest, the last post of this topic is over 60 days old and at this point you may not reply in this topic. If you wish to continue this conversation start a new topic.

  • You cannot reply to this topic
10 replies to this topic

#1 Mike_Stoddart   Members   -  Reputation: 182

Like
0Likes
Like

Posted 20 February 2013 - 08:15 PM

I think I already know the answer but I thought I'd double check anyway. Is it possible to reload a class in Java when its method signatures change? I think it's forbidden by the JVM but is this possible in any way?

 

I can dynamically create class definitions and add methods and member data with Javassist. 

 

Thanks


Edited by Mike_Stoddart, 20 February 2013 - 08:17 PM.


Sponsor:

#2 Bubsy   Members   -  Reputation: 407

Like
0Likes
Like

Posted 20 February 2013 - 08:58 PM

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)



#3 Mike_Stoddart   Members   -  Reputation: 182

Like
0Likes
Like

Posted 21 February 2013 - 07:16 AM

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.



#4 Bubsy   Members   -  Reputation: 407

Like
0Likes
Like

Posted 21 February 2013 - 09:38 AM

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



#5 Mike_Stoddart   Members   -  Reputation: 182

Like
0Likes
Like

Posted 21 February 2013 - 09:40 AM

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



#6 Bubsy   Members   -  Reputation: 407

Like
0Likes
Like

Posted 21 February 2013 - 03:13 PM

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.



#7 Mike_Stoddart   Members   -  Reputation: 182

Like
0Likes
Like

Posted 21 February 2013 - 04:29 PM

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

#8 Bubsy   Members   -  Reputation: 407

Like
0Likes
Like

Posted 22 February 2013 - 07:42 AM

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. 



#9 Mike_Stoddart   Members   -  Reputation: 182

Like
0Likes
Like

Posted 22 February 2013 - 08:48 AM

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.



#10 Mike_Stoddart   Members   -  Reputation: 182

Like
1Likes
Like

Posted 23 February 2013 - 08:18 AM

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

#11 Bubsy   Members   -  Reputation: 407

Like
0Likes
Like

Posted 23 February 2013 - 09:50 AM

Glad to help ;)






Old topic!
Guest, the last post of this topic is over 60 days old and at this point you may not reply in this topic. If you wish to continue this conversation start a new topic.



PARTNERS