OSGi DevCon 2008 Class.forName and ClassLoader.loadClass seem to be synonyms for the same basic operation: request a dynamic class load. Yet calling Class.forName does additional "checking" which is not very useful (certainly in OSGi). Class.forName does certain loader constraint checks to ensure type safety which are not necessary when doing dynamic class loads. This short talk will explore this issue and discuss a proposed "hack" to work around the issue for legacy code which can't easily be changed to avoid Class.forName.

1. Copyright © IBM Corp., 2008. All rights reserved. Source code in this presentation is made available under the EPL, v1.0, remainder of
the presentation is licensed under Creative Commons Att. Nc Nd 2.5 license
Why Class.forName sucks!
BJ Hargrave
Senior Technical Staff Member
IBM Lotus
http://blog.bjhargrave.com

2. Why Class.forName sucks! | Copyright © IBM Corp., 2008. All rights reserved. Source code in this presentation is made available
under the EPL, v1.0, remainder of the presentation is licensed under Creative Commons Att. Nc Nd 2.5 license.
Static Class Loading
•class loading constraints [1][2]
Foo f = FooFactory.createFoo()
•Foo.class
Class.forName("Foo")
class constant pool entry
• [1] http://java.sun.com/docs/books/jvms/second_edition/html/ConstantPool.doc.html#78621
• [2] http://www.bracha.org/classloaders.ps

3. Why Class.forName sucks! | Copyright © IBM Corp., 2008. All rights reserved. Source code in this presentation is made available
under the EPL, v1.0, remainder of the presentation is licensed under Creative Commons Att. Nc Nd 2.5 license.
Dynamic Class Loading
•Class.forName
•ClassLoader.loadClass

4. Why Class.forName sucks! | Copyright © IBM Corp., 2008. All rights reserved. Source code in this presentation is made available
under the EPL, v1.0, remainder of the presentation is licensed under Creative Commons Att. Nc Nd 2.5 license.
Initiating class loader
loadClass
Defining class loader
defineClass

5. Why Class.forName sucks! | Copyright © IBM Corp., 2008. All rights reserved. Source code in this presentation is made available
under the EPL, v1.0, remainder of the presentation is licensed under Creative Commons Att. Nc Nd 2.5 license.
Class.forName bleeds static
constraints onto dynamic loading
Same internal VM method used to
handle static class loads and
dynamic class loads via
Class.forName

6. Why Class.forName sucks! | Copyright © IBM Corp., 2008. All rights reserved. Source code in this presentation is made available
under the EPL, v1.0, remainder of the presentation is licensed under Creative Commons Att. Nc Nd 2.5 license.
Class.forName Issues
•Initiating class loader also caches
defined class along with loader
constraints
•If class is cached by initiating class
loader
initiatingClassLoader.loadClass
is never called

7. Why Class.forName sucks! | Copyright © IBM Corp., 2008. All rights reserved. Source code in this presentation is made available
under the EPL, v1.0, remainder of the presentation is licensed under Creative Commons Att. Nc Nd 2.5 license.
Class.forName(String)
not so bad since it will likely use bundle's class
loader as initiating class loader
Class.forName(String, true, callersClassLoader)
Class.forName(String, boolean,
ClassLoader)
When ClassLoader is the thread context Class
Loader is the issue!

8. Why Class.forName sucks! | Copyright © IBM Corp., 2008. All rights reserved. Source code in this presentation is made available
under the EPL, v1.0, remainder of the presentation is licensed under Creative Commons Att. Nc Nd 2.5 license.
Equinox's ContextFinder
•Thread Context Class Loader
•Never defines classes
•Delegates to bundle's class loaders
based upon the call context
•Wrong results returned when
multiple versions of classes present

9. Why Class.forName sucks! | Copyright © IBM Corp., 2008. All rights reserved. Source code in this presentation is made available
under the EPL, v1.0, remainder of the presentation is licensed under Creative Commons Att. Nc Nd 2.5 license.
Recommendation
•Use ClassLoader.loadClass in
your code
•Or Bundle.loadClass if you
know the bundle containing the
class

10. Why Class.forName sucks! | Copyright © IBM Corp., 2008. All rights reserved. Source code in this presentation is made available
under the EPL, v1.0, remainder of the presentation is licensed under Creative Commons Att. Nc Nd 2.5 license.
Hack?

11. Why Class.forName sucks! | Copyright © IBM Corp., 2008. All rights reserved. Source code in this presentation is made available
under the EPL, v1.0, remainder of the presentation is licensed under Creative Commons Att. Nc Nd 2.5 license.
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