I’m back at The Server Side Java Symposium this year and this is the first breakout session I’m attending. I think I’ve got good knowledge of ClassLoaders and Java Reflection API, but we’ll see if Jevgeni Kabanov, the speaker, can enlighten me with some new nuggets of knowledge. He’s Founder & CTO of ZeroTurnaround, a company I know next to nothing about, although I have heard of JRebel, one of their products. Anyway, enough background info… Let’s jump into the meat of the presentation.
Basics
public class A {
public void do() {
B b = new B();
b.do():
}
}
Instantiating Object B causes the following: A.class.getClassLoader().loadClass("B");
Problems & Solutions
1. Class Not Found
You build some code and get a NoClassDefFound exception when deploying and trying to run the servlet. How do we find out what the problem is?
Some solutions:
* IDE Class Lookup
* find *.jar -exec jar -tf ‘{}’ \; | grep MyClass
* URLClassLoader.getURLs()
2. Wrong Class Found
What if you get a NoSuchMethodError? What’s happening there? Well, the Class is found, but there’s something wrong with the class.
Solution:
* Get ClassLoader().getResource(Util2.class.replace(‘.’,’/’) + “.class” to find out where the ClassLoader is getting the class. Then you can use javap -private Util2 to inspect the contents of the file.
3. More than One Class Found
That error will typically occur if 2 different ClassLoaders are used and find different classes for the same name. They are not the same.
Helpful: -verbose:class, as well as ClassLoader.getResource() to identify the different classes.
Leaking ClassLoaders
If a Class has a reference to its ClassLoader, that object has a reference to all of its classes. To leak a ClassLoader, it’s enough to leak just one object loaded by that ClassLoader.
Hierarchy is not enough?
* Isolation: Difference versions of the same library
* Performance: Class lookup is very slow
* Restricted: Why can’t siblings see each other’s classes?
* OSGI, JBoss, NetBeans and others implement a different system
Follow the speaker on Twitter: @ekabanov