FULL PRODUCT VERSION :
1.6.0_01

ADDITIONAL OS VERSION INFORMATION :
Windows XP SP2 [5.1.2600]

A DESCRIPTION OF THE PROBLEM :
Java method ClassLoader.findSystemClass behavior changed starting in JDK 1.6:

Behavior pre-Java 1.6:
findSystemClass("[[Z")   -> returns "class [[Z" (an array of array primitive)
findSystemClass("[Z")   -> returns "class [Z" (an array primitive)

Behavior post-Java 1.6:
findSystemClass("[[Z")   -> throws ClassNotFoundException
findSystemClass("[Z")   ->  throws ClassNotFoundException


This caused custom serialization code in our application to fail. The idea is that this application can store plug-ins in the serialized objects - this way people who did not have the plug-ins installed could still use the serialized objects.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1 - run the source code in Java 5 and Java 6

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Expected: No difference between running in Java 5 and Java 6
ACTUAL -
Actual: Running in Java 6 throws a ClassNotFoundException, Java 5 works

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
// run this under both JDK 1.5 and JDK 1.6
public class TestClass {
	private static final class MyClassLoader extends ClassLoader {
		public Class myFindClassMethod(String name) throws ClassNotFoundException {
			return findSystemClass(name);
		}
	}

	public static void main(String[] args) {
		System.out.println(System.getProperty("java.version"));
		Object a = new boolean[4][4];
		System.out.println(a.getClass().getName()+ "");
		MyClassLoader loader = new MyClassLoader();
		try {
			Class cl = loader.myFindClassMethod(a.getClass().getName());
			System.out.println("This will only print in Java 1.5.x and lower "+cl);
		} catch (ClassNotFoundException e) {
			// throws exception in Java 1.6.0 and higher
			e.printStackTrace();
		}
	}
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Handle the ClassCastException and use the system class loader to look up primitive array classes.

Release Regression From : 5.0
The above release value was the last known release where this 
bug was not reproducible. Since then there has been a regression.