A class with an immutable boolean property "ascending", accessed by the public method isAscending(), cannot be encoded using XMLEncoder on Java 6, even though the encoding succeeds on Java 5. I attached TestBean.java and SortCriteria.java to reproduce the bug:

1. Set Java 5 (as root):
; rm /usr/java
; ln -s /usr/jdk/jdk1.5.0_08 /usr/java
; java -version
java version "1.5.0_08"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_08-b03)
Java HotSpot(TM) Server VM (build 1.5.0_08-b03, mixed mode)
;

2. Compile and run attached classes (as non-root):
; javac -cp . TestBean.java SortCriteria.java
; java -cp . TestBean
encoded: Count descending
decoded: Count descending
;

3. Set Java 6 (as root):
; rm /usr/java
; ln -s /usr/jdk/jdk1.6.0 /usr/java
; java -version
java version "1.6.0-rc"
Java(TM) SE Runtime Environment (build 1.6.0-rc-b100)
Java HotSpot(TM) Server VM (build 1.6.0-rc-b100, mixed mode)
;

4. Compile and run attached classes (as non-root):
; javac -cp . TestBean.java SortCriteria.java
; java -cp . TestBean
java.lang.NoSuchMethodException: SortCriteria.getAscending
Continuing ...
java.lang.IllegalArgumentException
Continuing ...
java.lang.Exception: XMLEncoder: discarding statement XMLEncoder.writeObject(SortCriteria);
Continuing ...
encoded: Count descending
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 0
        at com.sun.beans.ObjectHandler.dequeueResult(ObjectHandler.java:139)
        at java.beans.XMLDecoder.readObject(XMLDecoder.java:201)
        at TestBean.main(TestBean.java:44)
;

This was reproducible on sparc and amd64.