Some changes to the way the demos are built were made in b20.
SwingSet2's makefile make/mkdemo/jfc/SwingSet2 used to include ../DemoSwing.gmk
which had the (partial) rule :

ifeq ($(DEMONAME),SwingSet2)
        $(CP) -f $(DEMODST)/src/*.java $(DEMODST_CLASS)
endif

this copied the sources into the application jar file.
This rule is now gone and the demo doesn't work properly anymore.
OPERATING SYSTEM
----------------
All

FULL JDK VERSION
----------------
Reproducible with JDK 7b88. Not sure which built it started.


STEPS TO REPRODUCE
------------------

1. Launch the SwingSet2 demo:

   <JAVA_HOME>/jre/bin/java -jar <JAVA_HOME>/demo/jfc/SwingSet2/SwingSet2.jar

2. Once the application is launched, click on any of the demo components
   (InternalFrameDemo , ButtonDemo etc).

3. Click on the "source code" tab

4. Observe the error message "Could not load file: <demo>.java"

Expected result: the code should be loaded successfully.

The source code has not been present in the jar files since b20 - it seems to be this fact that breaks the source code functionality in the SwingSet2 demo.

Release Regression From : 7
The above release value was the last known release where this
bug was not reproducible. Since then there has been a regression.
*** (#1 of 1): 2010-04-13 07:42:45 PDT ###@###.###
Run SwingSet2:

   java -showversion -jar $JAVA_HOME/demo/jfc/SwingSet2/SwingSet2.jar SwingSet2

When the first panel "Internal Frames Demo" appears, select the 'Source Code' tab.

You will see:

Could not load file: InternalFrameDemo.java
*** (#1 of 1): 2008-07-24 22:30:00 PDT ###@###.###