http://www.nabble.com/Java-subclasses-in-Ruby-tf3344729.html

In the email:
http://www.nabble.com/Re%3A-Java-subclasses-in-Ruby-p9386203.html
Charlie explains:
Ok, I think I might know what's happening here. Inside JRuby, all
exceptions are RuntimeException, since that's what we pass around.
Exceptions in Java code would get wrapped as a NativeException and
propogated out. My guess is that we're probably not unwrapping and
re-throwing the contained Java exception correctly in these cases.

A complete test case is available, discussed in the mail thread. The test is:
http://backspaces.net/files/rtest.tar.gz
The readme has been a complete explaination, attached.. The simplest test is to simply uncompress rtest.tar.gz, cd to rtest, then do:

export CLASSPATH=. # should only catch stuff in ./processing/core
java RandBoxes # make sure java version still works and prints out messages
jruby testprocessing.rb # test ruby

This results in the desired dump in either java 1.4 or 1.5 Got this trace:
Test: Init() called
Test: Try start
Test: Try exception, e=org.jruby.exceptions.RaiseException: Native Exception: 'class java.lang.NullPointerException'; Message: null; StackTrace: java.lang.NullPointerException
at java.applet.Applet.getAppletContext(Applet.java:181)
at processing.core.PApplet$Proxy0.__super$getAppletContext(Unknown Source)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
...
The complete dump is http://backspaces.net/files/dump

Here's the readme file, with far more details than needed, but may be useful if you need to rebuild processing using 1.4:

1. Expand http://backspaces.net/files/rtest.tar.gz and cd to rtest dir
2. To simply test the precompiled versions, this works, both w/ 1.5 and 1.4 java.
   export CLASSPATH=. # should only catch stuff in ./processing/core
   java RandBoxes # make sure java version still works and prints out messages
   jruby testprocessing.rb # test ruby

1. Here's the full monty:
2. Start clean. Note, I'll reset CLASSPATH several times to make sure we don't
3. inadvertently use unexpected classes.
   unset CLASSPATH
   rm -rf *.class processing testsrc
   ls
4. Should look like:
5. JavaClass.java core.jar testjava.rb
6. JavaSubClass.java readme testprocessing.rb
7. PAppleTest.java src
8. RandBoxes.java testframe.rb

1. Note src contains all the processing core source files
2. PApplet.java PGraphics3D.java PPolygon.java
3. PConstants.java PGraphicsJava2D.java PShape.java
4. PFont.java PImage.java PTriangle.java
5. PGraphics.java PLine.java
6. PGraphics2D.java PMatrix.java

1. Switch to using 1.4 just to be sure. (Processing is built on 1.4)
   export JAVA_HOME=/System/Library/Frameworks/JavaVM.framework/Versions/1.4.2/Home
   export PATH=$JAVA_HOME/bin:$PATH
   java -version

1. Simple test to make sure subclassing and NPE works as expected
   javac JavaSubClass.java
   java JavaSubClass

1. Two preliminary jruby tests, one of frames, that we'll use for our Processing test
   jruby testframe.rb # make sure frames OK
2. .. and one mimicking the JavaSubClass test
   export CLASSPATH=. # so jruby sees JavaSubClass
   jruby testjava.rb # subclassing and NPE work as expected

1. Now build and run the Processing test in Java
   export CLASSPATH=core.jar
   javac RandBoxes.java
   export CLASSPATH=core.jar:.
   java RandBoxes

1. Now test a ruby version
   export CLASSPATH=core.jar
   jruby testprocessing.rb
2. change line 10 to use PApplet, not RandBoxes. No NPE!
3. This works because PApplet in not abstract and uses defaults for size etc
   sed 's/RandBoxes.new/PApplet.new/' <testprocessing.rb | jruby

1. Here's the jar file contents:
   jar tvf core.jar
2. 0 Mon Nov 13 18:57:26 MST 2006 processing/
3. 0 Thu Nov 30 16:29:44 MST 2006 processing/core/
4. 591 Thu Nov 30 17:33:08 MST 2006 processing/core/PApplet$1.class
5. 818 Thu Nov 30 17:33:08 MST 2006 processing/core/PApplet$2.class
6. 1140 Thu Nov 30 17:33:08 MST 2006 processing/core/PApplet$3.class
7. 752 Thu Nov 30 17:33:08 MST 2006 processing/core/PApplet$4.class
8. 1064 Thu Nov 30 17:33:08 MST 2006 processing/core/PApplet$5.class
9. 487 Thu Nov 30 17:33:08 MST 2006 processing/core/PApplet$6.class
10. 490 Thu Nov 30 17:33:08 MST 2006 processing/core/PApplet$7.class
11. 535 Thu Nov 30 16:36:18 MST 2006 processing/core/PApplet$8.class
12. 1286 Thu Nov 30 17:33:08 MST 2006 processing/core/PApplet$RegisteredMethods.class
13. 945 Thu Nov 30 17:33:08 MST 2006 processing/core/PApplet$Worker$1.class
14. 2025 Thu Nov 30 17:33:08 MST 2006 processing/core/PApplet$Worker.class
15. 499 Thu Nov 30 17:33:08 MST 2006 processing/core/PApplet$WorkerVar.class
16. 84537 Thu Nov 30 17:33:08 MST 2006 processing/core/PApplet.class
17. 5271 Thu Nov 30 17:33:08 MST 2006 processing/core/PConstants.class
18. 10927 Thu Nov 30 17:33:08 MST 2006 processing/core/PFont.class
19. 37829 Thu Nov 30 17:33:08 MST 2006 processing/core/PGraphics.class
20. 16786 Thu Nov 30 17:33:08 MST 2006 processing/core/PGraphics2D.class
21. 37665 Thu Nov 30 17:33:08 MST 2006 processing/core/PGraphics3D.class
22. 1898 Thu Nov 30 17:33:10 MST 2006 processing/core/PGraphicsJava2D$ImageCache.class
23. 16073 Thu Nov 30 17:33:10 MST 2006 processing/core/PGraphicsJava2D.class
24. 23073 Thu Nov 30 17:33:08 MST 2006 processing/core/PImage.class
25. 11827 Thu Nov 30 17:33:08 MST 2006 processing/core/PLine.class
26. 10391 Thu Nov 30 17:33:08 MST 2006 processing/core/PMatrix.class
27. 8824 Thu Nov 30 17:33:08 MST 2006 processing/core/PPolygon.class
28. 2972 Thu Nov 30 17:33:10 MST 2006 processing/core/PShape.class
29. 27970 Thu Nov 30 17:33:10 MST 2006 processing/core/PTriangle.class

1. Now rebuild the processing library from source. Make sure we're still on 1.4
   java -version
2. make a copy of the src folder and compile it to make sure all is well.
3. I get one deprecation warning.
   cp -Rp src testsrc
   javac -deprecation -d . testsrc/*
4. list the resulting classes
   ls -R processing

1. Note: I get one class not in the jar file: PApplet$Worker$1.class
2. Not sure why, but everything works

1. Test this vanilla rebuild with RandBoxes.class
   export CLASSPATH=. # should only catch stuff in ./processing/core
   java RandBoxes # should work as before

1. Edit testsrc/PApplet.java to have these changes:
2. public void init() {
3. System.out.println("Test: Init() called");
4. ..
5. try { # System.out.println("Test: Try start"); # getAppletContext(); # System.out.println("Test: Try OK"); # online = true; # } catch (Exception e) { // NullPointerException # System.out.println("Test: Try exception, e="+e); # online = false; # }
6. .. or just copy the one I used:
   cp PAppleTest.java testsrc/PApplet.java

1. Now rerun with new PApplet
   javac -deprecation -d . testsrc/PApplet.java
   java RandBoxes # should now report results of try/catch

1. Now run ruby test which should catch the exception:
   jruby testprocessing.rb # test ruby