Jhandles on Linux – java.lang.Exception: invalid handle

[Scott Haynes]

Hello all,

I compiled Jhandles-0.3.3 on my Fedora Linux machine using Java 
jdk1.6.0_06.

Anyway I have all the oct and jar files built and paths set correctly 
but surprise it still  doesn't work. If I just give it a simple command 
like plot(0:10) it dies with the following error message.

  octave:6> plot(0:10)

error: [java] java.lang.Exception: invalid handle - 1.0

error: evaluating assignment expression near line 18, column 7

error: called from `__get_object__' in file 
`/usr/share/octave/packages/jhandles-0.3.3/__get_object__.m'

error: evaluating assignment expression near line 20, column 9

error: evaluating if command near line 19, column 3

error: called from `drawnow' in file 
`/usr/share/octave/packages/jhandles-0.3.3/drawnow.m'

octave:7>

I first opened up the HandleObject.java to see what's going on. I don't 
know java at all so I'm not sort of in the dark. Pasted below is the 
code snipit of interest:

         public static HandleObject getHandleObject(double handle) 
throws Exception

         {

                 WeakReference ref = (WeakReference)handleMap.get(new 
Double(handle));

                 if (ref != null && ref.get() != null)

                 {

                         return (HandleObject)ref.get();

                 }

                 if (handle == 0)

                         return RootObject.getInstance();

                 throw new Exception("invalid handle - " + handle);

         }

Obviously, I'm not getting to the return(HandleObject) statement because 
of ref and ref.get.

Next at the octave command prompt I typed:

octave:7> obj = java_invoke ('java.lang.ref.WeakReference','get')

which returned


error: [java] java.lang.NullPointerException

error: evaluating assignment expression near line 7, column 5

octave:7>


I think this means ref.get is equal to null which is what is sending me 
in the wrong direction. I still don't know how to determine what ref is 
but I realize I'm in a bit over my head and it seems like a good point 
to ask the group for some help. Can someone briefly explain what ref and 
ref.get are?

I'd also be happy to share any info on how I managed to get this far if 
anyone else is interested in running jhandles on linux.

Best Regards
-- 
Scott Haynes