View source for VTK/Java Wrapping

== Configuration ==

Bartlomiej Wilkowski has created a nice tutorial of [http://www.spinet.pl/~wilku/vtk-howto/ configuring Java wrapping with VTK].

== Java Wrapper Refactoring (Oct 8, 2007) ==

There were a few problems with the old Java wrappers. One was that, as
you said, objects were being deleted before they were supposed to. We
hacked in a fix at one point about a year ago which basically made all
VTK objects accessed from Java stay around forever, but this was not
acceptable either.

Ref:
* http://vtk.org/cgi-bin/viewcvs.cgi/Wrapping/vtkWrapJava.c?r1=1.59&r2=1.60

The other major concern was that the map from Java objects to VTK
objects was in the C++ JNI layer, and while we tried to keep this map
synchronized with a mutex, race conditions could still occur because
other Java threads could advance while the JNI layer was being called
(a thread could access a C++ object just as it is being
garbage-collected and deleted). There does not seem to be a way to
atomically call a JNI method, or ensure the collector doesn't run
while a method is called. This second issue forced us to rethink how
the map is done, and the solution was to keep the map in Java instead
of C++. But we didn't want this Java map to prohibit objects from
being garbage collected. Fortunately, Java has a WeakReference class
for just this type of purpose. When accessed, the reference will
either be valid or null depending on whether it has been
garbage-collected.

http://java.sun.com/j2se/1.4.2/docs/api/java/lang/ref/WeakReference.html

Thus, the wrapper code can lookup objects in this map when returning
objects from methods, and if it is not there, or null, it creates a
new Java object representing that C++ object.

A final issue was that we wanted a way to guarantee all C++
destructors are called before the program exits. The natural place to
decrement the reference count of the C++ object is in finalize(),
which works when things are garbage-collected, but Java does not
guarantee that finalize will ever be called. So the method
vtkGlobalJavaHash.DeleteAll() will plow through the remaining VTK
objects and call Delete on them.