VTK/Java Wrapping

Configuration

You basically just need to turn VTK_WRAP_JAVA on in CMake and build.

Bartlomiej Wilkowski has created a nice tutorial of configuring Java wrapping with VTK.

Mac (Snow Leopard)

To build a sample application provided in VTK against your VTK build directory (with an installed VTK replace "bin" with "lib"):

$ export DYLD_LIBRARY_PATH=$DYLD_LIBRARY_PATH:your_vtk_build_dir/bin
$ javac -cp your_vtk_build_dir/bin/vtk.jar your_vtk_source_dir/Wrapping/Java/vtk/sample/Demo.java
$ java -cp your_vtk_build_dir/bin/vtk.jar:your_vtk_source_dir/Wrapping/Java vtk.sample.Demo

Sample Code (from VTK/Wrapping/Java/vtk/sample/SimpleVTK.java)

<source lang="java"> /**

* An application that displays a 3D cone. A button allows you to close the
* application.
*/

public class SimpleVTK extends JPanel implements ActionListener {

   private static final long serialVersionUID = 1L;
   private vtkPanel renWin;
   private JButton exitButton;

   // -----------------------------------------------------------------
   // Load VTK library and print which library was not properly loaded
   static {
       if (!vtkNativeLibrary.LoadAllNativeLibraries()) {
           for (vtkNativeLibrary lib : vtkNativeLibrary.values()) {
               if (!lib.IsLoaded()) {
                   System.out.println(lib.GetLibraryName() + " not loaded");
               }
           }
       }
       vtkNativeLibrary.DisableOutputWindow(null);
   }

   // -----------------------------------------------------------------
   public SimpleVTK() {
       super(new BorderLayout());

       // build VTK Pipeline
       vtkConeSource cone = new vtkConeSource();
       cone.SetResolution(8);

       vtkPolyDataMapper coneMapper = new vtkPolyDataMapper();
       coneMapper.SetInputConnection(cone.GetOutputPort());

       vtkActor coneActor = new vtkActor();
       coneActor.SetMapper(coneMapper);

       renWin = new vtkPanel();
       renWin.GetRenderer().AddActor(coneActor);

       // Add Java UI components
       exitButton = new JButton("Exit");
       exitButton.addActionListener(this);

       add(renWin, BorderLayout.CENTER);
       add(exitButton, BorderLayout.SOUTH);
   }

   /** An ActionListener that listens to the button. */
   public void actionPerformed(ActionEvent e) {
       if (e.getSource().equals(exitButton)) {
           System.exit(0);
       }
   }

   public static void main(String s[]) {
       SwingUtilities.invokeLater(new Runnable() {
           @Override
           public void run() {
               JFrame frame = new JFrame("SimpleVTK");
               frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
               frame.getContentPane().setLayout(new BorderLayout());
               frame.getContentPane().add(new SimpleVTK(), BorderLayout.CENTER);
               frame.setSize(400, 400);
               frame.setLocationRelativeTo(null);
               frame.setVisible(true);
           }
       });
   }

} </source>

Some key points from this code to note:

• vtkNativeLibrary.LoadAllNativeLibraries() is required to load the dynamic libraries from VTK's C++ core. This call must happen before any VTK code executes, which is why it is put in a static block in our application class.
• vtkNativeLibrary.DisableOutputWindow(null) simply hides any debugging information that may otherwise pop up in a dialog box when any warnings are reached. This is good to call when releasing an application.
• SwingUtilities.invokeLater(...) is called because technically all GUI code, including setting up and using a VTK render window, should happen in the Swing event thread.

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.