Java HotSpot Virtual Machine

The following code fails because JAWT_DrawingSurface *ds is null after the assignment ds = awt.GetDrawingSurface(env, canvas);
I don't understand why this is null and am struggling to debug the code with print statements. As I don't know how to debug JNI through xcode. think it's probably something simple I am missing as I downloaded a working example that uses the same setup code.

Thanks for all your help
David

   JAWT awt;
    JAWT_DrawingSurface* ds = NULL;
    JAWT_DrawingSurfaceInfo* dsi = NULL;
    JAWT_MacOSXDrawingSurfaceInfo* dsi_mac = NULL;
    jboolean result = JNI_FALSE;
    jint lock = 0;
    
    // get the AWT
    awt.version = JAWT_VERSION_1_4;
    result = JAWT_GetAWT(env, &awt);
    if (env->ExceptionOccurred()) {
        env->ExceptionDescribe();
    }
    assert(result != JNI_FALSE);
    
    // Get the drawing surface.  This can be safely cached.
    // Anything below the DS (DSI, contexts, etc) 
    // can possibly change/go away and should not be cached.
    ds = awt.GetDrawingSurface(env, canvas);
    if (env->ExceptionOccurred()) {
        env->ExceptionDescribe();
    }
	if(!ds)
	{
		std::cout << "ds is null" << std::endl;
		std::cout << "env address is : " << env << std::endl;
		std::cout << "canvas address is : " << &canvas << std::endl;
		return;
	}
    assert(ds != NULL);
    
    // Lock the drawing surface
    // You must lock EACH TIME before drawing
    lock = ds->Lock(ds); 
    if (env->ExceptionOccurred()) {
        env->ExceptionDescribe();
    }
    assert((lock & JAWT_LOCK_ERROR) == 0);
    
    // Get the drawing surface info
    dsi = ds->GetDrawingSurfaceInfo(ds);
    
    // Check DrawingSurfaceInfo.  This can be NULL on Mac OS X
    // if the windowing system is not ready
    if (dsi != NULL) {

        // Get the platform-specific drawing info
        // We will use this to get at Cocoa and CoreGraphics
        // See <JavaVM/jawt_md.h>
        dsi_mac = (JAWT_MacOSXDrawingSurfaceInfo*)dsi->platformInfo;
        if (env->ExceptionOccurred()) {
            env->ExceptionDescribe();
        }
        
        // Get the corresponding peer from the caller canvas
        NSView *view = dsi_mac->cocoaViewRef;
        		
        // Get the CoreGraphics context from the parent window.
        // DO NOT CACHE NSGraphicsContexts -- they may go away.
        NSWindow *window = [view window];
        NSGraphicsContext *ctxt = [NSGraphicsContext graphicsContextWithWindow:window];
        CGContextRef cg = static_cast<CGContextRef>([ctxt graphicsPort]);
        
        // Match Java's ctm
        NSRect windowRect = [window frame];
        CGContextConcatCTM(cg, CGAffineTransformMake(1, 0, 0, -1, dsi->bounds.x, windowRect.size.height-dsi->bounds.y));
        
        // Draw a pattern using CoreGraphics
        CGContextSetRGBFillColor(cg, 1.0f, 0.0f, 0.0f, 1.0f);
        CGContextFillRect(cg, CGRectMake(15, 15, dsi->bounds.width-30, dsi->bounds.height-30));
        
        // Free the DrawingSurfaceInfo
        ds->FreeDrawingSurfaceInfo(dsi);
        if (env->ExceptionOccurred()){
            env->ExceptionDescribe();
        }