Java AWT Native Interface
Introduction
The Java Native Interfaceenabled Developers to add platform-dependant functionality to Java Applications. It enables developers to add things like time-critical operations like mathematical calculations and 3-D Rendering. 3-D Rendering is interesting, because the native side don't know where to draw the graphics! The AWT Native Interface is designed to give developers a sort of a "canvas" for their native programs to "draw" on. The AWT Native Interface is very similar to the JNI, and, the steps are, in fact, the same as those of the JNI. So, it is highly recommended that you read the Java Native Interface article first before continuing.
Note that the AWT Interface only existed since JDK 1.3 ("Kestrel")
AWT Native Interface Example Walkthrough
1. Create the Java Side
Type in this in a .java file named JavaSideCanvas and compile:
import java.awt.*;
import java.awt.event.*;
public class JavaSideCanvas extends Canvas {
static {
System.loadLibrary("NativeSideCanvas");
}
public native void paint(Graphics g);
public static void main(String[] args) {
Frame f = new Frame();
f.setBounds(0, 0, 500, 500);
JavaSideCanvas jsc = new JavaSideCanvas();
f.add(jsc);
f.addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent ev) {
System.exit(0);
}
});
f.show();
}
}
If you read the Java Native Interface article you know the native keywords and the loadLibrary() method. The paint method will be simply invoked when the JVM "repaints" the screen.
2. Create Header File
Create Header File as Usual (Look at Java Native Interface for help) The Header File looks like this now:
/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class JavaSideCanvas */
#ifndef _Included_JavaSideCanvas
#define _Included_JavaSideCanvas
#ifdef __cplusplus
extern "C" {
#endif
#undef JavaSideCanvas_FOCUS_TRAVERSABLE_UNKNOWN
#define JavaSideCanvas_FOCUS_TRAVERSABLE_UNKNOWN 0L
#undef JavaSideCanvas_FOCUS_TRAVERSABLE_DEFAULT
#define JavaSideCanvas_FOCUS_TRAVERSABLE_DEFAULT 1L
#undef JavaSideCanvas_FOCUS_TRAVERSABLE_SET
#define JavaSideCanvas_FOCUS_TRAVERSABLE_SET 2L
#undef JavaSideCanvas_TOP_ALIGNMENT
#define JavaSideCanvas_TOP_ALIGNMENT 0.0f
#undef JavaSideCanvas_CENTER_ALIGNMENT
#define JavaSideCanvas_CENTER_ALIGNMENT 0.5f
#undef JavaSideCanvas_BOTTOM_ALIGNMENT
#define JavaSideCanvas_BOTTOM_ALIGNMENT 1.0f
#undef JavaSideCanvas_LEFT_ALIGNMENT
#define JavaSideCanvas_LEFT_ALIGNMENT 0.0f
#undef JavaSideCanvas_RIGHT_ALIGNMENT
#define JavaSideCanvas_RIGHT_ALIGNMENT 1.0f
#undef JavaSideCanvas_serialVersionUID
#define JavaSideCanvas_serialVersionUID -7644114512714619750i64
#undef JavaSideCanvass_serialVersionUID
#define JavaSideCanvas_serialVersionUID -2284879212465893870i64
/*
* Class: JavaSideCanvas
* Method: paint
* Signature: (Ljava/awt/Graphics;)V
*/
JNIEXPORT void JNICALL Java_JavaSideCanvas_paint
(JNIEnv *, jobject, jobject);
#ifdef __cplusplus
}
#endif
#endif
3. Implement the C++ side
Type this in a file named NativeSideCanvas.cpp and compile into a library: (See Java Native Interface)
Don't Forget to link this with jawt.lib and gdi32.lib (because it draws a rectangle)
Microsoft C++:
#include "jawt_md.h"
#include <assert.h>
#include "JavaSideCanvas.h"
JNIEXPORT void JNICALL
Java_MyCanvas_paint(JNIEnv* env, jobject canvas, jobject graphics)
{
JAWT awt;
JAWT_DrawingSurface* ds;
JAWT_DrawingSurfaceInfo* dsi;
JAWT_Win32DrawingSurfaceInfo* dsi_win;
jboolean result;
jint lock;
// Get the AWT
awt.version = JAWT_VERSION_1_3;
result = JAWT_GetAWT(env, &awt);
assert(result != JNI_FALSE);
// Get the drawing surface
ds = awt.GetDrawingSurface(env, canvas);
assert(ds != NULL);
// Lock the drawing surface
lock = ds->Lock(ds);
assert((lock & JAWT_LOCK_ERROR) == 0);
// Get the drawing surface info
dsi = ds->GetDrawingSurfaceInfo(ds);
// Get the platform-specific drawing info
dsi_win = (JAWT_Win32DrawingSurfaceInfo*)dsi->platformInfo;
//////////////////////////////
// !!! DO PAINTING HERE !!! //
//////////////////////////////
//Simple paints a rectangle (GDI32)
Rectangle(dsi_win->hdc, 50, 50, 200, 200);
// Free the drawing surface info
ds->FreeDrawingSurfaceInfo(dsi);
// Unlock the drawing surface
ds->Unlock(ds);
// Free the drawing surface
awt.FreeDrawingSurface(ds);
}
(Solaris Code and Other operating systems: look at links below)
The code above should be self-explanatory
4. Run the Example
Run the file as usual (See Java Native Interface)
It's interesting to note that the AWT Native Interface requires the jawt.dll (or jawt.so) to run with the application, so the easiest way to to that is copying the jawt.dll (should be in the .../jre/bin of the JDK's installation path)
You should see a window with a rectangle drawn in it.
Congratulations! You have made your first AWT Native Application!
Note about Native Painting
As you can see, you can paint as if it is a native application. In Windows, the JVM will pass a HWND and other window information to your native application so that your application will "know" where to draw. In this example, it uses GDI to draw a Rectangle. The window information your native side need will be in a JAWT_Win32DrawingSurfaceInfo structure (Depending on Operating System) which can be retrieved with this line:
dsi_win = (JAWT_Win32DrawingSurfaceInfo*)dsi->platformInfo;
dsi_win has the information, look in the jni.h file for details.
External Links
The AWT Native Interface