Java AWT Native Interface

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Java AWT Native Interface is an interface for the Java programming language that enables rendering libraries compiled to native code to draw directly to a Java Abstract Window Toolkit (AWT) Canvas object drawing surface.

The Java Native Interface (JNI) enabled developers to add platform-dependent functionality to Java applications. The JNI enables developers to add time-critical operations like mathematical calculations and 3D rendering. Previously, native 3D rendering was a problem because the native code didn't have access to the graphic context. The AWT Native Interface is designed to give developers access to an AWT Canvas for direct drawing by native code. In fact, the Java 3D API extension to the standard Java SE JDK relies heavily on the AWT Native Interface to render 3D objects in Java. The AWT Native Interface is very similar to the JNI, and, the steps are, in fact, the same as those of the JNI. See the Java Native Interface article for an explanation of the JNI techniques employed by the AWT Native Interface.

The AWT Native Interface was added to the Java platform with the J2SE 1.3 ("Kestrel") version.

Contents

[edit] AWT Native Interface example walkthrough

[edit] Create the Java application

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();
     }
 }

See the Java Native Interface article for an explanation of the native keyword and the loadLibrary() method. The paint() method will be simply invoked when the AWT event dispatching thread "repaints" the screen.

[edit] Create the C++ header file

Create the C++ header file as usual (See Java Native Interface for more complete explanation.)

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 JavaSideCanvas_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

[edit] Implement the C++ native code

Type this in a file named "NativeSideCanvas.cpp" and compile into a library. See Java Native Interface for a more complete explanation.

(Microsoft) Don't forget to link this with "jawt.lib" and "gdi32.lib". These libraries are needed because the code draws a rectangle using routines from these libraries.

Microsoft C++:

#include "jawt_md.h"
 #include <assert.h>
 #include "JavaSideCanvas.h"
 JNIEXPORT void JNICALL Java_JavaSideCanvas_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);
 }

(For Solaris code and other operating systems see links below.)

[edit] Run the example

Run the file as usual. (See Java Native Interface for complete instructions.)

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 do that is copying the "jawt.dll" (should be in the .../jre/bin file path 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!

[edit] Native painting

 This section may require cleanup to meet Wikipedia's quality standards.
Please improve this article if you can (May 2008).

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.

[edit] See also

[edit] External links