JNI - Free ByteBuffer from C ++

Summary

• Create a ByteBuffer in Java called a buffer with ByteBuffer.allocateDirect (someBufferSize)
• Fill the buffer with data
• Pass buffer in C ++ as jobject - jbuffer
• Get direct pointer to buffer with env-> GetDirectBufferAddress (jbuffer)
• Working with buffer data on the C ++ side. How can we prevent GC from flushing our buffer or it never happens?
• Job done - we don't need jbuffer.
• Release jbuffer? free (jbuffer) - Will result in an invalid address error.

Long part

I am using the following code to load PNG files with Java AssetManager to use them to generate Open GL ES 2.0 textures.

Java class PNG

import java.nio.ByteBuffer;
import android.graphics.Bitmap;

public class PNG 
{
 private static final int BYTES_PER_PIXEL_PNG = 4;
 private static final String LOG_TAG = "[PNG]";
 public int width;
 public int height;
 public ByteBuffer pixels;


 public PNG(Bitmap bitmap)
 {
  this.width = bitmap.getWidth();
  this.height = bitmap.getHeight();
  this.pixels = ByteBuffer.allocateDirect(this.width * this.height * BYTES_PER_PIXEL_PNG);
  bitmap.copyPixelsToBuffer(this.pixels);
 }
}

public static PNG loadPNG(String path)
{
    InputStream is = null;
    try
    {
        is = ASSETS_MANAGER.open(path);//get png file stream with AssetsManager instance
    }
    catch (IOException e)
    {
        Log.e(LOG_TAG, "Can't load png - " + path, e);
    }

    return new PNG(BitmapFactory.decodeStream(is));
}

      

        
        
        
      

    

C ++ side PNG

typedef struct png
{
int width;
int height;
char* pixels;
} png;

png* load_png(const char* path)
{
 png* res = (res*) malloc(sizeof(png);
 ...
 jobject _png = env->CallStaticObjectMethod(get_java_lib_class(), get_method_id(JAVA_LIB_LOAD_PNG, JAVA_LIB_LOAD_PNG_SIGN), _path);//Calling loadPng() from Java, get PNG jobject
 jobject _pixels =  env->GetObjectField(_png, PNG_FIELDS->PNG_PIXELS_ID);//Getting pixels field from Java PNG jobject
 res->pixels = (char*) env->GetDirectBufferAddress(_pixels);//Get direct pointer to our pixel data
 //Cleanup
 ...
 env->DeleteLocalRef(_png);
 env->DeleteLocalRef(_pixels);
 return res;
}

      

        
        
        
      

    

Then using png to create the texture

void test_create_tex(const char* path)
{
 ...
 png* source = load_png(path);
 glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, source->width, source->height, 0, GL_RGBA, GL_UNSIGNED_BYTE, source->pixels);
//We don't need source pixel data any more
//free(source->pixels);//INVALID HEAP ADDRESS (deadbaad)
free(source);
}

      

        
        
        
      

    

So how to free the byte buffer after using on the C ++ side with a direct pointer? It is allocated directly (like malloc is native) and must be deallocated or I will get an OutOfMemory error.