getting the memory location for a java byte array using sun.misc.unsafe

Question

I want to pass the memory location of a byte array to a jni call so that the array does not have to be copied. There is a question here that hints this is possible with sun.misc.Unsafe. Is there an example of how this can be done?

The use case is that I have a BufferedImage and I have a OpenGL method that takes in a pointer. I know that the obvious thing to do is to allocate a direct bytebuffer but I want to save on the copy.

Never use Unsafe. JNI doesn't need it – Mark Jeronimus Oct 04 '18 at 15:42 — Mark Jeronimus, Oct 04 '18 at 15:42

score 0 · Accepted Answer · answered Oct 04 '18 at 15:44

0

First off, if at all feasible, migrate to JNA, it's much more user friendly (doesn't require you to compile c files)

If you want to pass data directly to native code, don't use an array, but use a ByteBuffer instead. To create it, use ByteBuffer.allocateDirect() (the direct part signifies that it doesn't use an internal array but 'direct' memory.) JNI and JNA should automatically detect this and pass the pointer to the native library.

answered Oct 04 '18 at 15:44

Mark Jeronimus

9,278
3
37
50

How did you obtain the bufferedimage? Isn't it possible to load an image using the native lib? (e.g. LWJGL has STBImage) – Mark Jeronimus Oct 05 '18 at 08:40
yeah, I went with the alternative method of creating a DataBuffer wrapping a ByteBuffer. – zcaudate Oct 05 '18 at 14:43

score 0 · Answer 2 · answered Oct 05 '18 at 14:48

After doing some reading, it seems that it's not the best idea. I ended up using this to pass the native data to the BufferedImage.

import java.awt.image.DataBuffer;
import java.nio.ByteBuffer;

public class DirectDataBuffer extends DataBuffer {
    protected ByteBuffer directByteBuffer;

    public DirectDataBuffer(ByteBuffer directByteBuffer) {
        super(TYPE_BYTE, directByteBuffer.capacity());
        this.directByteBuffer = directByteBuffer;   
    }

    public int getElem(int bank, int i) {
        return directByteBuffer.get(i);
    }

    public void setElem(int bank, int i, int val) {
        directByteBuffer.put(i, (byte)val);
    }

    public ByteBuffer getDirectByteBuffer() {
        return directByteBuffer;
    }
}

getting the memory location for a java byte array using sun.misc.unsafe

2 Answers2