获取相对于原始ByteBuffer的直接ByteBuffer切片位置

Question

Is there any way of getting a direct ByteBuffer zero position relative to another, given only the two ByteBuffer objects, if it is known that it is a subsequence of the other buffer?

I know that this can be done with a non-direct array backed ByteBuffer using the arrayOffset() method like so:

int getRelativeBufferOffset(ByteBuffer parentBuffer, ByteBuffer childBuffer)
{
  return childBuffer.arrayOffset() - parentBuffer.arrayOffset();
}

void example()
{
  ByteBuffer buffer1 = ByteBuffer.allocate(10000);
  buffer1 .position(22);
  ByteBuffer buffer2 = buffer1.slice();
  buffer2.position(55);
  ByteBuffer buffer3 = buffer2.slice();

  // returns 22
  getRelativeBufferOffset(buffer1, buffer2);

  // returns 55
  getRelativeBufferOffset(buffer2, buffer3);

  // returns 77
  getRelativeBufferOffset(buffer1, buffer3);
}

I'm thinking that there isn't anything that exists for free for direct buffers. In order to get something similar the best option I can think of is to extend ByteBuffer to store a reference to the buffer from which it was created (the parent buffer) and the zero position relative to the parent's zero position at which it was created.

EDIT: Annoyingly it looks like I can't extend ByteBuffer because none of its constructors are visible. I guess I will have to write some kind of wrapper class instead.

Answer 1

It is possible, using reflection. But depending on what you want to achieve, you should consider alternative solutions. It's not clear for which purpose you need this offset. The "pragmatic" recommendation would be to wrap the buffers into a simple, own class, like

class SlicedBuffer {
    int getBuffer() { ... }
    Buffer getParent() { ... }
    int getOffsetToParent() { ... }
}

and work with this one, but it's not clear whether this is applicable in your case.

I'll post the code using reflection here, but note that

// Many things...
// ... can go ...
// ... wrong when...
// ... using reflection

so this is only a demonstration:

import java.lang.reflect.Field;
import java.nio.Buffer;
import java.nio.ByteBuffer;

public class DirectByteBufferSliceOffsetsTest
{
    public static void main(String[] args)
    {
        testArray();
        testDirect();
    }

    private static void testArray()
    {
        System.out.println("Array: ");

        ByteBuffer buffer1 = ByteBuffer.allocate(10000);
        buffer1.position(22);
        ByteBuffer buffer2 = buffer1.slice();
        buffer2.position(55);
        ByteBuffer buffer3 = buffer2.slice();

        // prints 22
        System.out.println(getRelativeBufferOffsetArray(buffer1, buffer2));

        // prints 55
        System.out.println(getRelativeBufferOffsetArray(buffer2, buffer3));

        // prints 77
        System.out.println(getRelativeBufferOffsetArray(buffer1, buffer3));
    }

    private static int getRelativeBufferOffsetArray(
        ByteBuffer parentBuffer, ByteBuffer childBuffer)
    {
        return childBuffer.arrayOffset() - parentBuffer.arrayOffset();
    }


    private static void testDirect()
    {
        System.out.println("Direct: ");

        ByteBuffer buffer1 = ByteBuffer.allocateDirect(10000);
        buffer1.position(22);
        ByteBuffer buffer2 = buffer1.slice();
        buffer2.position(55);
        ByteBuffer buffer3 = buffer2.slice();

        // prints 22
        System.out.println(getRelativeBufferOffsetDirect(buffer1, buffer2));

        // prints 55
        System.out.println(getRelativeBufferOffsetDirect(buffer2, buffer3));

        // prints 77
        System.out.println(getRelativeBufferOffsetDirect(buffer1, buffer3));
    }

    private static int getRelativeBufferOffsetDirect(
        ByteBuffer parentBuffer, ByteBuffer childBuffer)
    {
        long parentAddress = getAddress(parentBuffer);
        long childAddress = getAddress(childBuffer);
        int offset = (int)(childAddress - parentAddress);
        return offset;
    }

    private static long getAddress(Buffer buffer)
    {
        Field f = null;
        try
        {
            f = Buffer.class.getDeclaredField("address");
            f.setAccessible(true);
            return f.getLong(buffer);
        }
        catch (NoSuchFieldException e)
        {
            // Many things...
            e.printStackTrace();
        }
        catch (SecurityException e)
        {
            // ... can go ...
            e.printStackTrace();
        }
        catch (IllegalArgumentException e)
        {
            // ... wrong when...
            e.printStackTrace();
        }
        catch (IllegalAccessException e)
        {
            // ... using reflection
            e.printStackTrace();
        }
        finally
        {
            if (f != null)
            {
                f.setAccessible(false);
            }
        }
        return 0;
    }
}