Is there a way to create a direct ByteBuffer from a pointer only in Java?

Question

Or do I need to have a JNI helper function that calls env-> NewDirectByteBuffer (buffer, size)?

Answer 1

What I do is create a regular DirectByteBuffer and change its address.

Field address = Buffer.class.getDeclaredField("address"); address.setAccessible(true); Field capacity = Buffer.class.getDeclaredField("capacity"); capacity.setAccessible(true); ByteBuffer bb = ByteBuffer.allocateDirect(0).order(ByteOrder.nativeOrder()); address.setLong(bb, addressYouWantToSet); capacity.setInt(bb, theSizeOf); 

From now on, you can access ByteBuffer by referencing the base address. I did this to access memory on network adapters for a null copy and worked fine.

You can create a DirectByteBuffer for your address directly, but this is more obscure.

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An alternative is to use Unsafe (this only works in the OpenJDK / HotSpot JVM and in native byte order)

 Unsafe.getByte(address); Unsafe.getShort(address); Unsafe.getInt(address); Unsafe.getLong(address);