Question

Is there a way to expand the Java memory-mapped byte buffer such that the new size is reflected back to the mapped file on disk?

Answer 1

No, you will need to adjust the size of the underlying file and recreate the Memory Mapped Byte Buffer.

RandomAccessFile file = new RandomAccessFile(/* some file */);
MappedByteBuffer buffer = file.getChannel().map(MapMode.READ_WRITE, 0, file.length());

// Some stuff happens...

// adjust the size
file.setLength(newLength);

// recreate the memory mapped buffer
buffer = file.getChannel().map(MapMode.READ_WRITE, 0, file.length());

Note: Setting the file length has some slightly odd behaviour. If you write to the file via the map at a specific position that is beyond the end of the file (either using map.position() or map.putX(position, …)) the values will be appended to the end of the file and not written at the position you expect (on linux at least). If this is undesired behaviour you will need to append data to the file in order to truly grow the file.