FileChannel inChannel = new FileInputStream(fileName).getChannel();

ByteBuffer buffer = ByteBuffer.allocateDirect(CAPACITY);

while(inChannel.read(buffer) > 0)

    buffer.clear(); // do something with the data and clear/compact it.

inChannel.close();

FileChannel inChannel = new RandomAccessFile(fileName, "r").getChannel();

MappedByteBuffer buffer = inChannel.map(FileChannel.MapMode.READ_ONLY, 0, inChannel.size());

// access the buffer as you wish.

inChannel.close();