request
Request buffer range atLeast bytes length
There are the following reasons for this function to return null:
not enough bytes available yet (should be at least
atLeastbytes available)due to buffer fragmentation it is impossible to represent the requested range as a single buffer range
end of stream encountered and all bytes were consumed
Return
buffer for the requested range or null if it is impossible to provide such a buffer view
Throws
if the channel has been closed with an exception or cancelled