Java Reference
In-Depth Information
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DatagramSocket and MulticastSocket
The
DatagramSocket
and
MulticastSocket
classes let you perform UDP-
basedcommunicationsbetweenapairofhosts(
DatagramSocket
)orbetweenmul-
tiplehosts(
MulticastSocket
).Witheitherclass,youcommunicateone-waymes-
sagesvia
datagram packets
,whicharearraysofbytesassociatedwithinstancesofthe
DatagramPacket
class.
Note
Althoughyoumightthinkthat
Socket
and
ServerSocket
areallthatyou
need,
DatagramSocket
(and its
MulticastSocket
subclass) have their uses.
Forexample,considerascenariowhereagroupofmachinesneedtooccasionallytell
a server that they're alive. It shouldn't matter when the occasional message is lost or
evenwhenthemessagedoesn'tarriveontime.Anotherexampleisalow-prioritystock
tickerthatperiodicallybroadcastsstockprices.Whenapacketdoesn'tarrive,oddsare
thatthenextpacketwillarriveandyou'llthenreceivenotificationofthelatestprices.
Timely rather than reliable or orderly delivery is more important in realtime applica-
tions.
DatagramPacket
declares several constructors with
DatagramPack-
et(byte[] buf, int length)
being the simplest. This constructor requires
youtopassbytearrayandintegerargumentsto
buf
and
length
,where
buf
isadata
bufferthatstoresdatatobesentorreceived,and
length
(whichmustbelessthanor
equalto
buf.length
)specifiesthenumberofbytes(startingat
buf[0]
)tosendor
receive.
The following example demonstrates this constructor:
byte[] buffer = new byte[100];
DatagramPacket dgp = new DatagramPacket(buffer, buf-
fer.length);
Note
Additional constructors let you specify an offset into
buf
that identifies the
storagelocationofthefirstoutgoingorincomingbyte,and/orletyouspecifyadestin-
ation socket address.
