Java Reference
In-Depth Information
Given this class, UDP daytime, time, chargen, and quote of the day clients are almost
trivial. A time client is only slightly harder, and only because you need to convert the
four raw bytes returned by the server to a
java.util.Date
object. The same algorithm
as in
Example 8-3
will accomplish that as demonstrated in
Example 12-8
.
Example 12-8. A UDP time client
import
java.net.*
;
import
java.util.*
;
public
class
UDPTimeClient
{
public
final
static
int
PORT
=
37
;
public
final
static
String
DEFAULT_HOST
=
"time.nist.gov"
;
public
static
void
main
(
String
[]
args
)
{
InetAddress
host
;
try
{
if
(
args
.
length
>
0
)
{
host
=
InetAddress
.
getByName
(
args
[
0
]);
}
else
{
host
=
InetAddress
.
getByName
(
DEFAULT_HOST
);
}
}
catch
(
RuntimeException
|
UnknownHostException
ex
)
{
System
.
out
.
println
(
"Usage: java UDPTimeClient [host]"
);
return
;
}
UDPPoke
poker
=
new
UDPPoke
(
host
,
PORT
);
byte
[]
response
=
poker
.
poke
();
if
(
response
==
null
)
{
System
.
out
.
println
(
"No response within allotted time"
);
return
;
}
else
if
(
response
.
length
!=
4
)
{
System
.
out
.
println
(
"Unrecognized response format"
);
return
;
}
// The time protocol sets the epoch at 1900,
// the Java Date class at 1970. This number
// converts between them.
long
differenceBetweenEpochs
=
2208988800L
;
long
secondsSince1900
=
0
;
for
(
int
i
=
0
;
i
<
4
;
i
++)
{
secondsSince1900
=
(
secondsSince1900
<<
8
)
|
(
response
[
i
]
&
0x000000FF
);
}
long
secondsSince1970
