Sockets for Clients - Java Network Programming - page 241

• The string UTC(NIST) is a constant, and the OTM is almost a constant (an asterisk
unless something really weird has happened).
These details are all NIST specific. They are not part of the daytime standard. Although
they do offer a lot of data, if you have a real programmatic need to sync with a network
time server, you're better off using the NTP protocol defined in RFC 5905 instead.
I'm not sure how long this example is going to work as shown here.
These servers are overloaded, and I did have intermittent problems
connecting while writing this chapter. In early 2013, NIST an‐
nounced , “Users of the NIST DAYTIME protocol on tcp port 13 are
also strongly encouraged to upgrade to the network time protocol,
which provides greater accuracy and requires less network band‐
width. The NIST time client (nistime-32bit.exe) supports both proto‐
cols. We expect to replace the tcp version of this protocol with a udp-
based version near the end of 2013.” I'll show you how to access this
service over UDP in Chapter 11 .
Now let's see how to retrieve this same data programmatically using sockets. First, open
a socket to time.nist.gov on port 13:
Socket socket = new Socket ( "time.nist.gov" , 13 );
This doesn't just create the object. It actually makes the connection across the network.
If the connection times out or fails because the server isn't listening on port 13, then the
constructor throws an IOException , so you'll usually wrap this in a try block. In Java
7, Socket implements Autocloseable so you can use try-with-resources:
try ( Socket socket = new Socket ( "time.nist.gov" , 13 )) {
// read from the socket...
} catch ( IOException ex ) {
System . err . println ( "Could not connect to time.nist.gov" );
}
In Java 6 and earlier, you'll want to explicitly close the socket in a finally block to
release resources the socket holds:
Socket socket = null ;
try {
socket = new Socket ( hostname , 13 );
// read from the socket...
} catch ( IOException ex ) {
System . err . println ( ex );
} finally {
if ( socket != null ) {
try {
socket . close ();
} catch ( IOException ex ) {