You want to look up a host’s address name or number, or get the address at the other end of a network connection.
The InetAddress
object represents the Internet
address of a given computer or host. There are no public
constructors; you obtain an InetAddress
by calling
the static byName( )
method, passing in either a hostname like www.darwinsys.com or a network address as a
string, like “1.23.45.67”. All the “lookup”
methods in this class can throw the checked exception
UnknownHostException
, which must be caught or
declared on the calling method’s header. None of these methods
actually contacts the remote host, so they do not throw the other
exceptions related to network connections.
The method getHostAddress( )
gives
you the numeric IP address (as a string) corresponding to the
InetAddress
. The inverse is getHostName( )
, which reports the name of the
InetAddress
. This can be used to print the address
of a host given its name, or vice versa:
// From InetAddrDemo.java String ipNumber = "123.45.67.89"; String hostName = "www.darwinsys.com"; System.out.println(hostName + "'s address is " + InetAddress.byName(hostName).getHostAddress( )); System.out.println(ipNumber + "'s name is " + InetAddress.byName(ipNumber).getHostName( ));
You can also get an InetAddress
from a
Socket
by calling its getInetAddress( )
method. You can construct a
Socket
using an InetAddress
instead of a hostname string. So, to connect to port number
“myPortNumber” on the same host as an existing socket,
you’d use:
InetAddress remote = theSocket.getInetAddress( ); Socket anotherSocket = new Socket(remote, myPortNumber);
Finally, to look up all the
addresses associated with a host -- a
server may be on more than one network -- use the static method
getAllByName(host)
,
which returns an array of InetAddress
objects, one
for each IP address associated with the given name.
There is a static method getLocalHost( )
, which returns an
InetAddress
equivalent to “localhost”
or 127.0.0.1. This can be used to connect to a server on the same
machine as the client.
There is not yet a way to look up services, i.e., to find out that the
HTTP service is on port 80. Full implementations of TCP/IP have
always included an additional set of resolvers; in C, the call
getservbyname("http",
"tcp");
would look up the given service[32] and return a
servent
(service entry) structure whose
s
_port
member would contain the
value 80. The numbers of established services do not change, but when
services are new or installed in non-routine ways, it is convenient
to be able to change the service number for all programs on a machine
or network (regardless of programming language) just by changing the
services definitions. Java should provide this capability in a future
release.
[32] The location where it
is looked up varies. It might be in a file
named/etc/services
on Unix, or the
services
file under
windows
or winnt
under
MS-Windows; in a centralized registry such as Sun’s Network
Information Services (NIS, formerly YP); or in some other platform-
or network-dependent location.
3.144.9.147