Sockets for Servers - Java Network Programming - page 305

public void bind ( SocketAddress endpoint ) throws IOException
public void bind ( SocketAddress endpoint , int queueLength ) throws IOException
The primary use for this feature is to allow programs to set server socket options before
binding to a port. Some options are fixed after the server socket has been bound. The
general pattern looks like this:
ServerSocket ss = new ServerSocket ();
// set socket options...
SocketAddress http = new InetSocketAddress ( 80 );
ss . bind ( http );
You can also pass null for the SocketAddress to select an arbitrary port. This is like
passing 0 for the port number in the other constructors.
Getting Information About a Server Socket
The ServerSocket class provides two getter methods that tell you the local address and
port occupied by the server socket. These are useful if you've opened a server socket on
an anonymous port and/or an unspecified network interface. This would be the case,
for one example, in the data connection of an FTP session:
public InetAddress getInetAddress ()
This method returns the address being used by the server (the local host). If the local
host has a single IP address (as most do), this is the address returned by InetAd
dress.getLocalHost() . If the local host has more than one IP address, the specific
address returned is one of the host's IP addresses. You can't predict which address you
will get. For example:
ServerSocket httpd = new ServerSocket ( 80 );
InetAddress ia = httpd . getInetAddress ();
If the ServerSocket has not yet bound to a network interface, this method returns null:
public int getLocalPort ()
The ServerSocket constructors allow you to listen on an unspecified port by passing
0 for the port number. This method lets you find out what port you're listening on. You
might use this in a peer-to-peer multisocket program where you already have a means
to inform other peers of your location. Or a server might spawn several smaller servers
to perform particular operations. The well-known server could inform clients on what
ports they can find the smaller servers. Of course, you can also use getLocalPort() to
find a nonanonymous port, but why would you need to? Example 9-9 demonstrates.
Example 9-9. A random port
import java.io.* ;
import java.net.* ;