Sockets for Servers - Java Network Programming - page 306

public class RandomPort {
public static void main ( String [] args ) {
try {
ServerSocket server = new ServerSocket ( 0 );
System . out . println ( "This server runs on port "
+ server . getLocalPort ());
} catch ( IOException ex ) {
System . err . println ( ex );
}
}
}
Here's the output of several runs:
$ java RandomPort
This server runs on port 1154
D: \ JAVA \ JNP4 \ examples \ 9 > java RandomPort
This server runs on port 1155
D: \ JAVA \ JNP4 \ examples \ 9 > java RandomPort
This server runs on port 1156
At least on this system, the ports aren't truly random, but they are indeterminate until
runtime.
If the ServerSocket has not yet bound to a port, getLocalPort() returns -1.
As with most Java objects, you can also just print out a ServerSocket using its to
String() method. A String returned by a ServerSocket 's toString() method looks
like this:
ServerSocket [ addr = 0.0 . 0.0 , port = 0 , localport = 5776 ]
addr is the address of the local network interface to which the server socket is bound.
This will be 0.0.0.0 if it's bound to all interfaces, as is commonly the case. port is always
0. The localport is the local port on which the server is listening for connections. This
method is sometimes useful for debugging, but not much more. Don't rely on it.
Socket Options
Socket options specify how the native sockets on which the ServerSocket class relies
send and receive data. For server sockets, Java supports three options:
• SO_TIMEOUT
• SO_REUSEADDR
• SO_RCVBUF
It also allows you to set performance preferences for the socket's packets.