Nonblocking I/O - Java Network Programming - page 351

buffer . flip ();
output . write ( buffer );
You don't have to tell the output channel how many bytes to write. Buffers keep track
of how many bytes they contain. However, in general, the output channel is not guar‐
anteed to write all the bytes in the buffer. In this specific case, though, it's a blocking
channel and it will either do so or throw an IOException .
You shouldn't create a new buffer for each read and write. That would kill the perfor‐
mance. Instead, reuse the existing buffer. You'll need to clear the buffer before reading
into it again:
buffer . clear ();
This is a little different than flipping. Flipping leaves the data in the buffer intact, but
prepares it for writing rather than reading. Clearing resets the buffer to a pristine state.
(Actually that's a tad simplistic. The old data is still present; it's not overwritten, but it
will be overwritten with new data read from the source as soon as possible.)
Example 11-1 puts this together into a complete client. Because chargen is by design an
endless protocol, you'll need to kill the program using Ctrl-C.
Example 11-1. A channel-based chargen client
import java.nio.* ;
import java.nio.channels.* ;
import java.net.* ;
import java.io.IOException ;
public class ChargenClient {
public static int DEFAULT_PORT = 19 ;
public static void main ( String [] args ) {
if ( args . length == 0 ) {
System . out . println ( "Usage: java ChargenClient host [port]" );
return ;
}
int port ;
try {
port = Integer . parseInt ( args [ 1 ]);
} catch ( RuntimeException ex ) {
port = DEFAULT_PORT ;
}
try {
SocketAddress address = new InetSocketAddress ( args [ 0 ], port );
SocketChannel client = SocketChannel . open ( address );
ByteBuffer buffer = ByteBuffer . allocate ( 74 );