Nonblocking I/O - Java Network Programming - page 370

Compacting Buffers
Most writable buffers support a compact() method:
public abstract ByteBuffer compact ()
public abstract IntBuffer compact ()
public abstract ShortBuffer compact ()
public abstract FloatBuffer compact ()
public abstract CharBuffer compact ()
public abstract DoubleBuffer compact ()
(If it weren't for invocation chaining, these six methods could have been replaced by
one method in the common Buffer superclass.) Compacting shifts any remaining data
in the buffer to the start of the buffer, freeing up more space for elements. Any data that
was in those positions will be overwritten. The buffer's position is set to the end of the
data so it's ready for writing more data.
Compacting is an especially useful operation when you're copying —reading from one
channel and writing the data to another using nonblocking I/O. You can read some data
into a buffer, write the buffer out again, then compact the data so all the data that wasn't
written is at the beginning of the buffer, and the position is at the end of the data re‐
maining in the buffer, ready to receive more data. This allows the reads and writes to
be interspersed more or less at random with only one buffer. Several reads can take place
in a row, or several writes follow consecutively. If the network is ready for immediate
output but not input (or vice versa), the program can take advantage of that. This tech‐
nique can be used to implement an echo server as shown in Example 11-5 . The echo
protocol simply responds to the client with whatever data the client sent. Like chargen,
it's useful for network testing. Also like chargen, echo relies on the client to close the
connection. Unlike chargen, however, an echo server must both read and write from
the connection.
Example 11-5. Echo server
import java.nio.* ;
import java.nio.channels.* ;
import java.net.* ;
import java.util.* ;
import java.io.IOException ;
public class EchoServer {
public static int DEFAULT_PORT = 7 ;
public static void main ( String [] args ) {
int port ;
try {
port = Integer . parseInt ( args [ 0 ]);
} catch ( RuntimeException ex ) {
port = DEFAULT_PORT ;