UDP - Java Network Programming - page 432

not have to use it thereafter, and you don't have to use it all in Java 7 and later. Nor do
you ever use DatagramPacket . Instead, you read and write byte buffers, just as you do
with a SocketChannel .
Opening a socket
The java.nio.channels.DatagramChannel class does not have any public construc‐
tors. Instead, you create a new DatagramChannel object using the static open() method
For example:
DatagramChannel channel = DatagramChannel.open();
This channel is not initially bound to any port. To bind it, you access the channel's peer
DatagramSocket object using the socket() method. For example, this binds a channel
to port 3141:
SocketAddress address = new InetSocketAddress(3141);
DatagramSocket socket = channel.socket();
socket.bind(address);
Java 7 adds a convenient bind() method directly to DatagramChannel , so you don't have
to use a DatagramSocket at all. For example:
SocketAddress address = new InetSocketAddress(3141);
channel.bind(address);
Receiving
The receive() method reads one datagram packet from the channel into a ByteBuffer .
It returns the address of the host that sent the packet:
public SocketAddress receive ( ByteBuffer dst ) throws IOException
If the channel is blocking (the default), this method will not return until a packet has
been read. If the channel is nonblocking, this method will immediately return null if no
packet is available to read.
If the datagram packet has more data than the buffer can hold, the extra data is thrown
away with no notification of the problem . You do not receive a BufferOverflowExcep
tion or anything similar. Again you see that UDP is unreliable. This behavior introduces
an additional layer of unreliability into the system. The data can arrive safely from the
network and still be lost inside your own program.
Using this method, you can reimplement the discard server to log the host sending the
data as well as the data sent. Example 12-15 demonstrates. It avoids the potential loss of
data by using a buffer that's big enough to hold any UDP packet and clearing it before
it's used again.
Example 12-15. A UDPDiscardServer based on channels
import java.io.* ;
import java.net.* ;