Writing a Datagram Client and Server

The client application in this example is fairly simple. It sends a single datagram packet to the server indicating that the client would like to receive a quote of the moment. The client then waits for the server to send a datagram packet in response.

Two classes implement the server application: QuoteServer and QuoteServerThread. A single class implements the client application: QuoteClient.

Let's investigate these classes, starting with the class that contains the main method for the server application. Working With a Server-Side Application contains an applet version of the QuoteClient class.

The QuoteServer Class

import java.io.*;

public class QuoteServer {
    public static void main(String[] args) throws IOException {
        new QuoteServerThread().start();
    }
}

The QuoteServerThread Class

public QuoteServerThread() throws IOException {
    this("QuoteServer");
}
public QuoteServerThread(String name) throws IOException {
    super(name);
    socket = new DatagramSocket(4445);

    try {
        in = new BufferedReader(
                  new FileReader("one-liners.txt"));	
    } catch (FileNotFoundException e)
        System.err.println("Couldn't open quote file. " + 
                          "Serving time instead.");
    }
}  

The constructor also opens a BufferedReader on a file named one-liners.txt which contains a list of quotes. Each quote in the file is on a line by itself.

Now for the interesting part of the QuoteServerThread: its run method. The run method overrides run in the Thread class and provides the implementation for the thread. For information about threads, see Defining and Starting a Thread.

The run method contains a while loop that continues as long as there are more quotes in the file. During each iteration of the loop, the thread waits for a DatagramPacket to arrive over the DatagramSocket. The packet indicates a request from a client. In response to the client's request, the QuoteServerThread gets a quote from the file, puts it in a DatagramPacket and sends it over the DatagramSocket to the client that asked for it.

Let's look first at the section that receives the requests from clients:

byte[] buf = new byte[256];
DatagramPacket packet = new DatagramPacket(buf, buf.length);
socket.receive(packet);

The last statement in the previous code snippet receives a datagram from the socket (the information received from the client gets copied into the packet). The receive method waits forever until a packet is received. If no packet is received, the server makes no further progress and just waits.

Now assume that, the server has received a request from a client for a quote. Now the server must respond. This section of code in the run method constructs the response:

String dString = null;
if (in == null)
    dString = new Date().toString();
else
    dString = getNextQuote();
buf = dString.getBytes();

Now, the run method sends the response to the client over the DatagramSocket with this code:

InetAddress address = packet.getAddress();
int port = packet.getPort();
packet = new DatagramPacket(buf, buf.length, address, port);
socket.send(packet);

The third statement creates a new DatagramPacket object intended for sending a datagram message over the datagram socket. You can tell that the new DatagramPacket is intended to send data over the socket because of the constructor used to create it. This constructor requires four arguments. The first two arguments are the same required by the constructor used to create receiving datagrams: a byte array containing the message from the sender to the receiver and the length of this array. The next two arguments are different: an Internet address and a port number. These two arguments are the complete address of the destination of the datagram packet and must be supplied by the sender of the datagram. The last line of code sends the DatagramPacket on its way.

When the server has read all the quotes from the quote file, the while loop terminates and the run method cleans up:

socket.close();

The QuoteClient Class

The QuoteClient class contains one method, the main method for the client application. The top of the main method declares several local variables for its use:

int port;
InetAddress address;
DatagramSocket socket = null;
DatagramPacket packet;
byte[] sendBuf = new byte[256];

if (args.length != 1) {
     System.out.println("Usage: java QuoteClient <hostname>");
     return;
}

Next, the main method creates a DatagramSocket:

DatagramSocket socket = new DatagramSocket();

Next, the QuoteClient program sends a request to the server:

byte[] buf = new byte[256];
InetAddress address = InetAddress.getByName(args[0]);
DatagramPacket packet = new DatagramPacket(buf, buf.length, 
                                           address, 4445);
socket.send(packet);

Note that the code creates a DatagramPacket with an empty byte array. The byte array is empty because this datagram packet is simply a request to the server for information. All the server needs to know to send a response--the address and port number to which reply--is automatically part of the packet.

Next, the client gets a response from the server and displays it:

packet = new DatagramPacket(buf, buf.length);
socket.receive(packet);
String received = new String(packet.getData(), 0, packet.getLength());
System.out.println("Quote of the Moment: " + received);

When the client receives a reply from the server, the client uses the getData method to retrieve that data from the packet. The client then converts the data to a string and displays it.

Running the Server and Client

Once the server has started, you can run the client program. Remember to run the client program with one command-line argument: the name of the host on which the QuoteServer is running.

After the client sends a request and receives a response from the server, you should see output similar to this:

Quote of the Moment:
Good programming is 99% sweat and 1% coffee.