Java client server protocol

Java UDP Client Server Program Example

In this Java Network programming tutorial, you will learn how to code a client/server application based on UDP protocol.

First, let’s see how Java Network API is designed to support development of network applications that make use of UDP.

DatagramPacket and DatagramSocket are the two main classes that are used to implement a UDP client/server application. DatagramPacket is a data container and DatagramSocket is a mechanism to send and receive DatagramPacket s.

1. DatagramPacket

In UDP’s terms, data transferred is encapsulated in a unit called datagram. A datagram is an independent, self-contained message sent over the network whose arrival, arrival time, and content are not guaranteed. And in Java, DatagramPacket represents a datagram.

You can create a DatagramPacket object by using one of the following constructors:

• DatagramPacket(byte[] buf, int length)

• DatagramPacket(byte[] buf, int length, InetAddress address, int port)

The second constructor creates a DatagramPacket to be sent, so you need to specify the address and port number of the destination host.

The parameter length specifies the amount of data in the byte array to be used, usually is the length of the array ( buf.length ).

There are also other constructors that allow you to specify the offset in the byte array, as well as using a SocketAddress :

• DatagramPacket(byte[] buf, int offset, int length)

• DatagramPacket(byte[] buf, int offset, int length, SocketAddress address)

2. DatagramSocket

You use DatagramSocket to send and receive DatagramPacket s. DatagramSocket represents a UDP connection between two computers in a network.

In Java, we use DatagramSocket for both client and server. There are no separate classes for client and server like TCP sockets.

So you create a DatagramSocket object to establish a UDP connection for sending and receiving datagram, by using one of the following constructors:

• DatagramSocket()

• DatagramSocket(int port)

• DatagramSocket(int port, InetAddress laddr)

And the third constructor binds the server to the specified IP address (in case the computer has multiple IP addresses).

These constructors can throw SocketException if the socket could not be opened, or the socket could not bind to the specified port or address. So you have catch or re-throw this checked exception.

The key methods of the DatagramSocket include:

• send(DatagramPacket p) : sends a datagram packet.

• receive(DatagramPacket p) : receives a datagram packet.

• setSoTimeout(int timeout) : sets timeout in milliseconds, limiting the waiting time when receiving data. If the timeout expires, a SocketTimeoutException is raised.

• close() : closes the socket.

Now, let’s see some sample programs in action.

3. Java UDP Client Example

We will write code for a client program that requests for quotes from a server that implements the Quote of the Day (QOTD) service — an Internet standard. The following code snippet sends a DatagramPacket to a server specified by hostname and port:

String hostname = "djxmmx.net"; int port = 17; InetAddress address = InetAddress.getByName(hostname); DatagramSocket socket = new DatagramSocket(); byte[] buffer = new byte[512]; DatagramPacket request = new DatagramPacket(buffer, buffer.length, address, port); socket.send(request);

As you can see, the buffer has no data because QOTD service doesn’t require a client sends any specific message. And you have to specify the server’s address and port in the DatagramPacket . So this code just sends a signal to the server implying that “Hey, I’d like to get a quote from you”.

And the following code snippet receives a DatagramPacket from the server:

DatagramPacket response = new DatagramPacket(buffer, buffer.length); socket.receive(response); String quote = new String(buffer, 0, response.getLength()); System.out.println(quote);

As you can see, once the socket is opened, receiving a packet is very simple. And the code converts the byte array to a String to be printed in readable format.

And here is the code of the full client program that parameterizes the hostname and port number, handles exceptions and gets a quote from the server for every 10 seconds:

import java.io.*; import java.net.*; /** * This program demonstrates how to implement a UDP client program. * * * @author www.codejava.net */ public class QuoteClient < public static void main(String[] args) < if (args.length < 2) < System.out.println("Syntax: QuoteClient "); return; > String hostname = args[0]; int port = Integer.parseInt(args[1]); try < InetAddress address = InetAddress.getByName(hostname); DatagramSocket socket = new DatagramSocket(); while (true) < DatagramPacket request = new DatagramPacket(new byte[1], 1, address, port); socket.send(request); byte[] buffer = new byte[512]; DatagramPacket response = new DatagramPacket(buffer, buffer.length); socket.receive(response); String quote = new String(buffer, 0, response.getLength()); System.out.println(quote); System.out.println(); Thread.sleep(10000); >> catch (SocketTimeoutException ex) < System.out.println("Timeout error: " + ex.getMessage()); ex.printStackTrace(); >catch (IOException ex) < System.out.println("Client error: " + ex.getMessage()); ex.printStackTrace(); >catch (InterruptedException ex) < ex.printStackTrace(); >> >

java QuoteClient djxmmx.net 17

You would see the output something like this:

"When a stupid man is doing something he is ashamed of, he always declares that it is his duty." George Bernard Shaw (1856-1950) "Oh the nerves, the nerves; the mysteries of this machine called man! Oh the little that unhinges it, poor creatures that we are!" Charles Dickens (1812-70) In Heaven an angel is nobody in particular." George Bernard Shaw (1856-1950)

If you test this program yourself, you may see different quotes because the server returns random quotes. Press Ctrl + C to terminate the program.

4. Java UDP Server Example

The following sample program demonstrates how to implement a server for the above client. The following code creates a UDP server listening on port 17 and waiting for client’s request:

DatagramSocket socket = new DatagramSocket(17); byte[] buffer = new byte[256]; DatagramPacket request = new DatagramPacket(buffer, buffer.length); socket.receive(request);

The receive() method blocks until a datagram is received. And the following code sends a DatagramPacket to the client:

InetAddress clientAddress = request.getAddress(); int clientPort = request.getPort(); String data = "Message from server"; buffer = data.getBytes(); DatagramPacket response = new DatagramPacket(buffer, buffer.length, clientAddress, clientPort); socket.send(response);

As you can see, the server also needs to know client’s address and port to send the DatagramPacket . This information can be obtained from the DatagramPacket received from the client previously. And a String is converted to an array of bytes which then can be wrapped in a DatagramPacket .

And the following is a full-featured server program that reads quotes from a text file, and sends a random quote for every client’s request. The quote file and port number are given as program’s arguments. Here’s the code:

import java.io.*; import java.net.*; import java.util.*; /** * This program demonstrates how to implement a UDP server program. * * * @author www.codejava.net */ public class QuoteServer < private DatagramSocket socket; private ListlistQuotes = new ArrayList(); private Random random; public QuoteServer(int port) throws SocketException < socket = new DatagramSocket(port); random = new Random(); >public static void main(String[] args) < if (args.length < 2) < System.out.println("Syntax: QuoteServer "); return; > String quoteFile = args[0]; int port = Integer.parseInt(args[1]); try < QuoteServer server = new QuoteServer(port); server.loadQuotesFromFile(quoteFile); server.service(); >catch (SocketException ex) < System.out.println("Socket error: " + ex.getMessage()); >catch (IOException ex) < System.out.println("I/O error: " + ex.getMessage()); >> private void service() throws IOException < while (true) < DatagramPacket request = new DatagramPacket(new byte[1], 1); socket.receive(request); String quote = getRandomQuote(); byte[] buffer = quote.getBytes(); InetAddress clientAddress = request.getAddress(); int clientPort = request.getPort(); DatagramPacket response = new DatagramPacket(buffer, buffer.length, clientAddress, clientPort); socket.send(response); >> private void loadQuotesFromFile(String quoteFile) throws IOException < BufferedReader reader = new BufferedReader(new FileReader(quoteFile)); String aQuote; while ((aQuote = reader.readLine()) != null) < listQuotes.add(aQuote); >reader.close(); > private String getRandomQuote() < int randomIndex = random.nextInt(listQuotes.size()); String randomQuote = listQuotes.get(randomIndex); return randomQuote; >>

Whether you think you can or you think you can't, you're right - Henry Ford There are no traffic jams along the extra mile - Roger Staubach Build your own dreams, or someone else will hire you to build theirs - Farrah Gray What you do today can improve all your tomorrows - Ralph Marston Remember that not getting what you want is sometimes a wonderful stroke of luck - Dalai Lama

java QuoteServer Quotes.txt 17

java QuoteClient localhost 17

Both the client and server are running in an infinite loop, so you have to press Ctrl + C to terminate.

That’s the lesson about how to develop a network client/server application relying on UDP protocol. Based on this knowledge, you are able to develop client programs that communicate with servers via UDP, and developing your own UDP client/server applications.

API References:

Related Java Network Tutorials:

Other Java network tutorials:

About the Author:

Nam Ha Minh is certified Java programmer (SCJP and SCWCD). He started programming with Java in the time of Java 1.4 and has been falling in love with Java since then. Make friend with him on Facebook and watch his Java videos you YouTube.

Add comment

Comments

Hi Delaram,
For multiple clients, you modify the QuoteClient program to execute the code that calls server in a new thread.
Or just run multiple instances of the client program.

Excellent tutorial. The only problem I had to get server and client running was nothing to do with UDP, but the fact that I had the package call at the beginning of the programs. The package call needs to be removed for the programs to work outside your IDE. Thanks for sharing!

Definitely one of the best tutorials on UDP that I’ve seen on the Internet . very well done. Have you done a lesson on sending binary data over UDP? Preferably for files of any size?

Thanks for this!
Very interesting and easy to grasp because of your good way of explaining a quit complex thing for me =)

Источник