Java UDP Sockets

UDP Introduction

TCP/IP sockets explained in this article is a reliable form of communication. However there is additional overhead involved in terms of error handling, congestion control etc. in TCP/IP communication. The other alternative available is to use UDP (User Datagram Protocol). UDP protocol doesn't guarantee that the message sent is transmitted reliably. Since UDP involves less overhead it is used in applications like video conferencing where real time performance is needed.

Java supports APIs to work with datagrams. There are 2 key classes DatagramSocket and DatagramPacket which are key elements to work with UDP protocol.

1. DatagramSocket -A datagram socket is the sending or receiving point for a packet delivery service. Each packet sent or received on a datagram socket is individually addressed and routed. Multiple packets sent from one machine to another may be routed differently, and may arrive in any order.
2. DatagramPacket - Datagram packets are used to implement a connectionless packet delivery service. Each message is routed from one machine to another based solely on information contained within that packet. Multiple packets sent from one machine to another might be routed differently, and might arrive in any order. Packet delivery is not guaranteed.

UDP (Datagram) server example in Java

In this example we implement a simple Datagram server. We construct a DatagramSocket and bind to a specific port on the local machine. Then we construct a DatagramPacket to receive packets. The receive call is a blocking call and waits till any packet is received from the client. When the receive method returns, the DatagramPacket's buffer is filled with the data received. Then we print the received contents.

package com.sourcetricks.server;

import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;

public class UDPServer {

  public static void main(String[] args) {
    final int port = 2222;
    byte buffer[] = new byte[1024];
    
    try ( DatagramSocket socket = new DatagramSocket(port); ) {
      while ( true ) {
        DatagramPacket packet = new DatagramPacket(buffer, buffer.length);
        socket.receive(packet);
        System.out.println("Message received: " + new String(packet.getData()));
      }
    } catch (IOException e) {
      e.printStackTrace();
    } 
  }
}

UDP (Datagram) client example in Java

Datagram client example constructs a datagram packet for sending packets of specified length to the specified port number on the specified host. Finally we do the send call on the DatagramSocket to send the datagram to the server.

package com.sourcetricks.client;

import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;

public class UDPClient {

  public static void main(String[] args) {
    final int destPort = 2222;

    byte buffer[] = new byte[1024];
    
    try ( DatagramSocket socket = new DatagramSocket(); ) {
      String str = "Hello UDP server";
      buffer = str.getBytes();
      DatagramPacket packet = new DatagramPacket(buffer, buffer.length, 
          InetAddress.getLocalHost(), destPort);
      socket.send(packet);
      Thread.sleep(2000);
    } catch (IOException e) {
      e.printStackTrace();
    } catch (InterruptedException e) {
      e.printStackTrace();
    } 
  }
}

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