UDP协议（User Datagram Protocol,用户数据报协议）不同于TCP协议，它是不可能靠的，但是它比TCP协议具有更快的传输速度，UDP发送的数据单元称为数据报，当网络传输UDP传输UDP数据报是无法保证数据能够到达目的地，也无法保证按发送的顺序到达目的地，也就是说先发送了“hello”,再发送了“world”,但接收方可能会先收到“world”,再收到“hello”,也有可能收不到数据，为什么呢？因为它是不可能靠的，在传输途中可能丢失了。但UDP比TCP跟适合与传输实时音频。下面是一个简单的UPD传输数据报的例子

服务器端：

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

public class EchoServer {
	
	private DatagramSocket datagramSocket;
	
	private final int port = 8088;
	
	public static void main(String[] args) throws SocketException {
		new EchoServer().service();
	}
	
	public EchoServer() throws SocketException{
		datagramSocket = new DatagramSocket(port);
		System.out.println("服务器启动");
	}
	
	public String echo(String msg){
		return "echo:"+msg;
	}
	
	public void service(){
		while (true) {
			try {
				DatagramPacket packet = new DatagramPacket(new byte[512], 512);
				datagramSocket.receive(packet);
				String msg = new String(packet.getData(), 0, packet.getLength());
				System.out.println(packet.getAddress()+"/"+packet.getPort()+" msg:"+msg);
				packet.setData(echo(msg).getBytes());
				datagramSocket.send(packet);
			} catch (IOException e) {
				e.printStackTrace();
			}
			
		}
	}

}

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.net.SocketException;

public class EchoClient {

	private String remoteHost="localhost";
	private int remotePort=8088;
	private DatagramSocket datagramSocket;
	
	public EchoClient() throws SocketException{
		datagramSocket = new DatagramSocket();
	}
	
	public static void main(String[] args) throws SocketException {
		new EchoClient().talk();
	}

	public void talk(){
		try {
			BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
			String msg = null;
			InetAddress address = InetAddress.getByName(remoteHost);
			while ((msg=reader.readLine())!=null) {
				//发送数据报
				byte [] buffer = msg.getBytes();
				DatagramPacket packet = new DatagramPacket(buffer,buffer.length, address, remotePort);
				datagramSocket.send(packet);
				//接收数据报
				DatagramPacket inputPacket = new DatagramPacket(new byte[512], 512);
				datagramSocket.receive(inputPacket);
				System.out.println(new String(inputPacket.getData(), 0 , inputPacket.getLength()));
				if("bye".equals(msg)){
					break;
				}
			}

		} catch (IOException e) {
			e.printStackTrace();
		}finally{
			datagramSocket.close();
		}
	}

}