数据报的使用

首先是一个工具类：

public class Dgram {
	public static DatagramPacket toDatagram (
			String s ,InetAddress destIA, int destPort){
		byte [] buf = new byte[s.length()+1] ;//这对防止截尾现象是非常重要的
		s.getBytes(0, s.length(), buf, 0);
		return new DatagramPacket(buf, buf.length,
				destIA, destPort) ;
	}
	
	public static String toString (DatagramPacket p){
		return new String(p.getData(), 0, p.getLength()) ;
	}

}

然后是服务器端

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


public class ChartterServer {
	static final int INPORT =1711;
	private byte [] buf = new byte[1000];
	private DatagramPacket dp = 
			new DatagramPacket(buf, buf.length) ;
	private DatagramSocket socket ;
	
	public ChartterServer (){
		try {
			socket = new DatagramSocket(INPORT) ;
			System.out.println("server start") ;
			while (true) {
				socket.receive(dp) ;
				String rcvd = Dgram.toString(dp)+"from:"+
				dp.getAddress()+"port:"+ dp.getPort() ;
				System.out.println(rcvd) ;
				String echoString = "Echo:"+ rcvd ;
				DatagramPacket echo = Dgram.toDatagram(echoString,
						dp.getAddress(), dp.getPort()) ;
				socket.send(echo) ;
			}
		} catch (SocketException e) {
			System.err.println("can not open socked") ;
			System.exit(1) ;
 		}catch (IOException e) {
 			System.err.println("communication erro") ;
 			e.printStackTrace() ;
		}
	}
	
	public static void main (String [] args ){
		new ChartterServer() ;
	}
}

　　客户端

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


public class ChatterClient extends Thread  {
	static final int INPORT =1711;
	private DatagramSocket s ;
	private InetAddress hostAddress ;
	private byte [] buf = new byte[1000] ;
	private DatagramPacket dp = new 
			DatagramPacket(buf, buf.length) ;
	private int id ;
	
	public ChatterClient (int id ){
		this.id = id ;
		try {
			s = new DatagramSocket() ;
			hostAddress = InetAddress.getByName(null) ;
		} catch (SocketException e) {
			System.err.println("canot open socked") ;
			System.exit(1) ;
		} catch (UnknownHostException e) {
			System.out.println("can not find host") ;
			e.printStackTrace();
		}
		System.out.println("chatterclient start") ;
	}
	
	public void run (){
		try {
			for (int i =0; i<25; i++){
				String outMsg = "client id:"+id+"msg:"+ i ;
				s.send(Dgram.toDatagram(outMsg,
						hostAddress, INPORT)) ; 
				s.receive(dp) ;
				String rcvd = "client id :"+ id+"rcvd from:"+
				dp.getAddress()+":"+dp.getPort()+Dgram.toString(dp) ;
				System.out.println(rcvd) ; 
			}
		} catch (IOException e) {
			e.printStackTrace() ;
			System.exit(1) ;
		}
	}
	
	public static void main (String [] args){
		for (int i=0;i<10; i++){
			new ChatterClient(i).start() ;
		}
	}
}