關於UDP：將資料及源和目的封裝成資料包中，不需要建立連線；每個資料報的大小在限制在64k內；因無連線，是不可靠協議；不需要建立連線，速度快

使用udp協議主要涉及到兩個類：DatagramSocket，DatagramPacket

DatagramSocket: 用於建立一個數據包的出口或入口，構造方法中有IP的都是用於接收方的

DatagramPacket: 用於包裝資料，構造方法中有IP的都是用於傳送方的

可以將其比作傳送包裹，DatagramSocket就是快遞公司，選擇哪一個快遞公司並不會影響包裹的傳送；而DatagramPacket則是你的包裹，上面必須寫明地址（IP）和門牌號（port:埠號）.

下面的程式，兩臺電腦各執行一個，只需要將IP改為接收方的IP，埠號改為你電腦上面空餘的埠號（電腦的埠號是0--65535，1024以內是系統的，所以一般建議使用1024以外的埠號）

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

public class Chat {
	public static void main(String[] args) {
		DatagramSocket sendsocket = null;
		DatagramSocket receiceSocket = null;
		try {
			sendsocket = new DatagramSocket();
			receiceSocket = new DatagramSocket(8888,InetAddress.getByName("127.0.0.1"));
		} catch (SocketException e) {
			e.printStackTrace();
		} catch (UnknownHostException e) {
			e.printStackTrace();
		}
		new Thread(new Send(sendsocket)).start();
		new Thread(new Receive(receiceSocket)).start();
	}
}
class Send implements Runnable{
	DatagramSocket socket = null;
	public Send(DatagramSocket socket) {
		this.socket = socket;
	}
	@Override
	public void run() {
		byte[] buf = new byte[1024];
		DatagramPacket packet = null;
		Scanner scanner = new Scanner(System.in);
		while(true) {
			String data = scanner.nextLine();
			if("over".equalsIgnoreCase(data)) {
				break;
			}
			buf = data.getBytes();
			try {
				//接收方的IP和埠號
				packet = new DatagramPacket(buf, buf.length, InetAddress.getByName("127.0.0.1"), 9999);
				socket.send(packet);
			} catch (UnknownHostException e) {
				e.printStackTrace();
			} catch (IOException e) {
				e.printStackTrace();
			}
		}
		scanner.close();
	}
}
class Receive implements Runnable{
	DatagramSocket socket;
	public Receive(DatagramSocket socket) {
		this.socket = socket;
	}
	@Override
	public void run() {
		while(true) {
			byte[]buf = new byte[1024];
			DatagramPacket packet = new DatagramPacket(buf, buf.length);
			try {
				socket.receive(packet);
			} catch (IOException e) {
				e.printStackTrace();
				break;
			}
			String text = new String(packet.getData(), 0, packet.getLength());
			String ip = packet.getAddress().getHostAddress();
			if("over".equalsIgnoreCase(text)) {
				break;
			}
			System.out.println(ip+" : "+text);
		}
		socket.close();
	}
	
}
 

-------------------------------#以下是補充內容，2018-10-16 #----------------------------------------

在傳送端，要在資料包物件中明確目的地IP及埠

DatagramSocket ds = new DatagramSocket();
byte[] by = “hello,udp”.getBytes();
DatagramPacket dp = new DatagramPacket(by,0,by.length,
	InetAddress.getByName(“127.0.0.1”),10000);
ds.send(dp);
ds.close();
 

在接收端，要指定監聽的埠

DatagramSocket ds = new DatagramSocket(10000);
byte[] by = new byte[1024];
DatagramPacket dp = new DatagramPacket(by,by.length);
ds.receive(dp);
String str = new String(dp.getData(),0,dp.getLength());
System.out.println(str+"--"+dp.getAddress());
ds.close();