Java实现UDP通信

TCP和UDP的区别是：

TCP协议是面向连接的，UDP是无连接的。

TCP主要是ServerSocket、Socket。UDP主要是DatagramSocket、DatagramPacket。

TCP用ServerSocket来监听收数据的端口，用Socket来建立连接，用输入输出流来通信（inputstream、outputstream）。UDP是把数据、目标IP和端口都封装在DatagramPacket中，然后由DatagramSocket发送出去。

构建DatagramSocket：

DatagramSocket()：如果没有特定监听的端口，就随机使用一个临时的端口发送数据。 

DatagramSocket(int port)：创建实例，并固定监听Port端口的报文。

DatagramSocket实例的receive方法接收DatagramPacket，DatagramPacket中间就包含了通信的内容和目标的IP和端口。 

DatagramSocket的send发送DatagramPacket

服务端代码：

package com.example.androidudptest;

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

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.util.Log;
import android.widget.Toast;

public class MainActivity extends Activity {

public final static String TAG = "UDP_TEST";

public final static int SHOW_MSG = 1001;

public final static int SHOW_ERROR = 1002;

private DatagramSocket datagramSocket = null;

private DatagramPacket datagramPacket = null;

private final static int PORT = 8090;

private byte[] dataBuf;

private Handler mHandler = new Handler() {
public void handleMessage(android.os.Message msg) {
if(msg.what == SHOW_ERROR || msg.what == SHOW_MSG){
Toast.makeText(MainActivity.this, (String)msg.obj, Toast.LENGTH_LONG).show();
}
}
};

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
init();
}

private void init() {
dataBuf = new byte[1024];
try {
datagramSocket = new DatagramSocket(PORT);
datagramPacket = new DatagramPacket(dataBuf, dataBuf.length);
// datagramSocket.setSoTimeout(10000);
new Thread(new ReceiveRunnable()).start();
}
catch (SocketException e) {
Log.d(TAG, "init(), SocketException : ", e);
showMsg(SHOW_ERROR,"init(), SocketException");
}
}

class ReceiveRunnable implements Runnable {
public void run() {
while (true) {
try {
Log.d(TAG, "ReceiveRunnable , start receive...");
datagramPacket.setData(dataBuf);
datagramSocket.receive(datagramPacket); // 接收数据报文到datagramPacket中。receive方法产生阻塞(阻塞的时间为datagramSocket.setSoTimeout(10000)设置的时间)

int length = datagramPacket.getLength();
byte [] data = datagramPacket.getData();
String strData = new String(data,0,length);

String clientIp = datagramPacket.getAddress().getHostAddress(); // 消息发送者的IP
int clientPort = datagramPacket.getPort(); // 消息发送者的端口
Log.d(TAG, "ReceiveRunnable , strData = " + strData + ", clientIp = " + clientIp + ", clientPort = " + clientPort);
showMsg(SHOW_MSG,strData);

// 向客户端返回消息
datagramPacket.setSocketAddress(new InetSocketAddress(clientIp, clientPort));
datagramPacket.setData("nihao_client".getBytes());
datagramSocket.send(datagramPacket);
}
catch (IOException e) {
Log.d(TAG, "ReceiveRunnable , IOException : ", e);
showMsg(SHOW_ERROR,"ReceiveRunnable(), IOException");
//break;
}
}
}
}

private void showMsg(int msgWhat,String strMsg){
Message msg = mHandler.obtainMessage(msgWhat);
msg.obj = strMsg;
mHandler.sendMessage(msg);
}

}


客户单代码：

package com.java.test.udp;

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 UdpTestClient {

public static void main(String[] args){
String serverHost = "172.27.223.3";
int serverPort = 8090;
byte[] sendMsg = "服务端你好，我这里是客户端!".getBytes();

DatagramPacket datagramPacket = null;
DatagramSocket datagramSocket = null;
try {
datagramPacket = new DatagramPacket(sendMsg,
sendMsg.length,
InetAddress.getByName(serverHost),
serverPort);

//datagramSocket = new DatagramSocket(8891); // 监听本地的端口
datagramSocket = new DatagramSocket(); // 没有监听本地的端口
datagramSocket.send(datagramPacket);

/*
* 等待服务端返回来的消息
*/
datagramSocket.receive(datagramPacket) ;
int length = datagramPacket.getLength();
byte [] data = datagramPacket.getData();

String info = new String(data,0,length);

System.out.println("服务端回应数据：" + info);
}
catch (UnknownHostException e) {
e.printStackTrace();
}
catch (SocketException e) {
e.printStackTrace();
}

catch (IOException e) {
e.printStackTrace();
}
}
}