需求：應用A（通常有多個）和應用B（1個）進行 socket通訊，應用A必須知道應用B的ip地址（在應用A的配置文件中寫死的），這個時候就必須把應用B的ip設成固定ip（但是某些時候如更換路由後要重新設置網絡，但是操作人員不知道這個規則），就有可能造成應用A和應用B無法進行正常通訊，所以要改成應用A動態獲取應用B的ip地址。

經過討論決定采用udp協議實現，upd是一種無連接的傳輸層協議。應用A在不知道應用B的 ip情況下 可以使用廣播地址255.255.255.255，將消息發送到在同一廣播網絡上的B。從而獲取B的ip。

實現代碼：

B應用為服務端：將udp監聽放到一個線程中，當有客戶端請求時就會進行響應
 
/**
 * udp連接，用於動態ip, pos向255.255.255.255：5060發送請求即可
 * **/
public class UdpServer extends Thread implements Runnable {
    private final int MAX_LENGTH = 1024;
    private final int PORT = 5060;
    private DatagramSocket datagramSocket;
    
    public void run() {
        try {
            init();
            
 
while(true){
                try {
                    byte[] buffer = new byte[MAX_LENGTH];
                    DatagramPacket packet = new DatagramPacket(buffer, buffer.length);
                    receive(packet);
                    String receStr = new String(packet.getData(), 0 , packet.getLength());
                    System.out.println(
 
"接收數據包" + receStr);
                    byte[] bt = new byte[packet.getLength()];
                    
                    System.arraycopy(packet.getData(), 0, bt, 0, packet.getLength());
                    System.out.println(packet.getAddress().getHostAddress() + "：" + packet.getPort() + "：" + Arrays.toString(bt));
                    packet.setData(bt);
                    response(packet);
                    
                } catch (Exception e) {
                    e.printStackTrace();
                    LoggerUtil.error("udp線程出現異常：" + e.toString());
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public void receive(DatagramPacket packet) throws Exception {
        datagramSocket.receive(packet);
    }

    public void response(DatagramPacket packet) throws Exception {
        datagramSocket.send(packet);
    }
    
    /**
     * 初始化連接
     */
    public void init(){
        try {
            datagramSocket = new DatagramSocket(PORT);
            System.out.println("udp服務端已經啟動！");
        } catch (Exception e) {
            datagramSocket = null;
            System.out.println("udp服務端啟動失敗！");
            e.printStackTrace();
        }
    }
}

客戶端：本來客戶端是使用pb來實現的，但是這裏使用java來模擬
/***
 * UDP Client端
 ***/
public class UdpClient {
   
    private String sendStr = "hello";
    private String netAddress = "255.255.255.255";
    private final int PORT = 5060;
   
    private DatagramSocket datagramSocket;
    private DatagramPacket datagramPacket;
   
    public UdpClient(){
        try { 
            datagramSocket = new DatagramSocket();
            byte[] buf = sendStr.getBytes();
            InetAddress address = InetAddress.getByName(netAddress);
            datagramPacket = new DatagramPacket(buf, buf.length, address, PORT);
            datagramSocket.send(datagramPacket);
           
            byte[] receBuf = new byte[1024];
            DatagramPacket recePacket = new DatagramPacket(receBuf, receBuf.length);
            datagramSocket.receive(recePacket);
           
            String receStr = new String(recePacket.getData(), 0 , recePacket.getLength());
            
　　　　　　　//獲取服務端ip
            String serverIp = recePacket.getAdress();
        } catch (SocketException e) {
            e.printStackTrace();
        } catch (UnknownHostException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            // 關閉socket
            if(datagramSocket != null){
                datagramSocket.close();
            }
        }
    }
    
    public static void main(String[] args) {
        for (int i = 0; i < 100; i++) {
            new Thread(new Runnable() {
                @Override
                public void run() {
                    UdpClient udpClient = new UdpClient();
                }
            }).start();
        }
    }
}

java 實現udp通訊