java写一段程序 测试服务器UDP端口是否开放

你可以使用 Java 的 `DatagramSocket` 类来测试一个 UDP 端口是否开放。

这是一个示例代码：

import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.net.SocketTimeoutException;

public class UDPSocketClient {
    public static void main(String[] args) {
        String hostname = "localhost";
        int port = 8080;

        try (DatagramSocket socket = new DatagramSocket()) {
            socket.setSoTimeout(1000); // Set a timeout of 1 second
            InetAddress address = InetAddress.getByName(hostname);
            byte[] data = "TEST".getBytes();
            DatagramPacket packet = new DatagramPacket(data, data.length, address, port);
            socket.send(packet);
            System.out.println("Packet sent to " + hostname + ":" + port);

            byte[] receiveData = new byte[1024];
            DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
            socket.receive(receivePacket);
            System.out.println("Packet received from " + receivePacket.getAddress().getHostAddress() + ":" + receivePacket.getPort());
        } catch (SocketTimeoutException ex) {
            System.out.println("UDP Port " + port + " on host " + hostname + " is closed.");
        } catch (IOException ex) {
            System.out.println("Error occurred while communicating with UDP port " + port + " on host " + hostname + ".");
            ex.printStackTrace();
        }
    }
}

这段代码将向给定的主机名和端口发送一个 "TEST" 的数据包，并等待一秒钟的响应。如果在这段时间内没有收到响应，就说明该端口是关闭的。如果收到了响应，则说明该端口是开放的。

请注意，这只是一个示例代码，实际上并不一定能够确切地判断一个端口是否开放。因为有些防火墙可能会拦截或修改这些数据包，因此该代码不