Java 获取可用 UDP 端口号的方法。TCP 获取的办法类似于这个。

方法一：如果你不介意获取的端口号范围，可以使用 DatagramSocket 的构造方法定义 0 为其端口号，系统将为其分配一个闲置的端口号：

public static DatagramSocket getRandomPort() throws SocketException {
DatagramSocket s = new DatagramSocket(0);
return s;
}

方法二：如果你想使用一套特定范围的端口号，最简单的办法就是依次遍历这些端口号直到有一个可用：

public static DatagramSocket getRangePort(int[] ports) throws IOException {
for (int port : ports) {
try {
return new DatagramSocket(port);
} catch (IOException ex) {
continue; // try next port
}
}

// if the program gets here, no port in the range was found
throw new IOException("no free port found");
}

测试代码：

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

public class UdpPortTest {

public static void main(String[] args) throws IOException {
DatagramSocket socket = getRandomPort();
System.out.println("__________socket.getLocalPort():" + socket.getLocalPort());

DatagramSocket socket2 = getRangePort(new int[] { 3843, 4584, 4843 });
System.out.println("__________socket2.getLocalPort():" + socket2.getLocalPort());
}

public static DatagramSocket getRandomPort() throws SocketException {
DatagramSocket s = new DatagramSocket(0);
return s;
}

public static DatagramSocket getRangePort(int[] ports) throws IOException {
for (int port : ports) {
try {
return new DatagramSocket(port);
} catch (IOException ex) {
continue; // try next port
}
}

// if the program gets here, no port in the range was found
throw new IOException("no free port found");
}

}

打印结果：
__________socket.getLocalPort():2156
__________socket2.getLocalPort():3843