java获取跨网段公网的mac地址

要获取跨网段公网的mac地址需要使用网络层的协议，例如ARP协议。Java中可以通过使用Jpcap或JNetPcap等库来实现对ARP协议的操作。

以下是一个示例代码：

import java.net.InetAddress;
import jpcap.JpcapCaptor;
import jpcap.NetworkInterface;
import jpcap.PacketReceiver;
import jpcap.packet.ARPPacket;
import jpcap.packet.Packet;

public class MacAddressResolver {
    public static void main(String[] args) throws Exception {
        // 获取所有网络接口
        NetworkInterface[] devices = JpcapCaptor.getDeviceList();

        // 选择一个网络接口
        NetworkInterface device = devices[0];

        // 构造ARP请求包
        ARPPacket arp = new ARPPacket();
        arp.hardtype = ARPPacket.HARDTYPE_ETHER;
        arp.prototype = ARPPacket.PROTOTYPE_IP;
        arp.operation = ARPPacket.ARP_REQUEST;
        arp.hlen = 6;
        arp.plen = 4;
        arp.sender_hardaddr = device.mac_address;
        arp.sender_protoaddr = InetAddress.getByName("192.168.0.1").getAddress();
        arp.target_hardaddr = new byte[] { 0, 0, 0, 0, 0, 0 };
        arp.target_protoaddr = InetAddress.getByName("8.8.8.8").getAddress();

        // 发送ARP请求包
        JpcapCaptor captor = JpcapCaptor.openDevice(device, 2000, false, 20);
        captor.sendPacket(arp);

        // 接收ARP响应包
        captor.loopPacket(-1, new PacketReceiver() {
            public void receivePacket(Packet packet) {
                if (packet instanceof ARPPacket) {
                    ARPPacket arp = (ARPPacket) packet;
                    if (arp.operation == ARPPacket.ARP_REPLY) {
                        System.out.printf("MAC地址为 %s\n", macToString(arp.sender_hardaddr));
                    }
                }
            }
        });
    }

    private static String macToString(byte[] mac) {
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < mac.length; i++) {
            sb.append(String.format("%02X", mac[i]));
            if (i != mac.length - 1) {
                sb.append(":");
            }
        }
        return sb.toString();
    }
}

该代码会向IP地址为8.8.8.8的主机发送ARP请求包，并等待ARP响应包。如果收到响应包，就输出响应包中的MAC地址。在实际使用中，应该将目标IP地址替换成要查询的主机的IP地址。