Java面试题 - 《李俊扬的技术分享》

数据库中char与varchar的区别
- 过时的技术
java 通过发送 UDP－NetBIOS－NS 获取客户端的Mac地址
java通过执行系统命令，获取对方的mac地址

数据库中char与varchar的区别

他们两个的底层数据结构都是线性表，但char是定长的，varchar是链表结构的。

过时的技术

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GWT Google Web Toolkit 是一种允许开发人员使用 Java 编程语言快速构建和维护复杂但性能高的 JavaScript 前端应用程序的工具集，能够降低开发难度，尤其是与 Eclipse Google 插件结合使用时，优势更明显。有了 GWT可以使用 Java 编程语言编写 AJAX 前端，然后 GWT 会交叉编译到优化的JavaScript 中，而 JavaScript 可以自动在所有主要浏览器上运行。

k8s (全称：kubernetes)
Docker
https://my.oschina.net/jamesview/blog/2994112
serverless
https://www.npmjs.com/package/serverless
**
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java 通过发送 UDP－NetBIOS－NS 获取客户端的Mac地址

UdpGetClientMacAddr umac = new UdpGetClientMacAddr(sip);
String smac = umac.GetRemoteMacAddr();

final UdpGetClientMacAddr umac = new UdpGetClientMacAddr(sip);
//---长时间获取不到MAC地址则放弃
ExecutorService exec = Executors.newFixedThreadPool(1);
Callable<String> call = new Callable<String>() {
    public String call() throws Exception {
        return umac.GetRemoteMacAddr();
    }
};
try {
    Future<String> future = exec.submit(call);
    String smac = future.get(1000 * 1, TimeUnit.MILLISECONDS);
    loginMonitor.setMacAddress(smac);
} catch (TimeoutException ex) {
    loginMonitor.setMacAddress("获取失败");
    logger.info("获取MAC地址超时");
    ex.printStackTrace();
}
// 关闭线程池  
exec.shutdown();
//---

package shmc.commonsys.security.controller;
import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
/** 
 * 主机A向主机B发送“UDP－NetBIOS－NS”询问包，即向主机B的137端口，发Query包来询问主机B的NetBIOS Names信息。 
 * 其次，主机B接收到“UDP－NetBIOS－NS”询问包，假设主机B正确安装了NetBIOS服务........... 而且137端口开放，则主机B会向主机A发送一个“UDP－NetBIOS－NS”应答包，即发Answer包给主机A。 
 * 并利用UDP(NetBIOS Name Service)来快速获取远程主机MAC地址的方法 
 *  
 */  
public class UdpGetClientMacAddr {  
    private String sRemoteAddr;  
    private int iRemotePort=137;  
    private byte[] buffer = new byte[1024];  
    private DatagramSocket ds=null;  
    public UdpGetClientMacAddr(String strAddr) throws Exception{  
        sRemoteAddr = strAddr;  
        ds = new DatagramSocket();  
    }  
    public final DatagramPacket send(final byte[] bytes) throws IOException {  
        DatagramPacket dp = new DatagramPacket(bytes,bytes.length,InetAddress.getByName(sRemoteAddr),iRemotePort);  
        ds.send(dp);  
        return dp;  
    }  
    public final DatagramPacket receive() throws Exception {  
        DatagramPacket dp = new DatagramPacket(buffer,buffer.length);  
        ds.receive(dp);  
        return dp;  
    }  
    public byte[] GetQueryCmd() throws Exception {  
        byte[] t_ns = new byte[50];  
        t_ns[0] = 0x00;  
        t_ns[1] = 0x00;  
        t_ns[2] = 0x00;  
        t_ns[3] = 0x10;  
        t_ns[4] = 0x00;  
        t_ns[5] = 0x01;  
        t_ns[6] = 0x00;  
        t_ns[7] = 0x00;  
        t_ns[8] = 0x00;  
        t_ns[9] = 0x00;  
        t_ns[10] = 0x00;  
        t_ns[11] = 0x00;  
        t_ns[12] = 0x20;  
        t_ns[13] = 0x43;  
        t_ns[14] = 0x4B;  
        for(int i = 15; i < 45; i++){  
            t_ns[i] = 0x41;  
        }  
        t_ns[45] = 0x00;  
        t_ns[46] = 0x00;  
        t_ns[47] = 0x21;  
        t_ns[48] = 0x00;  
        t_ns[49] = 0x01;  
        return t_ns;  
    }  
    public final String GetMacAddr(byte[] brevdata) throws Exception {  
        // 获取计算机名  
        int i = brevdata[56] * 18 + 56;  
        String sAddr="";  
        StringBuffer sb = new StringBuffer(17);  
        // 先从第56字节位置，读出Number Of Names（NetBIOS名字的个数，其中每个NetBIOS Names Info部分占18个字节）  
        // 然后可计算出“Unit ID”字段的位置＝56＋Number Of Names×18，最后从该位置起连续读取6个字节，就是目的主机的MAC地址。  
        for(int j = 1; j < 7;j++)  
        {  
            sAddr = Integer.toHexString(0xFF & brevdata[i+j]);  
            if(sAddr.length() < 2)  
            {  
                sb.append(0);  
            }  
            sb.append(sAddr.toUpperCase());  
            if(j < 6) sb.append(':');  
        }  
        return sb.toString();  
    }  
    public final void close() throws Exception {  
        ds.close();  
    }  
    public final String GetRemoteMacAddr() throws Exception {  
        byte[] bqcmd = GetQueryCmd();  
        send(bqcmd);  
        DatagramPacket dp = receive();  
        String smac = GetMacAddr(dp.getData());  
        close();  
        return smac;  
    }  
    public static void main(String args[]) throws Exception{  
        UdpGetClientMacAddr umac=new UdpGetClientMacAddr("172.19.1.198");  
        umac=new UdpGetClientMacAddr("192.168.16.83");  
        System.out.println(umac.GetRemoteMacAddr());  
    }  
}

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java通过执行系统命令，获取对方的mac地址

// 1.获取IP方式
public static String getIpAddress(HttpServletRequest request) {
        String ip = request.getHeader("X-real-ip");
        if (ip == null || ip.length() == 0 || "unknown".equalsIgnoreCase(ip)) {
            ip = request.getHeader("x-forwarded-for");
        }
        if (ip == null || ip.length() == 0 || "unknown".equalsIgnoreCase(ip)) {
            ip = request.getHeader("Proxy-Client-IP");
        }
        if (ip == null || ip.length() == 0 || "unknown".equalsIgnoreCase(ip)) {
            ip = request.getHeader("WL-Proxy-Client-IP");
        }
        if (ip == null || ip.length() == 0 || "unknown".equalsIgnoreCase(ip)) {
            ip = request.getHeader("HTTP_CLIENT_IP");
        }
        if (ip == null || ip.length() == 0 || "unknown".equalsIgnoreCase(ip)) {
            ip = request.getHeader("HTTP_X_FORWARDED_FOR");
        }
        if (ip == null || ip.length() == 0 || "unknown".equalsIgnoreCase(ip)) {
            ip = request.getRemoteAddr();
        }
        return ip;
}
// 2.通过IP获取Mac地址；
static String getMacAddrByIp(String ip) {
    String macAddr = null;
    try {
        Process process = Runtime.getRuntime().exec("nbtstat -a " + ip);
        BufferedReader br = new BufferedReader(
                new InputStreamReader(process.getInputStream()));
        Pattern pattern = Pattern.compile("([A-F0-9]{2}-){5}[A-F0-9]{2}");
        Matcher matcher;
        for (String strLine = br.readLine(); strLine != null;
             strLine = br.readLine()) {
            matcher = pattern.matcher(strLine);
            if (matcher.find()) {
                macAddr = matcher.group();
                break;
            }
        }
    } catch (IOException e) {
        e.printStackTrace();
    }
    System.out.println(macAddr);
    return macAddr;
}
public static void main(String[] args) {
    String ip = "192.168.8.59";
    getMacAddrByIp(ip);
}

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原文链接：https://blog.csdn.net/qing_mei_xiu/java/article/details/80745557