java 生成GUID，可以用UUID类来生成GUID

　　全局唯一标识符（GUID，Globally Unique Identifier）是一种由算法生成的二进制长度为128位的数字标识符，一般用16进制表示。在理想情况下，任何计算机和计算机集群都不会生成两个相同的GUID。算法的核心思想是结合机器的网卡、当地时间、一个随机数来生成GUID。从理论上讲，如果一台机器每秒产生10000000个GUID，则可以保证（概率意义上）3240年不重复

第一种：可以用UUID类来生成GUID

import java.util.UUID;
public class GuidTest {
    public static void main(String[] args) {
        UUID uuid = UUID.randomUUID();
        System.out.println (uuid);
    }
}

输出：

第二种：直接生成

import java.net.InetAddress;
import java.net.UnknownHostException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;
import java.util.Random;
public class GUID {
    public String valueBeforeMD5 = "";
    public String valueAfterMD5 = "";
    private static final Random myRand;
    private static final SecureRandom mySecureRand;
    private static String s_id;
    static {
        mySecureRand = new SecureRandom();
        long secureInitializer = mySecureRand.nextLong();
        myRand = new Random(secureInitializer);
        try {
            s_id = InetAddress.getLocalHost().toString();
        } catch (UnknownHostException e) {
            e.printStackTrace();
        }
    }
    public GUID() {
        getRandomGUID(false);
    }
    public GUID(boolean secure) {
        getRandomGUID(secure);
    }
    private void getRandomGUID(boolean secure) {
        MessageDigest md5 = null;
        StringBuilder sbValueBeforeMD5 = new StringBuilder();
        try {
            md5 = MessageDigest.getInstance("MD5");
        } catch (NoSuchAlgorithmException e) {
            System.out.println("Error: " + e);
        }
        try {
            long time = System.currentTimeMillis();
            long rand = myRand.nextLong();
            if (secure) {
                rand = mySecureRand.nextLong();
            }
            sbValueBeforeMD5.append(s_id);
            sbValueBeforeMD5.append(":");
            sbValueBeforeMD5.append(time);
            sbValueBeforeMD5.append(":");
            sbValueBeforeMD5.append(rand);
            valueBeforeMD5 = sbValueBeforeMD5.toString();
            md5.update(valueBeforeMD5.getBytes());
            byte[] array = md5.digest();
            StringBuilder sb = new StringBuilder();
            for (byte value : array) {
                int b = value & 0xFF;
                if (b < 0x10) sb.append('0');
                sb.append(Integer.toHexString(b));
            }
            valueAfterMD5 = sb.toString();
        } catch (Exception e) {
            System.out.println("Error:" + e);
        }
    }
    public String toString() {
        String raw = valueAfterMD5.toUpperCase();
        return raw.substring(0, 8) +
                "-" +
                raw.substring(8, 12) +
                "-" +
                raw.substring(12, 16) +
                "-" +
                raw.substring(16, 20) +
                "-" +
                raw.substring(20);
    }
    public static void main(String[] args) {
        GUID guid = new GUID();
        System.out.println(guid);
    }
}