java 生成GUID

　　全局唯一標識符（GUID，Globally Unique Identifier）是一種由算法生成的二進制長度為128位的數字標識符，一般用16進制表示。在理想情況下，任何計算機和計算機集群都不會生成兩個相同的GUID。算法的核心思想是結合機器的網卡、當地時間、一個隨機數來生成GUID。從理論上講，如果一台機器每秒產生10000000個GUID，則可以保證（概率意義上）3240年不重復

第一種：可以用UUID類來生成GUID

import java.util.UUID;

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

輸出：

 

 

第二種：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);
    }
}

輸出：