这个也是根据前辈的博客改写的代码，附上以下链接：
https://blog.csdn.net/sskicgah/article/details/12904881
package main;
//import java.util.Date;
import java.util.UUID;
public class UUIDBits {
/**
* 采用URL Base64字符，即把“+/”换成“-_”
*/
static private char[] alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_=".toCharArray();
/**
* Base64 编码
* @param data
* @return
*/
private char[] encode(byte[] data) {
char[] out = new char[((data.length + 2) / 3) * 4];
boolean quad, trip;
for (int i = 0, index = 0; i < data.length; i += 3, index += 4) {
quad = trip = false;
int val = (0xFF & (int) data[i]);
val <<= 8;
if ((i + 1) < data.length) {
val |= (0xFF & (int) data[i + 1]);
trip = true;
}
val <<= 8;
if ((i + 2) < data.length) {
val |= (0xFF & (int) data[i + 2]);
quad = true;
}
out[index + 3] = alphabet[(quad ? (val & 0x3F) : 64)];
val >>= 6;
out[index + 2] = alphabet[(trip ? (val & 0x3F) : 64)];
val >>= 6;
out[index + 1] = alphabet[val & 0x3F];
val >>= 6;
out[index + 0] = alphabet[val & 0x3F];
}
return out;
}
/**
* 转成字节
* @return
*/
// private byte[] toBytes(String u) {
// UUID uuid = UUID.fromString(u);
private byte[] toBytes() {
UUID uuid = UUID.randomUUID();
long msb = uuid.getMostSignificantBits();
long lsb = uuid.getLeastSignificantBits();
byte[] buffer = new byte[16];
for (int i = 0; i < 8; i++) {
buffer[i] = (byte) ((msb >>> 8 * (7 - i)) & 0xFF);
buffer[i + 8] = (byte) ((lsb >>> 8 * (7 - i)) & 0xFF);
}
return buffer;
}
// public String getUUID(String u) {
// char[] res = encode(toBytes(u));
public String getUUID() {
char[] res = encode(toBytes());
System.out.println(new String(res));
return new String(res, 0, res.length - 2);
}
public static void main(String[] args) {
System.out.println(getUUIDBits(16)); //指定16位
// System.out.println(getUUID("c19b9de1-f33a-494b-afbe-f06817218d64"));
// System.out.println(getUUID22("c19b9de1-f33a-494b-afbe-f06817218d64"));
// Date d1 = new Date();
// for(int i = 0; i < 1000000; i++) {
// UUID.randomUUID().toString();
// getUUID22();
// }
// Date d2 = new Date();
// System.out.print(d2.getTime() - d1.getTime());
}
/**
* 将随机UUID转换成指定位数的字符串
* @param bits 指定位数
* @return
*/
// public static String getUUID22(String u) {
// UUID uuid = UUID.fromString(u);
public static String getUUIDBits(int bits) {
UUID uuid = UUID.randomUUID();
// System.out.println(uuid.toString());
long msb = uuid.getMostSignificantBits();
long lsb = uuid.getLeastSignificantBits();
char[] out = new char[24];
int tmp = 0, idx = 0;
// 基础写法
/*tmp = (int) ((msb >>> 40) & 0xffffff);
out[idx + 3] = alphabet[tmp & 0x3f];
tmp >>= 6;
out[idx + 2] = alphabet[tmp & 0x3f];
tmp >>= 6;
out[idx + 1] = alphabet[tmp & 0x3f];
tmp >>= 6;
out[idx] = alphabet[tmp & 0x3f];
idx += 4;
tmp = (int) ((msb >>> 16) & 0xffffff);
out[idx + 3] = alphabet[tmp & 0x3f];
tmp >>= 6;
out[idx + 2] = alphabet[tmp & 0x3f];
tmp >>= 6;
out[idx + 1] = alphabet[tmp & 0x3f];
tmp >>= 6;
out[idx] = alphabet[tmp & 0x3f];
idx += 4;
tmp = (int) (((msb & 0xffff) << 8) | (lsb >>> 56 & 0xff));
out[idx + 3] = alphabet[tmp & 0x3f];
tmp >>= 6;
out[idx + 2] = alphabet[tmp & 0x3f];
tmp >>= 6;
out[idx + 1] = alphabet[tmp & 0x3f];
tmp >>= 6;
out[idx] = alphabet[tmp & 0x3f];
idx += 4;
tmp = (int) ((lsb >>> 32) & 0xffffff);
out[idx + 3] = alphabet[tmp & 0x3f];
tmp >>= 6;
out[idx + 2] = alphabet[tmp & 0x3f];
tmp >>= 6;
out[idx + 1] = alphabet[tmp & 0x3f];
tmp >>= 6;
out[idx] = alphabet[tmp & 0x3f];
idx += 4;
tmp = (int) ((lsb >>> 8) & 0xffffff);
out[idx + 3] = alphabet[tmp & 0x3f];
tmp >>= 6;
out[idx + 2] = alphabet[tmp & 0x3f];
tmp >>= 6;
out[idx + 1] = alphabet[tmp & 0x3f];
tmp >>= 6;
out[idx] = alphabet[tmp & 0x3f];
idx += 4;
tmp = (int) (lsb & 0xff);
out[idx + 3] = alphabet[64];
out[idx + 2] = alphabet[64];
tmp <<= 4;
out[idx + 1] = alphabet[tmp & 0x3f];
tmp >>= 6;
out[idx] = alphabet[tmp & 0x3f];
idx += 4;*/
// 循环写法
int bit = 0, bt1 = 8, bt2 = 8;
int mask = 0x00, offsetm = 0, offsetl = 0;
for(; bit < 16; bit += 3, idx += 4) {
offsetm = 64 - (bit + 3) * 8;
offsetl = 0;
tmp = 0;
if(bt1 > 3) {
mask = (1 << 8 * 3) - 1;
} else if(bt1 >= 0) {
mask = (1 << 8 * bt1) - 1;
bt2 -= 3 - bt1;
} else {
mask = (1 << 8 * ((bt2 > 3) ? 3 : bt2)) - 1;
bt2 -= 3;
}
if(bt1 > 0) {
bt1 -= 3;
tmp = (int) ((offsetm < 0) ? msb : (msb >>> offsetm) & mask);
if(bt1 < 0) {
tmp <<= Math.abs(offsetm);
mask = (1 << 8 * Math.abs(bt1)) - 1;
}
}
if(offsetm < 0) {
offsetl = 64 + offsetm;
tmp |= ((offsetl < 0) ? lsb : (lsb >>> offsetl)) & mask;
}
if(bit == 15) {
out[idx + 3] = alphabet[64];
out[idx + 2] = alphabet[64];
tmp <<= 4;
} else {
out[idx + 3] = alphabet[tmp & 0x3f];
tmp >>= 6;
out[idx + 2] = alphabet[tmp & 0x3f];
tmp >>= 6;
}
out[idx + 1] = alphabet[tmp & 0x3f];
tmp >>= 6;
out[idx] = alphabet[tmp & 0x3f];
}
return new String(out, 0, bits);
}
}
其中，//注释的代码是测试代码和部分注释。/**/注释的是最简单的写法，等价于下面的for循环写法。只是自己看着不“优雅”改了。
经过测试，改造完的getUUID22方法生成字符串的速度，比原生的UUID.randomUUID().toString()方法快，在100W次的测试中，输出是时间大概是原生1850+ms:改造1040+ms的速度。
究其原因，应该是toString()方法拼接字符串"-"导致速度慢了。拼接字符串用+一般比较没效率。
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