UUID.randomUUID() 生成的 UUID 是无序的，如果作为数据主键，不利于索引

Hibernate 的 UUIDHexGenerator.generate() 方法可以生成有序的 UUID, 本文参考其实现：

public class SequentialUuidHexGenerator extends AbstractUUIDGenerator{

private static final String sep = "-";

public static String generate() {

return

format( getJVM() ) + sep

+ format( getHiTime() ) + sep

+ format( getLoTime() ) + sep

+ format( getIP() ) + sep

+ format( getCount() );

}

protected static String format(int intValue) {

String formatted = Integer.toHexString( intValue );

StringBuilder buf = new StringBuilder( "00000000" );

buf.replace( 8 - formatted.length(), 8, formatted );

return buf.toString();

}

protected static String format(short shortValue) {

String formatted = Integer.toHexString( shortValue );

StringBuilder buf = new StringBuilder( "0000" );

buf.replace( 4 - formatted.length(), 4, formatted );

return buf.toString();

}

}

UUIDHexGenerator.generate() 将 IP 放在首位，考虑到在不同的机器生成，本文将时间戳放在了首位

AbstractUUIDGenerator 代码：

public abstract class AbstractUUIDGenerator {

private static final int IP;

static {

int ipadd;

try {

ipadd = BytesHelper.toInt( InetAddress.getLocalHost().getAddress() );

}

catch (Exception e) {

ipadd = 0;

}

IP = ipadd;

}

private static short counter = (short) 0;

private static final int JVM = (int) ( System.currentTimeMillis() >>> 8 );

public AbstractUUIDGenerator() {

}

protected static int getJVM() {

return JVM;

}

protected static short getCount() {

synchronized(AbstractUUIDGenerator.class) {

if ( counter < 0 ) {

counter=0;

}

return counter++;

}

}

protected static int getIP() {

return IP;

}

protected static short getHiTime() {

return (short) ( System.currentTimeMillis() >>> 32 );

}

protected static int getLoTime() {

return (int) System.currentTimeMillis();

}

}

BytesHelper 代码：

public final class BytesHelper {

private BytesHelper() {

}

public static int toInt(byte[] bytes) {

int result = 0;

for ( int i = 0; i < 4; i++ ) {

result = ( result << 8 ) - Byte.MIN_VALUE + (int) bytes[i];

}

return result;

}

}

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