UUID.randomUUID() 生成的 UUID 是無序的，如果作為數據主鍵，不利于索引

Hibernate 的 UUIDHexGenerator.generate() 方法可以生成有序的 UUID, 本文參考其實現：

public class SequentialUuidHexGenerator extends AbstractUUIDGenerator{private static final String sep = "-";public static String generate() {returnformat( 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() 將 IP 放在首位，考慮到在不同的機器生成，本文將時間戳放在了首位

AbstractUUIDGenerator 代碼：

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 代碼：

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;} }

完整代碼：GitHub

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轉載于:https://www.cnblogs.com/victorbu/p/11098647.html

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