JAVA ID生成器生成逻辑主键
2024年03月01日
在一个数据库设计里,假如使用了逻辑主键,那么你一般都需要一个ID生成器去生成逻辑主键。在许多数据库里面,都提供了ID生成的机制,如Oracle中的sequence,MSSQL中的identity,可惜这些方法各种数据库都不同的,所以很多人愿意找寻一种通用的方式。
编写代码,1、2、3……这样一直累加是最直接的想法,JAVA用以下方式去实现
private static AtomicInteger uniqueId=new AtomicInteger(0);
public static String nextId(){
return Integer.toString(uniqueId.incrementAndGet());
}
当然,这样太简单了,并且一重新启动,计数器就归0了,一般的做法可以用时间+计数器的方式,
private static final long ONE_STEP=10;
private static final long BASE=1129703383453l;
private static final Lock LOCK=new ReentrantLock();
private static long lastTime=System.currentTimeMillis();
private static short lastCount=0;
/**
*a time(as returned by{ link System#currentTimeMillis()})at which
*the VM that this UID was generated in was alive
* serial
*/
private final long time;
/**
*16-bit number to distinguish UID instances created
*in the same VM with the same time value
* serial
*/
private final short count;
/**
*Generates a UID that is unique over time with
*respect to the host that it was generated on.
*/
public UID(){
LOCK.lock();
try{
if(lastCount==ONE_STEP){
boolean done=false;
while(!done){
long now=System.currentTimeMillis();
if(now==lastTime){
//pause for a second to wait for time to change
try{
Thread.currentThread().sleep(1);
}
catch(java.lang.InterruptedException e){
}//ignore exception
continue;
}
else{
lastTime=now;
lastCount=0;
done=true;
}
}
}
time=lastTime;
count=lastCount++;
}
finally{
LOCK.unlock();
}
}
在一个群集的环境里面,通常还需要加上IP的前缀,即IP+时间+计数器,这个就是JAVA原版本的实现了。