java 以纳秒生成20位流水号

public class SequenceNumber {
public static String getSequenceNumber(){
Date d=new Date();
SimpleDateFormat sdf=new SimpleDateFormat("yyyyMMddhhmmss");
String str=sdf.format(d);
String haomiao=String.valueOf(System.nanoTime());
str=str+haomiao.substring(haomiao.length()-6,haomiao.length());
return str;
}
public static void main(String[] args) {
System.out.println(getSequenceNumber());
}

}