public final class UUID implements java.io.Serializable, Comparable {

    /**
     * Explicit serialVersionUID for interoperability.
     */
    private static final long serialVersionUID = -4856846361193249489L;
}

@Override
public String genId() {
    return UUID.randomUUID().toString()
            .replaceAll(PunctuationConst.MIDDLE_LINE, PunctuationConst.EMPTY);
}

package com.github.houbb.heaven.util.id.impl;

import com.github.houbb.heaven.annotation.ThreadSafe;
import com.github.houbb.heaven.util.id.Id;
import com.github.houbb.heaven.util.util.DateUtil;
import com.github.houbb.heaven.util.util.RandomUtil;

import java.util.Date;

/**
 * 基于随机数生成的随机数字标识
 *
 * 组成方式：前缀+15位时间戳+后续随机数字
 *
 * 重复概率：1 Mills 内重复的概率为 1/(10)^10
 *
 * 应用场景：平时生成随机的标识。
 *
 * 优点：便于阅读，生成方便。
 * 缺点：重复的概率和后续随机的长度有关，有时候可能会过长。
 *
 * @author binbin.hou
 * @since 0.1.12
 * @see com.github.houbb.heaven.util.util.DateUtil#TIMESTAMP_FORMAT_15 15 位时间戳
 */
@ThreadSafe
public class RandomNumId implements Id {

    @Override
    public String genId() {
        final int randomLength = 10;
        final String timestamp = DateUtil.getDateFormat(new Date(), DateUtil.TIMESTAMP_FORMAT_15);
        return timestamp+ RandomUtil.randomNumber(randomLength);
    }

}

/**
 * 
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not
 * use this file except in compliance with the License. You may obtain a copy of
 * the License at
 * 
 * http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations under
 * the License.
 */
import java.sql.Timestamp;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicLong;

/**
 * 
 * 高并发场景下System.currentTimeMillis()的性能问题的优化
 * 
 * 
 * System.currentTimeMillis()的调用比new一个普通对象要耗时的多（具体耗时高出多少我还没测试过，有人说是100倍左右）
 * System.currentTimeMillis()之所以慢是因为去跟系统打了一次交道
 * 后台定时更新时钟，JVM退出时，线程自动回收
 * 10亿：43410,206,210.72815533980582%
 * 1亿：4699,29,162.0344827586207%
 * 1000万：480,12,40.0%
 * 100万：50,10,5.0%
 * 
 *
 * @author lry
 */
public class SystemClock {

    private final long period;
    private final AtomicLong now;

    private SystemClock(long period) {
        this.period = period;
        this.now = new AtomicLong(System.currentTimeMillis());
        scheduleClockUpdating();
    }

    private static SystemClock instance() {
        return InstanceHolder.INSTANCE;
    }

    public static long now() {
        return instance().currentTimeMillis();
    }

    public static String nowDate() {
        return new Timestamp(instance().currentTimeMillis()).toString();
    }

    private void scheduleClockUpdating() {
        ScheduledExecutorService scheduler = Executors.newSingleThreadScheduledExecutor(new ThreadFactory() {

            @Override
            public Thread newThread(Runnable runnable) {
                Thread thread = new Thread(runnable, "System Clock");
                thread.setDaemon(true);
                return thread;
            }
        });
        scheduler.scheduleAtFixedRate(new Runnable() {

            @Override
            public void run() {
                now.set(System.currentTimeMillis());
            }
        }, period, period, TimeUnit.MILLISECONDS);
    }

    private long currentTimeMillis() {
        return now.get();
    }

    private static class InstanceHolder {
        public static final SystemClock INSTANCE = new SystemClock(1);
    }

}