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/**

* Copyright (c) 2011-2020, hubin (jobob@qq.com).

* <p>

* 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

* <p>

* http://www.apache.org/licenses/LICENSE-2.0

* <p>

* 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.

*/

package com.utils.common;

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;

/**

* <p>

* 高并发场景下System.currentTimeMillis()的性能问题的优化

* </p>

* <p>

* 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）<br>

* System.currentTimeMillis()之所以慢是因为去跟系统打了一次交道<br>

* 后台定时更新时钟，JVM退出时，线程自动回收<br>

* 10亿：43410,206,210.72815533980582%<br>

* 1亿：4699,29,162.0344827586207%<br>

* 1000万：480,12,40.0%<br>

* 100万：50,10,5.0%<br>

* </p>

*/

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() {

public Thread newThread(Runnable runnable) {

Thread thread = new Thread(runnable, "System Clock");

thread.setDaemon(true);

return thread;

}

});

scheduler.scheduleAtFixedRate(new Runnable() {

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);

}

}