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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 io.hmit.utils;
import java.sql.Timestamp;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
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>
*
* @author hubin
* @since 2016-08-01
*/
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(runnable -> {
Thread thread = new Thread(runnable, "System Clock");
thread.setDaemon(true);
return thread;
});
scheduler.scheduleAtFixedRate(() -> 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);
}
}