TimeUnit源码走读及基本使用
Posted amcomputer
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1 背景介绍
笔者遇到一个场景，用户输入的时间和数据库里面时间做对比，由于数据库里面是timestamp类型，（如 2021-08-23 08:28:41），而用户输入一般为小时，分钟，或者毫秒。代码规约规定不能使用java.sql.Time, java.sql.Date, 和java.sql.timestamp, 因为在jdk8中，这3个类有缺陷。
现在假设用户输入是毫秒 (long startTime = System.currentTimeMillis() )，笔者需要把这个毫秒转变为数据库里面是timestamp类型，笔者尝试了很多方法。最后还是不能避免使用java.sql.Timestamp，核心代码如下：
new Timestamp(new Date(System.currentTimeMillis()).getTime()),
即先把用户输入毫秒转为Date类型，在得到Time，最后转为Timestamp。
虽然能完成任务，需求，可是犯规了，笔者该怎么办？
备注：因为源码比较长, 就放到最后部分了。
2 基本使用
非常好用的TimeUnit类！！！！！
类A是pojo请求类：
class A{
Date StartTime;
Date endTime;
//getter or setter ignore
}
先设置时间：毫秒到秒
```java
A.setStartTime(TimeUnit.MILLISECONDS.toSeconds(System.currentTimeMillis()));
转化为目标查询参数：
Date startTime = new Date(TimeUnit.SECONDS.toMillis(A.getStartTime()))；
startTime就可以传入sql进行查询了。
下面是具体startTime的数值：
上面只是一个基本demo使用，毫秒，纳秒，微秒，小时，分钟，天等基本单位都很容易实现，只需要TimeUnit.XX.toXXs()转化就行了。
TimeUnit.DAYS //天
TimeUnit.HOURS //小时
TimeUnit.MINUTES //分钟
TimeUnit.SECONDS //秒
TimeUnit.MILLISECONDS //毫秒
TimeUnit.MICROSECONDS //微秒
TimeUnit.NANOSECONDS //纳秒
时间颗粒度转换 ：
public long toMillis(long d) ;//转化成毫秒
public long toSeconds(long d) ;//转化成秒
public long toMinutes(long d); //转化成分钟
public long toHours(long d) ;//转化成小时
public long toDays(long d); //转化天
同时，TimeUnit还能延时（与thread.sleep()一样），如thread.sleep(1000)是线程暂定1秒，但thread.sleep(80000)是多少呢？
TimeUnit.SECONDS.sleep( 5 );//暂停5秒
TimeUnit.MINUTES.sleep( 1 );//暂停1分钟
TimeUnit是不是可读性很高？
接下来看源码如何实现。
3 源码
！！！！最重要的部分！！！！
// Handy constants for conversion methods
static final long C0 = 1L;
static final long C1 = C0 * 1000L;
static final long C2 = C1 * 1000L;
static final long C3 = C2 * 1000L;
static final long C4 = C3 * 60L;
static final long C5 = C4 * 60L;
static final long C6 = C5 * 24L;
static final long MAX = Long.MAX_VALUE;
/**
* Scale d by m, checking for overflow.
* This has a short name to make above code more readable.
*/
static long x(long d, long m, long over) {
if (d > over) return Long.MAX_VALUE;
if (d < -over) return Long.MIN_VALUE;
return d * m;
}
```
其他的比较简单
c0,C1,C2,...,C6分别对应前面介绍的纳秒，微秒，。。。到天，即以纳秒为最小单位，C1 = C0 * 1000L;表上了一微秒等于1000纳秒。C6 = C5 * 24L;表示来一天等于24小时。
static final long MAX = Long.MAX_VALUE; 为最大的Long类型数，我们知道long是8个字节（32bit）所以最大的MAX为2^31-1，第一位为符号位。
x(d,m,over)方法为计算由d转为目标级别后换算单位，即d=5,由分钟转秒demo:
```java
TimeUnit.MINUTES.toSeconds(5)
那么此时会具体执行的函数为：
x(5,60,MAX/(60)），然后判断不超过最大范围，
返回了5*60，即得到了300秒
源代码如下：
/*
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/
/*
* This file is available under and governed by the GNU General Public
* License version 2 only, as published by the Free Software Foundation.
* However, the following notice accompanied the original version of this
* file:
*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/publicdomain/zero/1.0/
*/
package java.util.concurrent;
/**
* A {@code TimeUnit} represents time durations at a given unit of
* granularity and provides utility methods to convert across units,
* and to perform timing and delay operations in these units. A
* {@code TimeUnit} does not maintain time information, but only
* helps organize and use time representations that may be maintained
* separately across various contexts. A nanosecond is defined as one
* thousandth of a microsecond, a microsecond as one thousandth of a
* millisecond, a millisecond as one thousandth of a second, a minute
* as sixty seconds, an hour as sixty minutes, and a day as twenty four
* hours.
*
* <p>A {@code TimeUnit} is mainly used to inform time-based methods
* how a given timing parameter should be interpreted. For example,
* the following code will timeout in 50 milliseconds if the {@link
* java.util.concurrent.locks.Lock lock} is not available:
*
* <pre> {@code
* Lock lock = ...;
* if (lock.tryLock(50L, TimeUnit.MILLISECONDS)) ...}</pre>
*
* while this code will timeout in 50 seconds:
* <pre> {@code
* Lock lock = ...;
* if (lock.tryLock(50L, TimeUnit.SECONDS)) ...}</pre>
*
* Note however, that there is no guarantee that a particular timeout
* implementation will be able to notice the passage of time at the
* same granularity as the given {@code TimeUnit}.
*
* @since 1.5
* @author Doug Lea
*/
public enum TimeUnit {
/**
* Time unit representing one thousandth of a microsecond
*/
NANOSECONDS {
public long toNanos(long d) { return d; }
public long toMicros(long d) { return d/(C1/C0); }
public long toMillis(long d) { return d/(C2/C0); }
public long toSeconds(long d) { return d/(C3/C0); }
public long toMinutes(long d) { return d/(C4/C0); }
public long toHours(long d) { return d/(C5/C0); }
public long toDays(long d) { return d/(C6/C0); }
public long convert(long d, TimeUnit u) { return u.toNanos(d); }
int excessNanos(long d, long m) { return (int)(d - (m*C2)); }
},
/**
* Time unit representing one thousandth of a millisecond
*/
MICROSECONDS {
public long toNanos(long d) { return x(d, C1/C0, MAX/(C1/C0)); }
public long toMicros(long d) { return d; }
public long toMillis(long d) { return d/(C2/C1); }
public long toSeconds(long d) { return d/(C3/C1); }
public long toMinutes(long d) { return d/(C4/C1); }
public long toHours(long d) { return d/(C5/C1); }
public long toDays(long d) { return d/(C6/C1); }
public long convert(long d, TimeUnit u) { return u.toMicros(d); }
int excessNanos(long d, long m) { return (int)((d*C1) - (m*C2)); }
},
/**
* Time unit representing one thousandth of a second
*/
MILLISECONDS {
public long toNanos(long d) { return x(d, C2/C0, MAX/(C2/C0)); }
public long toMicros(long d) { return x(d, C2/C1, MAX/(C2/C1)); }
public long toMillis(long d) { return d; }
public long toSeconds(long d) { return d/(C3/C2); }
public long toMinutes(long d) { return d/(C4/C2); }
public long toHours(long d) { return d/(C5/C2); }
public long toDays(long d) { return d/(C6/C2); }
public long convert(long d, TimeUnit u) { return u.toMillis(d); }
int excessNanos(long d, long m) { return 0; }
},
/**
* Time unit representing one second
*/
SECONDS {
public long toNanos(long d) { return x(d, C3/C0, MAX/(C3/C0)); }
public long toMicros(long d) { return x(d, C3/C1, MAX/(C3/C1)); }
public long toMillis(long d) { return x(d, C3/C2, MAX/(C3/C2)); }
public long toSeconds(long d) { return d; }
public long toMinutes(long d) { return d/(C4/C3); }
public long toHours(long d) { return d/(C5/C3); }
public long toDays(long d) { return d/(C6/C3); }
public long convert(long d, TimeUnit u) { return u.toSeconds(d); }
int excessNanos(long d, long m) { return 0; }
},
/**
* Time unit representing sixty seconds
*/
MINUTES {
public long toNanos(long d) { return x(d, C4/C0, MAX/(C4/C0)); }
public long toMicros(long d) { return x(d, C4/C1, MAX/(C4/C1)); }
public long toMillis(long d) { return x(d, C4/C2, MAX/(C4/C2)); }
public long toSeconds(long d) { return x(d, C4/C3, MAX/(C4/C3)); }
public long toMinutes(long d) { return d; }
public long toHours(long d) { return d/(C5/C4); }
public long toDays(long d) { return d/(C6/C4); }
public long convert(long d, TimeUnit u) { return u.toMinutes(d); }
int excessNanos(long d, long m) { return 0; }
},
/**
* Time unit representing sixty minutes
*/
HOURS {
public long toNanos(long d) { return x(d, C5/C0, MAX/(C5/C0)); }
public long toMicros(long d) { return x(d, C5/C1, MAX/(C5/C1)); }
public long toMillis(long d) { return x(d, C5/C2, MAX/(C5/C2)); }
public long toSeconds(long d) { return x(d, C5/C3, MAX/(C5/C3)); }
public long toMinutes(long d) { return x(d, C5/C4, MAX/(C5/C4)); }
public long toHours(long d) { return d; }
public long toDays(long d) { return d/(C6/C5); }
public long convert(long d, TimeUnit u) { return u.toHours(d); }
int excessNanos(long d, long m) { return 0; }
},
/**
* Time unit representing twenty four hours
*/
DAYS {
public long toNanos(long d) { return x(d, C6/C0, MAX/(C6/C0)); }
public long toMicros(long d) { return x(d, C6/C1, MAX/(C6/C1)); }
public long toMillis(long d) { return x(d, C6/C2, MAX/(C6/C2)); }
public long toSeconds(long d) { return x(d, C6/C3, MAX/(C6/C3)); }
public long toMinutes(long d) { return x(d, C6/C4, MAX/(C6/C4)); }
public long toHours(long d) { return x(d, C6/C5, MAX/(C6/C5)); }
public long toDays(long d) { return d; }
public long convert(long d, TimeUnit u) { return u.toDays(d); }
int excessNanos(long d, long m) { return 0; }
};
// Handy constants for conversion methods
static final long C0 = 1L;
static final long C1 = C0 * 1000L;
static final long C2 = C1 * 1000L;
static final long C3 = C2 * 1000L;
static final long C4 = C3 * 60L;
static final long C5 = C4 * 60L;
static final long C6 = C5 * 24L;
static final long MAX = Long.MAX_VALUE;
/**
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