JDK8时间新API-2

1 获取时间戳不再使用System.currentTimeMillis

@Test
    public void testClock () {
        Clock clock = Clock.systemUTC();
        System.out.println("Clock : " + clock.millis());
        System.out.println("Clock : " + System.currentTimeMillis());
        // Returns time based on system clock zone
        Clock defaultClock = Clock.systemDefaultZone();
        System.out.println("Clock : " + defaultClock.millis());

    }

这两个方法

 Clock.systemUTC()   Clock.systemDefaultZone() 的区别是什么？

So although both will return the same instant if you ask them what the current instant is, 
they will not necessarily return the same LocalDateTime, or LocalDate, or ZonedDateTime. 
For example, suppose my system timezone is Asia/Shanghai, which is at an offset of UTC+8 all year round. 
And it is now 5 am on 2020-07-29 in Shanghai. 
If I do LocalDate.now(Clock.systemUTC()), it will tell me 2020-07-28, because it's still the 28th in the UTC timezone. 
If I do LocalDate.now(Clock.systemDefaultZone()), however, it will tell me 2020-07-29, 
because it is 2020-07-29 in Shanghai (which is the system timezone).

Now you should see why the parameterless now of date/time related classes use systemDefaultZone. 
It would be weird if I did LocalTime.now() in Shanghai and saw a time that's 8 hours earlier!

所以最好使用Clock.systemDefaultZone() 拿到本JVM的时钟

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