ZoneOffset 表示 UTC 时区的偏移量，而 ZoneId 表示 UTC 的时区
ZoneOffset 继承自 ZoneId
偏移量 VS 时区
一个 UTC 偏移量 仅仅只记录了时分秒而已，除此之外没有任何其他信息。举个例子 ，+08:00 的意思时超前于 UTC 八个小时，而 -05:45 意思是落后于 UTC 五小时四十五分钟
对于特定地区的人来说,时区是过去、现在以及未来的偏移量的历史集合。像夏令时会导致特定时间段内的偏移量会随时间变化。
比如美国的夏令时，半年左右的时间会再原来的基础上偏移一个小时，然后下半年再调整回这一个小时的偏移量。时区的意义就是记录这些所有的会造成偏移的情况。
因此只看偏移量没有意义的，同一个时区下，不同的时间，偏移量可能不一样,而且未来还会随着政府的法令进行更改。
举个例子，巴黎时间 (Europe/Paris)，在一年的一小部分时间内偏移量为 +01:00，而由于夏令时的缘故，大部分时间 (三月末到十月末）的偏移量都是 +02:00。
// 2020年2月，Europe/Paris(欧洲/巴黎)时区 偏移量为 +01:00
LocalDateTime dateTime1 = LocalDateTime.of(LocalDate.of(2020, 2, 01), LocalTime.now());
ZonedDateTime zonedDateTime1 = ZonedDateTime.of(dateTime1, ZoneId.of("Europe/Paris"));
zonedDateTime1.getOffset(); // +01:00
// 2020年5月，Europe/Paris(欧洲/巴黎)时区 偏移量为 +02:00
LocalDateTime dateTime2 = LocalDateTime.of(LocalDate.of(2020, 5, 01), LocalTime.now());
ZonedDateTime zonedDateTime2 = ZonedDateTime.of(dateTime2, ZoneId.of("Europe/Paris"));
zonedDateTime2.getOffset(); // +02:00
java 中如何处理时区和偏移量
由于同一个时区下，不同的时间，偏移量可能不一样,而且未来还会随着政府的法令进行更改，因此处理起来特别麻烦。好在有一个名为 IANA 时区数据库的国际组织在维护全球各时区的规则，包括历史规则。
在 java 中，时区偏移量规则是由 java.time.zone.ZoneRulesProvider 来处理的。
/**
* Provider of time-zone rules to the system.
* <p>
* This class manages the configuration of time-zone rules.
* The static methods provide the public API that can be used to manage the providers.
* The abstract methods provide the SPI that allows rules to be provided.
* <p>
* ZoneRulesProvider may be installed in an instance of the Java Platform as
* extension classes, that is, jar files placed into any of the usual extension
* directories. Installed providers are loaded using the service-provider loading
* facility defined by the {@link ServiceLoader} class. A ZoneRulesProvider
* identifies itself with a provider configuration file named
* {@code java.time.zone.ZoneRulesProvider} in the resource directory
* {@code META-INF/services}. The file should contain a line that specifies the
* fully qualified concrete zonerules-provider class name.
* Providers may also be made available by adding them to the class path or by
* registering themselves via {@link #registerProvider} method.
* <p>
* The Java virtual machine has a default provider that provides zone rules
* for the time-zones defined by IANA Time Zone Database (TZDB). If the system
* property {@code java.time.zone.DefaultZoneRulesProvider} is defined then
* it is taken to be the fully-qualified name of a concrete ZoneRulesProvider
* class to be loaded as the default provider, using the system class loader.
* If this system property is not defined, a system-default provider will be
* loaded to serve as the default provider.
* <p>
* Rules are looked up primarily by zone ID, as used by {@link ZoneId}.
* Only zone region IDs may be used, zone offset IDs are not used here.
* <p>
* Time-zone rules are political, thus the data can change at any time.
* Each provider will provide the latest rules for each zone ID, but they
* may also provide the history of how the rules changed.
*
* @implSpec
* This interface is a service provider that can be called by multiple threads.
* Implementations must be immutable and thread-safe.
* <p>
* Providers must ensure that once a rule has been seen by the application, the
* rule must continue to be available.
* <p>
* Providers are encouraged to implement a meaningful {@code toString} method.
* <p>
* Many systems would like to update time-zone rules dynamically without stopping the JVM.
* When examined in detail, this is a complex problem.
* Providers may choose to handle dynamic updates, however the default provider does not.
*
* @since 1.8
*/
public abstract class ZoneRulesProvider {
...
}
Java虚拟机有一个默认提供程序，为IANA时区数据库(TZDB)定义的时区提供区域规则。如果系统属性{@code java.time.zone.DefaultZoneRulesProvider} 那么它将被视为具体ZoneRulesProvider类的完全限定名，使用系统类加载器作为默认提供程序加载。如果未定义此系统属性，则将加载系统默认提供程序作为默认提供程序。
许多系统都希望动态更新时区规则，而不停止JVM，详细分析一下，这是一个复杂的问题，提供程序可以选择处理动态更新，但是默认提供程序不会这样做。
