Java-更改时区我班上的所有属性

Question

I need change every time zone of my DTO at runtime . Today the time zone is informed by parameter when the User performs request on my web-service , I wonder if it is possible to apply the new time zone for all dates attributes . The only thing I can not use is " TimeZone.setDefault ( myTimeZone ) " because that way apply to all JVM and how exists users of different time zones this solution is unfeasible .

I was trying something like this:

Query query = em.createNativeQuery(SQL.toString(), AgendamentoDTO.class);
collection = query.setParameter(1, idEmpresa).getResultList();

for (Field atributo : AgendamentoDTO.class.getDeclaredFields()) {
    if (atributo.getType().isAssignableFrom(Date.class)) {
        //Change time zone here
    }
}

Tks

Answer 1

Avoid setting default time zone

As wisely advised in the Question, you should set the JVM's current default time zone only as a last resort in the most desperate situation. Setting the default affects all code in all threads of all apps running within that JVM, and affects them immediately as they execute(!).

Instead, in all your date-time work, always pass the optional time zone argument to the various methods. Never rely implicitly on the JVM's current default zone.

Avoid old date-time classes

The old legacy date-time classes bundled with the earliest versions of Java have proven to be poorly designed, troublesome, and confusing. Avoid them. Now supplanted by the java.time classes.

So instead of java.util.Date , use java.time.Instant . The Instant class represents a moment on the timeline in UTC with a resolution of nanoseconds . This Instant class is the basic building block of date-time handling. Use this class frequently, as much of your business logic, data storage, data exchange, and database work should all be in UTC . Do not think of UTC as just another variation on time zone, rather, think of UTC as the One True Time. While programming, forget about your own local time zone as that parochial thinking will confuse your programming.

Instant

Generally speaking, your web service should take and give UTC values. The Instant class can directly parse and generate strings to represent these values in standard ISO 8601 format.

Instant instant = Instant.parse( "2016-09-09T22:34:08Z" );
String output = instant.toString();  // Generates: 2016-09-09T22:34:08Z

So no need to manipulate these UTC values. Keep them around. A data-providing service should stick with UTC for the most part.

In your case the DTOs, being DTOs , should stick to storing UTC values (either Instant object or a string in ISO 8601 format in UTC with the Z on the end). By definition , a DTO should be 'dumb' in the sense of lacking business object and instead should be merely transporting basic data elements. The other objects consuming those DTOs should handle any needed time zone assignments.

ZonedDateTime

Generate strings in other time zones only for presentation to users. Here we assign a time zone of Québec to view the moment through the lens of a different wall-clock time . Apply a ZoneId to get a ZonedDateTime . The ZonedDateTime and the Instant both represent the very same moment in history, the same point on the timeline.

ZoneId z = ZoneId.of( "America/Montreal" );
ZonedDateTime zdt = instant.atZone( z );

Notice that we are keeping the Instant object around in our business object, unmodified. We generate a distinct separate object, the ZonedDateTime , for a different wall-clock time.

• When making these time zone assignments within your code, pass around ZoneId objects.
• When specifying these time zone assignments through your web service API, pass the name of the time zone as a string. Always use proper IANA 'tz' time zone names in the format of continent/region such as America/Montreal or Pacific/Auckland . Never use the 3-4 letter abbreviation such as EST or IST as they are not true time zones, not standardized, and not even unique(!).

Generating strings

When your web service is serving data to be consumed as data rather than presentation, generate strings in ISO 8601 format. The java.time classes use these standard formats by default for parsing and generating strings. Simply call toString to generate a string in standard format. Note that the ZonedDateTime extends the standard format by appending the name of the time zone in square brackets.

String output = instant.toString();  // 2016-09-09T22:34:08Z
String output = zdt.toString();  // 2016-09-09T19:34:08-03:00[America/Montreal]

When your web service is serving information for presentation to a user rather than for consumption as data, generate strings in a format appropriate to the user's human language and cultural norms. You can specify a specific format. But generally best to let java.time automatically localize for you.

Locale locale = Locale.CANADA_FRENCH;
DateTimeFormatter f = DateTimeFormatter.ofLocalizedDateTime( FormatStyle.FULL ).withLocale( l );
String output = zdt.format( f );