Java应用程序中的独立时间

Question

I would like to set arbitrary time in application. Time is downloaded from server in milliseconds format- it should be independent from locale and other system preferences. But application reacquire thread safety solution, and object like standard not thread safety Calendar object.

Whats is the best way?

Today I use:

Calendar calendar = Calendar.getInstance(TimeZone.getTimeZone("UTC"));
calendar.setTimeInMillis(serverTime);

But is not good way for me beacuse, is't thread safe.

tl;dr Program have to contain own internal clock fetching time from external server. Clock must be thread-safe.

Answer 1

The time in milliseconds that a Java application uses is

the specified number of milliseconds since the standard base time known as "the epoch", namely January 1, 1970, 00:00:00 GMT.

This number is based on the GMT time zone. If you need to print it in another time zone you can use any formatting class you want, say SimpleDateFormat . If you need to make the variable that holds it thread safe, just synchronize on it, possibly by wrapping it in a class.

public class TimeInMillis {
    private volatile long time;

    public void setTime(long time) {this.time = time;} 

    public time getTime() {return time;}
}

Whenever you need to display it, just get the TimeInMillis object, get the time and create a Calendar object with it. Then use a formatting class to print the time in the format, locale, timezone, you require.

Answer 2

Time is downloaded from server in milliseconds format- it should be independent from locale and other system preferences

That isn't "time". That is a timestamp, meaning a particular time value reported by a particular piece of software at a particular point in time.

Now, if you are trying to say that, in future communications with this server, you want to translate time as reported by the device into the timebase as known by the server, that makes at least a bit of sense. In that case, you compute the delta between the device time when you receive the timestamp and the time value in the timestamp itself. Then, you apply that delta to future times you report back to the server.

Program have to contain own internal clock fetching time from external server

That makes no sense whatsoever.

In this universe, based on our current knowledge of physics, time is continuous and linear. Time does not change only when you are "fetching time from an external server". Again, what you are "fetching... from an external server" is a timestamp, a statement of what the clock on the server thought the time was at the time you made the request. You can use that timestamp for comparison purposes with other timestamps from that server, and you can use that timestamp to compare to the device's current time to determine the difference.

However, you cannot create hardware in Java code, and so you cannot create "internal clock" in Java code. The only clock is the device's clock. As noted earlier, you can translate the device's clock's time to the timebase of the server by applying the aforementioned difference, but the passage of time is still being marked by the device's own clock.

Since the difference is going to be an int or long , you can use AtomicInteger or AtomicLong if you are concerned about multiple threads working with the value at once.

Answer 3

I would just the the time in milli-second with GMT (BTW computers don't support UTC as such)

long serverTime = System.currentTimeMillis(); // millis since 1/1/1970 GMT.

To get/set this in a thread safe manner you can make it volatile

BTW Calender is pretty slow even on a PC. I would avoid using it on a phone.