SystemClockpublic final class SystemClock extends Object | Core timekeeping facilities.
Three different clocks are available, and they should not be confused:
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{@link System#currentTimeMillis System.currentTimeMillis()}
is the standard "wall" clock (time and date) expressing milliseconds
since the epoch. The wall clock can be set by the user or the phone
network (see {@link #setCurrentTimeMillis}), so the time may jump
backwards or forwards unpredictably. This clock should only be used
when correspondence with real-world dates and times is important, such
as in a calendar or alarm clock application. Interval or elapsed
time measurements should use a different clock.
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{@link #uptimeMillis} is counted in milliseconds since the
system was booted. This clock stops when the system enters deep
sleep (CPU off, display dark, device waiting for external input),
but is not affected by clock scaling, idle, or other power saving
mechanisms. This is the basis for most interval timing
such as {@link Thread#sleep(long) Thread.sleep(millls)},
{@link Object#wait(long) Object.wait(millis)}, and
{@link System#nanoTime System.nanoTime()}. This clock is guaranteed
to be monotonic, and is the recommended basis for the general purpose
interval timing of user interface events, performance measurements,
and anything else that does not need to measure elapsed time during
device sleep. Most methods that accept a timestamp value expect the
{@link #uptimeMillis} clock.
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{@link #elapsedRealtime} is counted in milliseconds since the
system was booted, including deep sleep. This clock should be used
when measuring time intervals that may span periods of system sleep.
There are several mechanisms for controlling the timing of events:
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Standard functions like {@link Thread#sleep(long)
Thread.sleep(millis)} and {@link Object#wait(long) Object.wait(millis)}
are always available. These functions use the {@link #uptimeMillis}
clock; if the device enters sleep, the remainder of the time will be
postponed until the device wakes up. These synchronous functions may
be interrupted with {@link Thread#interrupt Thread.interrupt()}, and
you must handle {@link InterruptedException}.
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{@link #sleep SystemClock.sleep(millis)} is a utility function
very similar to {@link Thread#sleep(long) Thread.sleep(millis)}, but it
ignores {@link InterruptedException}. Use this function for delays if
you do not use {@link Thread#interrupt Thread.interrupt()}, as it will
preserve the interrupted state of the thread.
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The {@link android.os.Handler} class can schedule asynchronous
callbacks at an absolute or relative time. Handler objects also use the
{@link #uptimeMillis} clock, and require an {@link android.os.Looper
event loop} (normally present in any GUI application).
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The {@link android.app.AlarmManager} can trigger one-time or
recurring events which occur even when the device is in deep sleep
or your application is not running. Events may be scheduled with your
choice of {@link java.lang.System#currentTimeMillis} (RTC) or
{@link #elapsedRealtime} (ELAPSED_REALTIME), and cause an
{@link android.content.Intent} broadcast when they occur.
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| Constructors Summary |
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private SystemClock()This class is uninstantiable.
// This space intentionally left blank.
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| Methods Summary |
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| public static native long | currentThreadTimeMillis()Returns milliseconds running in the current thread.
| | public static native long | elapsedRealtime()Returns milliseconds since boot, including time spent in sleep.
| | public static native boolean | setCurrentTimeMillis(long millis)Sets the current wall time, in milliseconds. Requires the calling
process to have appropriate permissions.
| | public static void | sleep(long ms)Waits a given number of milliseconds (of uptimeMillis) before returning.
Similar to {@link java.lang.Thread#sleep(long)}, but does not throw
{@link InterruptedException}; {@link Thread#interrupt()} events are
deferred until the next interruptible operation. Does not return until
at least the specified number of milliseconds has elapsed.
long start = uptimeMillis();
long duration = ms;
boolean interrupted = false;
do {
try {
Thread.sleep(duration);
}
catch (InterruptedException e) {
interrupted = true;
}
duration = start + ms - uptimeMillis();
} while (duration > 0);
if (interrupted) {
// Important: we don't want to quietly eat an interrupt() event,
// so we make sure to re-interrupt the thread so that the next
// call to Thread.sleep() or Object.wait() will be interrupted.
Thread.currentThread().interrupt();
}
| | public static native long | uptimeMillis()Returns milliseconds since boot, not counting time spent in deep sleep.
Note: This value may get reset occasionally (before it would
otherwise wrap around).
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