java.lang.Object | |
↳ | android.os.SystemClock |
Core timekeeping facilities.
Three different clocks are available, and they should not be confused:
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 setCurrentTimeMillis(long)
), 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. If you are using
System.currentTimeMillis(), consider listening to the
ACTION_TIME_TICK
,
ACTION_TIME_CHANGED
and ACTION_TIMEZONE_CHANGED
Intent
broadcasts to find out when the time changes.
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 Thread.sleep(millls)
,
Object.wait(millis)
, and
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
uptimeMillis()
clock.
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.
Standard functions like Thread.sleep(millis)
and Object.wait(millis)
are always available. These functions use the 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 Thread.interrupt()
, and
you must handle InterruptedException
.
SystemClock.sleep(millis)
is a utility function
very similar to Thread.sleep(millis)
, but it
ignores InterruptedException
. Use this function for delays if
you do not use Thread.interrupt()
, as it will
preserve the interrupted state of the thread.
The Handler
class can schedule asynchronous
callbacks at an absolute or relative time. Handler objects also use the
uptimeMillis()
clock, and require an event loop
(normally present in any GUI application).
The 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 currentTimeMillis()
(RTC) or
elapsedRealtime()
(ELAPSED_REALTIME), and cause an
Intent
broadcast when they occur.
Public Methods | |||||||||||
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Returns milliseconds running in the current thread.
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Returns milliseconds since boot, including time spent in sleep.
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Sets the current wall time, in milliseconds.
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Waits a given number of milliseconds (of uptimeMillis) before returning.
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Returns milliseconds since boot, not counting time spent in deep sleep.
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Inherited Methods | |||||||||||
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From class java.lang.Object
|
Returns milliseconds running in the current thread.
Returns milliseconds since boot, including time spent in sleep.
Sets the current wall time, in milliseconds. Requires the calling process to have appropriate permissions.
Waits a given number of milliseconds (of uptimeMillis) before returning.
Similar to sleep(long)
, but does not throw
InterruptedException
; interrupt()
events are
deferred until the next interruptible operation. Does not return until
at least the specified number of milliseconds has elapsed.
ms | to sleep before returning, in milliseconds of uptime. |
---|
Returns milliseconds since boot, not counting time spent in deep sleep. Note: This value may get reset occasionally (before it would otherwise wrap around).