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SystemClock

public final class SystemClock
extends Object

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 suitable for interval timing when the interval does not span device sleep. Most methods that accept a timestamp value currently expect the uptimeMillis() clock.

  • elapsedRealtime() and elapsedRealtimeNanos() return the time since the system was booted, and include deep sleep. This clock is guaranteed to be monotonic, and continues to tick even when the CPU is in power saving modes, so is the recommend basis for general purpose interval timing.

There are several mechanisms for controlling the timing of events:

Summary

Public methods

static long currentThreadTimeMillis()

Returns milliseconds running in the current thread.

static long elapsedRealtime()

Returns milliseconds since boot, including time spent in sleep.

static long elapsedRealtimeNanos()

Returns nanoseconds since boot, including time spent in sleep.

static boolean setCurrentTimeMillis(long millis)

Sets the current wall time, in milliseconds.

static void sleep(long ms)

Waits a given number of milliseconds (of uptimeMillis) before returning.

static long uptimeMillis()

Returns milliseconds since boot, not counting time spent in deep sleep.

Inherited methods

From class java.lang.Object

Public methods

currentThreadTimeMillis

Added in API level 1
long currentThreadTimeMillis ()

Returns milliseconds running in the current thread.

Returns
long elapsed milliseconds in the thread

elapsedRealtime

Added in API level 1
long elapsedRealtime ()

Returns milliseconds since boot, including time spent in sleep.

Returns
long elapsed milliseconds since boot.

elapsedRealtimeNanos

Added in API level 17
long elapsedRealtimeNanos ()

Returns nanoseconds since boot, including time spent in sleep.

Returns
long elapsed nanoseconds since boot.

setCurrentTimeMillis

Added in API level 1
boolean setCurrentTimeMillis (long millis)

Sets the current wall time, in milliseconds. Requires the calling process to have appropriate permissions.

Parameters
millis long
Returns
boolean if the clock was successfully set to the specified time.

sleep

Added in API level 1
void sleep (long ms)

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.

Parameters
ms long: to sleep before returning, in milliseconds of uptime.

uptimeMillis

Added in API level 1
long uptimeMillis ()

Returns milliseconds since boot, not counting time spent in deep sleep.

Returns
long milliseconds of non-sleep uptime since boot.
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