SystemClock

public final class SystemClock extends Object

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.

• 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:

• 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.

Public Method Summary

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 Method Summary

From class java.lang.Object

Object	clone()
boolean	equals(Object arg0)
void	finalize()
final Class<?>	getClass()
int	hashCode()
final void	notify()
final void	notifyAll()
String	toString()
final void	wait(long arg0, int arg1)
final void	wait(long arg0)
final void	wait()

Public Methods

public static long elapsedRealtime ()

Returns milliseconds since boot, including time spent in sleep.

Returns

• elapsed milliseconds since boot.

public static long elapsedRealtimeNanos ()

Returns nanoseconds since boot, including time spent in sleep.

Returns

• elapsed nanoseconds since boot.

public static boolean setCurrentTimeMillis (long millis)

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

Parameters

millis

Returns

• if the clock was successfully set to the specified time.

public static void sleep (long ms)

Waits a given number of milliseconds (of uptimeMillis) before returning. Similar to Thread.sleep(long), but does not throw InterruptedException; Thread.interrupt() events are deferred until the next interruptible operation. Does not return until at least the specified number of milliseconds has elapsed.

Parameters

ms	to sleep before returning, in milliseconds of uptime.

public static long uptimeMillis ()

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

Returns

• milliseconds of non-sleep uptime since boot.