android.os
Class SystemClock

java.lang.Object
  android.os.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 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.

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.

• 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 System.currentTimeMillis() (RTC) or elapsedRealtime() (ELAPSED_REALTIME), and cause an Intent broadcast when they occur.

Method Summary

static long	currentThreadTimeMillis() Returns milliseconds running in the current thread.
static long	elapsedRealtime() Returns milliseconds 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.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Method Detail

sleep

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.

setCurrentTimeMillis

public static boolean setCurrentTimeMillis(long millis)

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

Returns:

if the clock was successfully set to the specified time.

uptimeMillis

public static 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).

Returns:

milliseconds of non-sleep uptime since boot.

elapsedRealtime

public static long elapsedRealtime()

Returns milliseconds since boot, including time spent in sleep.

Returns:

elapsed milliseconds since boot.

currentThreadTimeMillis

public static long currentThreadTimeMillis()

Returns milliseconds running in the current thread.

Returns:

elapsed milliseconds in the thread