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 (seesetCurrentTimeMillis(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 theACTION_TIME_TICK
,ACTION_TIME_CHANGED
andACTION_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 asThread.sleep(millls)
,Object.wait(millis)
, andSystem.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 theuptimeMillis()
clock. -
elapsedRealtime()
andelapsedRealtimeNanos()
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.
-
Standard functions like
Thread.sleep(millis)
andObject.wait(millis)
are always available. These functions use theuptimeMillis()
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 withThread.interrupt()
, and you must handleInterruptedException
. -
SystemClock.sleep(millis)
is a utility function very similar toThread.sleep(millis)
, but it ignoresInterruptedException
. Use this function for delays if you do not useThread.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 theuptimeMillis()
clock, and require anevent 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 ofSystem.currentTimeMillis()
(RTC) orelapsedRealtime()
(ELAPSED_REALTIME), and cause anIntent
broadcast when they occur.
Summary
Public methods | |
---|---|
static
Clock
|
currentGnssTimeClock()
Returns a |
static
Clock
|
currentNetworkTimeClock()
Returns a |
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. |
static
long
|
uptimeNanos()
Returns nanoseconds since boot, not counting time spent in deep sleep. |
Inherited methods | |
---|---|
Public methods
currentGnssTimeClock
public static Clock currentGnssTimeClock ()
Returns a Clock
that starts at January 1, 1970 00:00:00.0 UTC,
synchronized using the device's location provider.
Returns | |
---|---|
Clock |
This value cannot be null . |
Throws | |
---|---|
DateTimeException |
when the location provider has not had a location fix since boot. |
currentNetworkTimeClock
public static Clock currentNetworkTimeClock ()
Returns a Clock
that starts at January 1, 1970 00:00:00.0 UTC,
synchronized using a remote network source outside the device.
While the time returned by System#currentTimeMillis()
can be
adjusted by the user, the time returned by this method cannot be adjusted
by the user.
This performs no blocking network operations and returns values based on a recent successful synchronization event; it will either return a valid time or throw.
Note that synchronization may occur using an insecure network protocol,
so the returned time should not be used for security purposes.
The device may resynchronize with the same or different network source
at any time. Due to network delays, variations between servers, or local
(client side) clock drift, the accuracy of the returned times cannot be
guaranteed. In extreme cases, consecutive calls to Clock.millis()
on the returned Clock
could return times that are
out of order.
Returns | |
---|---|
Clock |
This value cannot be null . |
Throws | |
---|---|
DateTimeException |
when no network time can be provided. |
currentThreadTimeMillis
public static long currentThreadTimeMillis ()
Returns milliseconds running in the current thread.
Returns | |
---|---|
long |
elapsed milliseconds in the thread |
elapsedRealtime
public static long elapsedRealtime ()
Returns milliseconds since boot, including time spent in sleep.
Returns | |
---|---|
long |
elapsed milliseconds since boot. |
elapsedRealtimeNanos
public static long elapsedRealtimeNanos ()
Returns nanoseconds since boot, including time spent in sleep.
Returns | |
---|---|
long |
elapsed nanoseconds since boot. |
setCurrentTimeMillis
public static 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
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 |
long : to sleep before returning, in milliseconds of uptime. |
uptimeMillis
public static long uptimeMillis ()
Returns milliseconds since boot, not counting time spent in deep sleep.
Returns | |
---|---|
long |
milliseconds of non-sleep uptime since boot. |
uptimeNanos
public static long uptimeNanos ()
Returns nanoseconds since boot, not counting time spent in deep sleep.
Returns | |
---|---|
long |
nanoseconds of non-sleep uptime since boot. |