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
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
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.
Standard functions like
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
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
Returns milliseconds since boot, including time spent in sleep.
Returns nanoseconds since boot, including time spent in sleep.
Sets the current wall time, in milliseconds.
Waits a given number of milliseconds (of uptimeMillis) before returning.