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Class 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. 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:
    • 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.
        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.
        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).
        milliseconds of non-sleep uptime since boot.
      • elapsedRealtime

        public static long elapsedRealtime()
        Returns milliseconds since boot, including time spent in sleep.
        elapsed milliseconds since boot.
      • elapsedRealtimeNanos

        public static long elapsedRealtimeNanos()
        Returns nanoseconds since boot, including time spent in sleep.
        elapsed nanoseconds since boot.
      • currentThreadTimeMillis

        public static long currentThreadTimeMillis()
        Returns milliseconds running in the current thread.
        elapsed milliseconds in the thread
      • currentThreadTimeMicro

        public static long currentThreadTimeMicro()
        Returns microseconds running in the current thread.
        elapsed microseconds in the thread
      • currentTimeMicro

        public static long currentTimeMicro()
        Returns current wall time in microseconds.
        elapsed microseconds in wall time


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