Android中各种Time API详细

// Milliseconds since Unix epoch (00:00:00 UTC on 1 January 1970)
System.currentTimeMillis()
// Nanoseconds since the VM started.
System.nanoTime()

// (API 29) Clock that starts at Unix epoch.
// Synchronized using the device's location provider.
SystemClock.currentGnssTimeClock()
// Milliseconds running in the current thread.
SystemClock.currentThreadTimeMillis()
// Milliseconds since boot, including time spent in sleep.
SystemClock.elapsedRealtime()
// Nanoseconds since boot, including time spent in sleep.
SystemClock.elapsedRealtimeNanos()
// Milliseconds since boot, not counting time spent in deep sleep.
SystemClock.uptimeMillis()

@LargeTest
@RunWith(AndroidJUnit4::class)
class TimingBenchmark {
    @get:Rule
    val benchmarkRule = BenchmarkRule()

    @Test
    fun nanoTime() {
        benchmarkRule.measureRepeated {
            System.nanoTime()
        }
    }

    @Test
    fun uptimeMillis() {
        benchmarkRule.measureRepeated {
            SystemClock.uptimeMillis()
        }
    }
}

public final class SystemClock {
    @CriticalNative
    native public static long uptimeMillis();
}

int64_t uptimeMillis()
{
    int64_t when = systemTime(SYSTEM_TIME_MONOTONIC);
    return (int64_t) nanoseconds_to_milliseconds(when);
}

nsecs_t systemTime(int clock) {
    static constexpr clockid_t clocks[] = {
        CLOCK_REALTIME,
        CLOCK_MONOTONIC,
        CLOCK_PROCESS_CPUTIME_ID,
        CLOCK_THREAD_CPUTIME_ID,
        CLOCK_BOOTTIME
    };
    timespec t = {};
    clock_gettime(clocks[clock], &t);
    return nsecs_t(t.tv_sec)*1000000000LL + t.tv_nsec;
}

public final class System {
    @CriticalNative
    public static native long nanoTime();
}

static jlong System_nanoTime() {
  struct timespec now;
  clock_gettime(CLOCK_MONOTONIC, &now);
  return now.tv_sec * 1000000000LL + now.tv_nsec;
}