https://source.android.com/ 
/*
 * Copyright (C) 2006 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package android.os;

import android.annotation.NonNull;
import android.app.IAlarmManager;
import android.content.Context;
import android.util.Slog;

import dalvik.annotation.optimization.CriticalNative;

import java.time.Clock;
import java.time.DateTimeException;
import java.time.ZoneOffset;


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), 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:


/**
 * Core timekeeping facilities.
 *
 * <p> Three different clocks are available, and they should not be confused:
 *
 * <ul>
 *     <li> <p> {@link System#currentTimeMillis 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 {@link #setCurrentTimeMillis}), 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
 *     {@link android.content.Intent#ACTION_TIME_TICK ACTION_TIME_TICK},
 *     {@link android.content.Intent#ACTION_TIME_CHANGED ACTION_TIME_CHANGED}
 *     and {@link android.content.Intent#ACTION_TIMEZONE_CHANGED
 *     ACTION_TIMEZONE_CHANGED} {@link android.content.Intent Intent}
 *     broadcasts to find out when the time changes.
 *
 *     <li> <p> {@link #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 {@link Thread#sleep(long) Thread.sleep(millls)},
 *     {@link Object#wait(long) Object.wait(millis)}, and
 *     {@link System#nanoTime 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 {@link #uptimeMillis} clock.
 *
 *     <li> <p> {@link #elapsedRealtime} and {@link #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.
 *
 * </ul>
 *
 * There are several mechanisms for controlling the timing of events:
 *
 * <ul>
 *     <li> <p> Standard functions like {@link Thread#sleep(long)
 *     Thread.sleep(millis)} and {@link Object#wait(long) Object.wait(millis)}
 *     are always available.  These functions use the {@link #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 {@link Thread#interrupt Thread.interrupt()}, and
 *     you must handle {@link InterruptedException}.
 *
 *     <li> <p> {@link #sleep SystemClock.sleep(millis)} is a utility function
 *     very similar to {@link Thread#sleep(long) Thread.sleep(millis)}, but it
 *     ignores {@link InterruptedException}.  Use this function for delays if
 *     you do not use {@link Thread#interrupt Thread.interrupt()}, as it will
 *     preserve the interrupted state of the thread.
 *
 *     <li> <p> The {@link android.os.Handler} class can schedule asynchronous
 *     callbacks at an absolute or relative time.  Handler objects also use the
 *     {@link #uptimeMillis} clock, and require an {@link android.os.Looper
 *     event loop} (normally present in any GUI application).
 *
 *     <li> <p> The {@link android.app.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 {@link java.lang.System#currentTimeMillis} (RTC) or
 *     {@link #elapsedRealtime} (ELAPSED_REALTIME), and cause an
 *     {@link android.content.Intent} broadcast when they occur.
 * </ul>
 */
public final class SystemClock {
    private static final String TAG = "SystemClock";

    
This class is uninstantiable.

/**
     * This class is uninstantiable.
     */
    private SystemClock() {
        // This space intentionally left blank.
    }

    
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. 
Params:
  • ms – to sleep before returning, in milliseconds of uptime.
/**
     * Waits a given number of milliseconds (of uptimeMillis) before returning.
     * Similar to {@link java.lang.Thread#sleep(long)}, but does not throw
     * {@link InterruptedException}; {@link Thread#interrupt()} events are
     * deferred until the next interruptible operation.  Does not return until
     * at least the specified number of milliseconds has elapsed.
     *
     * @param ms to sleep before returning, in milliseconds of uptime.
     */
    public static void sleep(long ms)
    {
        long start = uptimeMillis();
        long duration = ms;
        boolean interrupted = false;
        do {
            try {
                Thread.sleep(duration);
            }
            catch (InterruptedException e) {
                interrupted = true;
            }
            duration = start + ms - uptimeMillis();
        } while (duration > 0);

        if (interrupted) {
            // Important: we don't want to quietly eat an interrupt() event,
            // so we make sure to re-interrupt the thread so that the next
            // call to Thread.sleep() or Object.wait() will be interrupted.
            Thread.currentThread().interrupt();
        }
    }

    
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.
/**
     * Sets the current wall time, in milliseconds.  Requires the calling
     * process to have appropriate permissions.
     *
     * @return if the clock was successfully set to the specified time.
     */
    public static boolean setCurrentTimeMillis(long millis) {
        final IAlarmManager mgr = IAlarmManager.Stub
                .asInterface(ServiceManager.getService(Context.ALARM_SERVICE));
        if (mgr == null) {
            return false;
        }

        try {
            return mgr.setTime(millis);
        } catch (RemoteException e) {
            Slog.e(TAG, "Unable to set RTC", e);
        } catch (SecurityException e) {
            Slog.e(TAG, "Unable to set RTC", e);
        }

        return false;
    }

    
Returns milliseconds since boot, not counting time spent in deep sleep.

Returns:milliseconds of non-sleep uptime since boot.
/**
     * Returns milliseconds since boot, not counting time spent in deep sleep.
     *
     * @return milliseconds of non-sleep uptime since boot.
     */
    @CriticalNative
    native public static long uptimeMillis();

    
@removed
/**
     * @removed
     */
    @Deprecated
    public static @NonNull Clock uptimeMillisClock() {
        return uptimeClock();
    }

    
Return Clock that starts at system boot, not counting time spent in deep sleep. 
@removed
/**
     * Return {@link Clock} that starts at system boot, not counting time spent
     * in deep sleep.
     *
     * @removed
     */
    public static @NonNull Clock uptimeClock() {
        return new SimpleClock(ZoneOffset.UTC) {
            @Override
            public long millis() {
                return SystemClock.uptimeMillis();
            }
        };
    }

    
Returns milliseconds since boot, including time spent in sleep.

Returns:elapsed milliseconds since boot.
/**
     * Returns milliseconds since boot, including time spent in sleep.
     *
     * @return elapsed milliseconds since boot.
     */
    @CriticalNative
    native public static long elapsedRealtime();

    
Return Clock that starts at system boot, including time spent in sleep. 
@removed
/**
     * Return {@link Clock} that starts at system boot, including time spent in
     * sleep.
     *
     * @removed
     */
    public static @NonNull Clock elapsedRealtimeClock() {
        return new SimpleClock(ZoneOffset.UTC) {
            @Override
            public long millis() {
                return SystemClock.elapsedRealtime();
            }
        };
    }

    
Returns nanoseconds since boot, including time spent in sleep.

Returns:elapsed nanoseconds since boot.
/**
     * Returns nanoseconds since boot, including time spent in sleep.
     *
     * @return elapsed nanoseconds since boot.
     */
    @CriticalNative
    public static native long elapsedRealtimeNanos();

    
Returns milliseconds running in the current thread.

Returns:elapsed milliseconds in the thread
/**
     * Returns milliseconds running in the current thread.
     *
     * @return elapsed milliseconds in the thread
     */
    @CriticalNative
    public static native long currentThreadTimeMillis();

    
Returns microseconds running in the current thread.

Returns:elapsed microseconds in the thread
@hide
/**
     * Returns microseconds running in the current thread.
     *
     * @return elapsed microseconds in the thread
     *
     * @hide
     */
    @CriticalNative
    public static native long currentThreadTimeMicro();

    
Returns current wall time in  microseconds.

Returns:elapsed microseconds in wall time
@hide
/**
     * Returns current wall time in  microseconds.
     *
     * @return elapsed microseconds in wall time
     *
     * @hide
     */
    @CriticalNative
    public static native long currentTimeMicro();

    
Returns milliseconds since 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. Note that synchronization may occur using an insecure network protocol, so the returned time should not be used for security purposes. 

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.

Throws:
@hide
/**
     * Returns milliseconds since January 1, 1970 00:00:00.0 UTC, synchronized
     * using a remote network source outside the device.
     * <p>
     * While the time returned by {@link System#currentTimeMillis()} can be
     * adjusted by the user, the time returned by this method cannot be adjusted
     * by the user. Note that synchronization may occur using an insecure
     * network protocol, so the returned time should not be used for security
     * purposes.
     * <p>
     * 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.
     *
     * @throws DateTimeException when no accurate network time can be provided.
     * @hide
     */
    public static long currentNetworkTimeMillis() {
        final IAlarmManager mgr = IAlarmManager.Stub
                .asInterface(ServiceManager.getService(Context.ALARM_SERVICE));
        if (mgr != null) {
            try {
                return mgr.currentNetworkTimeMillis();
            } catch (ParcelableException e) {
                e.maybeRethrow(DateTimeException.class);
                throw new RuntimeException(e);
            } catch (RemoteException e) {
                throw e.rethrowFromSystemServer();
            }
        } else {
            throw new RuntimeException(new DeadSystemException());
        }
    }

    
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. Note that synchronization may occur using an insecure network protocol, so the returned time should not be used for security purposes. 

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.

Throws:
@hide
/**
     * Returns a {@link Clock} that starts at January 1, 1970 00:00:00.0 UTC,
     * synchronized using a remote network source outside the device.
     * <p>
     * While the time returned by {@link System#currentTimeMillis()} can be
     * adjusted by the user, the time returned by this method cannot be adjusted
     * by the user. Note that synchronization may occur using an insecure
     * network protocol, so the returned time should not be used for security
     * purposes.
     * <p>
     * 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.
     *
     * @throws DateTimeException when no accurate network time can be provided.
     * @hide
     */
    public static @NonNull Clock currentNetworkTimeClock() {
        return new SimpleClock(ZoneOffset.UTC) {
            @Override
            public long millis() {
                return SystemClock.currentNetworkTimeMillis();
            }
        };
    }
}