SystemClock.javaAPI DocAndroid 1.5 API6562Wed May 06 22:41:56 BST 2009android.os


public final class SystemClock extends Object
Core timekeeping facilities.

Three different clocks are available, and they should not be confused:

  • {@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.

  • {@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 the recommended basis for the general purpose interval timing of user interface events, performance measurements, and anything else that does not need to measure elapsed time during device sleep. Most methods that accept a timestamp value expect the {@link #uptimeMillis} clock.

  • {@link #elapsedRealtime} is counted in milliseconds since the system was booted, including deep sleep. This clock should be used when measuring time intervals that may span periods of system sleep.

There are several mechanisms for controlling the timing of events:
  • 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}.

  • {@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.

  • 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).

  • The {@link} 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.

Fields Summary
Constructors Summary
private SystemClock()
This class is uninstantiable.

        // This space intentionally left blank.
Methods Summary
public static native longcurrentThreadTimeMillis()
Returns milliseconds running in the current thread.

elapsed milliseconds in the thread

public static native longelapsedRealtime()
Returns milliseconds since boot, including time spent in sleep.

elapsed milliseconds since boot.

public static native booleansetCurrentTimeMillis(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.

public static voidsleep(long ms)
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.

ms to sleep before returning, in milliseconds of uptime.

        long start = uptimeMillis();
        long duration = ms;
        boolean interrupted = false;
        do {
            try {
            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.
public static native longuptimeMillis()
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.