Android example source code file (SystemClock.java)

This example Android source code file (SystemClock.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Android by Example" ^TM.

Java - Android tags/keywords

interruptedexception, systemclock

The SystemClock.java Android example source code

/*
 * 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;


/**
 * Core timekeeping facilities.
 *
 * <p> Three different clocks are available, and they should not be confused:
 *
 * <ul>
 *     <li> 
 {@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> 
 {@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.
 *
 *     <li> 
 {@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.
 * </ul>
 *
 * There are several mechanisms for controlling the timing of events:
 *
 * <ul>
 *     <li> 
 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> 
 {@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> 
 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> 
 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 {
    /**
     * This class is uninstantiable.
     */
    private SystemClock() {
        // This space intentionally left blank.
    }

    /**
     * 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.
     *
     * @return if the clock was successfully set to the specified time.
     */
    native public static boolean setCurrentTimeMillis(long millis);

    /**
     * Returns milliseconds since boot, not counting time spent in deep sleep.
     * <b>Note: This value may get reset occasionally (before it would
     * otherwise wrap around).
     *
     * @return milliseconds of non-sleep uptime since boot.
     */
    native public static long uptimeMillis();

    /**
     * Returns milliseconds since boot, including time spent in sleep.
     *
     * @return elapsed milliseconds since boot.
     */
    native public static long elapsedRealtime();
    
    /**
     * Returns milliseconds running in the current thread.
     * 
     * @return elapsed milliseconds in the thread
     */
    public static native long currentThreadTimeMillis();
}