timer.java

用于移动设备上的java虚拟机源代码

/*
 * @(#)Timer.java	1.22 02/07/24 @(#)
 *
 * Copyright (c) 1999-2001 Sun Microsystems, Inc.  All rights reserved.
 * PROPRIETARY/CONFIDENTIAL
 * Use is subject to license terms.
 */

package java.util;
import java.util.Date;

/**
 * A facility for threads to schedule tasks for future execution in a
 * background thread.  Tasks may be scheduled for one-time execution, or for
 * repeated execution at regular intervals.
 *
 * <p>Corresponding to each <tt>Timer</tt> object is a single background
 * thread that is used to execute all of the timer's tasks, sequentially.
 * Timer tasks should complete quickly.  If a timer task takes excessive time
 * to complete, it "hogs" the timer's task execution thread.  This can, in
 * turn, delay the execution of subsequent tasks, which may "bunch up" and
 * execute in rapid succession when (and if) the offending task finally
 * completes.
 *
 * <p>After the last live reference to a <tt>Timer</tt> object goes away
 * <i>and</i> all outstanding tasks have completed execution, the timer's task
 * execution thread terminates gracefully (and becomes subject to garbage
 * collection).  However, this can take arbitrarily long to occur.  By
 * default, the task execution thread does not run as a <i>daemon thread</i>,
 * so it is capable of keeping an application from terminating.  If a caller
 * wants to terminate a timer's task execution thread rapidly, the caller
 * should invoke the timer's <tt>cancel</tt> method.
 *
 * <p>If the timer's task execution thread terminates unexpectedly, for
 * example, because its <tt>stop</tt> method is invoked, any further
 * attempt to schedule a task on the timer will result in an
 * <tt>IllegalStateException</tt>, as if the timer's <tt>cancel</tt>
 * method had been invoked.
 *
 * <p>This class is thread-safe: multiple threads can share a single
 * <tt>Timer</tt> object without the need for external synchronization.
 *
 * <p>This class does <i>not</i> offer real-time guarantees: it schedules
 * tasks using the <tt>Object.wait(long)</tt> method.
 * <p>
 * Timers function only within a single VM and are cancelled when the VM exits.
 * When the VM is started no timers exist, they are created only by
 * application request.
 *
 * @see     TimerTask
 * @see     Object#wait(long)
 * @since   1.3
 */

public class Timer {
    /**
     * The timer task queue.  This data structure is shared with the timer
     * thread.  The timer produces tasks, via its various schedule calls,
     * and the timer thread consumes, executing timer tasks as appropriate,
     * and removing them from the queue when they're obsolete.
     */
    private TaskQueue queue = new TaskQueue();

    /**
     * The timer thread.
     */
    private TimerThread thread = new TimerThread(queue);

    /**
     * Creates a new timer.  The associated thread does <i>not</i> run as
     * a daemon thread, which may prevent an application from terminating.
     *
     * @see Thread
     * @see #cancel()
     */
    public Timer() {
        thread.start();
    }

    /**
     * Schedules the specified task for execution after the specified delay.
     *
     * @param task  task to be scheduled.
     * @param delay delay in milliseconds before task is to be executed.
     * @throws IllegalArgumentException if <tt>delay</tt> is negative, or
     *         <tt>delay + System.currentTimeMillis()</tt> is negative.
     * @throws IllegalStateException if task was already scheduled or
     *         cancelled, or timer was cancelled.
     */
    public void schedule(TimerTask task, long delay) {
        if (delay < 0)
            throw new IllegalArgumentException("Negative delay.");
        sched(task, System.currentTimeMillis()+delay, 0);
    }

    /**
     * Schedules the specified task for execution at the specified time.  If
     * the time is in the past, the task is scheduled for immediate execution.
     *
     * @param task task to be scheduled.
     * @param time time at which task is to be executed.
     * @throws IllegalArgumentException if <tt>time.getTime()</tt> is negative.
     * @throws IllegalStateException if task was already scheduled or
     *         cancelled, timer was cancelled, or timer thread terminated.
     */
    public void schedule(TimerTask task, Date time) {
        sched(task, time.getTime(), 0);
    }

    /**
     * Schedules the specified task for repeated <i>fixed-delay execution</i>,
     * beginning after the specified delay.  Subsequent executions take place
     * at approximately regular intervals separated by the specified period.
     *
     * <p>In fixed-delay execution, each execution is scheduled relative to
     * the actual execution time of the previous execution.  If an execution
     * is delayed for any reason (such as garbage collection or other
     * background activity), subsequent executions will be delayed as well.
     * In the long run, the frequency of execution will generally be slightly
     * lower than the reciprocal of the specified period (assuming the system
     * clock underlying <tt>Object.wait(long)</tt> is accurate).
     *
     * <p>Fixed-delay execution is appropriate for recurring activities
     * that require "smoothness."  In other words, it is appropriate for
     * activities where it is more important to keep the frequency accurate
     * in the short run than in the long run.  This includes most animation
     * tasks, such as blinking a cursor at regular intervals.  It also includes
     * tasks wherein regular activity is performed in response to human
     * input, such as automatically repeating a character as long as a key
     * is held down.
     *
     * @param task   task to be scheduled.
     * @param delay  delay in milliseconds before task is to be executed.
     * @param period time in milliseconds between successive task executions.
     * @throws IllegalArgumentException if <tt>delay</tt> is negative, or
     *         <tt>delay + System.currentTimeMillis()</tt> is negative.
     * @throws IllegalStateException if task was already scheduled or
     *         cancelled, timer was cancelled, or timer thread terminated.
     */
    public void schedule(TimerTask task, long delay, long period) {
        if (delay < 0)
            throw new IllegalArgumentException("Negative delay.");
        if (period <= 0)
            throw new IllegalArgumentException("Non-positive period.");
        sched(task, System.currentTimeMillis()+delay, -period);
    }

    /**
     * Schedules the specified task for repeated <i>fixed-delay execution</i>,
     * beginning at the specified time. Subsequent executions take place at
     * approximately regular intervals, separated by the specified period.
     *
     * <p>In fixed-delay execution, each execution is scheduled relative to
     * the actual execution time of the previous execution.  If an execution
     * is delayed for any reason (such as garbage collection or other
     * background activity), subsequent executions will be delayed as well.
     * In the long run, the frequency of execution will generally be slightly
     * lower than the reciprocal of the specified period (assuming the system
     * clock underlying <tt>Object.wait(long)</tt> is accurate).
     *
     * <p>Fixed-delay execution is appropriate for recurring activities
     * that require "smoothness."  In other words, it is appropriate for
     * activities where it is more important to keep the frequency accurate
     * in the short run than in the long run.  This includes most animation
     * tasks, such as blinking a cursor at regular intervals.  It also includes
     * tasks wherein regular activity is performed in response to human
     * input, such as automatically repeating a character as long as a key
     * is held down.
     *
     * @param task   task to be scheduled.
     * @param firstTime First time at which task is to be executed.
     * @param period time in milliseconds between successive task executions.
     * @throws IllegalArgumentException if <tt>time.getTime()</tt> is negative.
     * @throws IllegalStateException if task was already scheduled or
     *         cancelled, timer was cancelled, or timer thread terminated.
     */
    public void schedule(TimerTask task, Date firstTime, long period) {
        if (period <= 0)
            throw new IllegalArgumentException("Non-positive period.");
        sched(task, firstTime.getTime(), -period);
    }

    /**
     * Schedules the specified task for repeated <i>fixed-rate execution</i>,
     * beginning after the specified delay.  Subsequent executions take place
     * at approximately regular intervals, separated by the specified period.
     *
     * <p>In fixed-rate execution, each execution is scheduled relative to the
     * scheduled execution time of the initial execution.  If an execution is
     * delayed for any reason (such as garbage collection or other background
     * activity), two or more executions will occur in rapid succession to
     * "catch up."  In the long run, the frequency of execution will be
     * exactly the reciprocal of the specified period (assuming the system
     * clock underlying <tt>Object.wait(long)</tt> is accurate).
     *
     * <p>Fixed-rate execution is appropriate for recurring activities that
     * are sensitive to <i>absolute</i> time, such as ringing a chime every
     * hour on the hour, or running scheduled maintenance every day at a
     * particular time.  It is also appropriate for for recurring activities
     * where the total time to perform a fixed number of executions is
     * important, such as a countdown timer that ticks once every second for
     * ten seconds.  Finally, fixed-rate execution is appropriate for
     * scheduling multiple repeating timer tasks that must remain synchronized
     * with respect to one another.
     *
     * @param task   task to be scheduled.
     * @param delay  delay in milliseconds before task is to be executed.
     * @param period time in milliseconds between successive task executions.
     * @throws IllegalArgumentException if <tt>delay</tt> is negative, or
     *         <tt>delay + System.currentTimeMillis()</tt> is negative.
     * @throws IllegalStateException if task was already scheduled or
     *         cancelled, timer was cancelled, or timer thread terminated.
     */
    public void scheduleAtFixedRate(TimerTask task, long delay, long period) {
        if (delay < 0)
            throw new IllegalArgumentException("Negative delay.");
        if (period <= 0)
            throw new IllegalArgumentException("Non-positive period.");
        sched(task, System.currentTimeMillis()+delay, period);
    }

    /**
     * Schedules the specified task for repeated <i>fixed-rate execution</i>,
     * beginning at the specified time. Subsequent executions take place at
     * approximately regular intervals, separated by the specified period.
     *
     * <p>In fixed-rate execution, each execution is scheduled relative to the
     * scheduled execution time of the initial execution.  If an execution is
     * delayed for any reason (such as garbage collection or other background
     * activity), two or more executions will occur in rapid succession to
     * "catch up."  In the long run, the frequency of execution will be
     * exactly the reciprocal of the specified period (assuming the system
     * clock underlying <tt>Object.wait(long)</tt> is accurate).
     *
     * <p>Fixed-rate execution is appropriate for recurring activities that
     * are sensitive to <i>absolute</i> time, such as ringing a chime every
     * hour on the hour, or running scheduled maintenance every day at a
     * particular time.  It is also appropriate for for recurring activities
     * where the total time to perform a fixed number of executions is
     * important, such as a countdown timer that ticks once every second for
     * ten seconds.  Finally, fixed-rate execution is appropriate for
     * scheduling multiple repeating timer tasks that must remain synchronized
     * with respect to one another.
     *
     * @param task   task to be scheduled.
     * @param firstTime First time at which task is to be executed.
     * @param period time in milliseconds between successive task executions.
     * @throws IllegalArgumentException if <tt>time.getTime()</tt> is negative.
     * @throws IllegalStateException if task was already scheduled or
     *         cancelled, timer was cancelled, or timer thread terminated.
     */
    public void scheduleAtFixedRate(TimerTask task, Date firstTime,
                                    long period) {
        if (period <= 0)
            throw new IllegalArgumentException("Non-positive period.");
        sched(task, firstTime.getTime(), period);
    }

    /**
     * Schedule the specified timer task for execution at the specified
     * time with the specified period, in milliseconds.  If period is
     * positive, the task is scheduled for repeated execution; if period is
     * zero, the task is scheduled for one-time execution. Time is specified
     * in Date.getTime() format.  This method checks timer state, task state,
     * and initial execution time, but not period.
     *
     * @param task   task to be scheduled.
     * @param time time at which task is to be executed.
     * @param period time in milliseconds between successive task executions.
     * @throws IllegalArgumentException if <tt>time()</tt> is negative.
     * @throws IllegalStateException if task was already scheduled or
     *         cancelled, timer was cancelled, or timer thread terminated.
     */
    private void sched(TimerTask task, long time, long period) {
        if (time < 0)
            throw new IllegalArgumentException("Illegal execution time.");
        
        synchronized (queue) {
            if (!thread.newTasksMayBeScheduled) {
                throw new IllegalStateException("Timer already cancelled.");
            }

	    /*
	     * If the TimerThread has exited without an error
	     * it is restarted. See the commentary in TimerThread.run.
	     */
	    if (!(thread.isAlive())) {
		thread = new TimerThread(queue);
		thread.start();
	    }