timer.java.svn-base

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package net.java.sipmack.common.scheduler;

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 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/>
 * Implementation note: This class scales to large numbers of concurrently
 * scheduled tasks (thousands should present no problem). Internally, it uses a
 * binary heap to represent its task queue, so the cost to schedule a task is
 * O(log n), where n is the number of concurrently scheduled tasks.
 *
 * @author Josh Bloch
 * @version 1.9, 01/23/03
 * @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);

    /**
     * ReSchedules the specified task to perform after "delay" milliseconds
     *
     * @param task the task to reschedule
     * @param date the time/date this task is scheduled for
     */
    public void reschedule(TimerTask task, java.util.Date date) {
        queue.reschedule(task, date.getTime());
    }

    /**
     * ReSchedules the specified task to perform after "delay" milliseconds
     *
     * @param task  the task to reschedule
     * @param delay delay in milliseconds before task is to be executed.
     */
    public void reschedule(TimerTask task, long delay) {
        queue.reschedule(task, delay);
    }

    /**
     * This object causes the timer's task execution thread to exit gracefully
     * when there are no live references to the Timer object and no tasks in the
     * timer queue. It is used in preference to a finalizer on Timer as such a
     * finalizer would be susceptible to a subclass's finalizer forgetting to
     * call it.
     */
    private Object threadReaper = new Object() {
        protected void finalize() throws Throwable {
            synchronized (queue) {
                thread.newTasksMayBeScheduled = false;
                queue.notify(); // In case queue is empty.
            }
        }
    };

    /**
     * Creates a new timer. The associated thread does <i>not</i> run as a
     * daemon.
     *
     * @see Thread
     * @see #cancel()
     */
    public Timer() {
        thread.start();
    }

    /**
     * Creates a new timer whose associated thread may be specified to run as a
     * daemon. A deamon thread is called for if the timer will be used to
     * schedule repeating "maintenance activities", which must be performed as
     * long as the application is running, but should not prolong the lifetime
     * of the application.
     *
     * @param isDaemon true if the associated thread should run as a daemon.
     * @see Thread
     * @see #cancel()
     */
    public Timer(boolean isDaemon) {
        thread.setDaemon(isDaemon);
        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/>
     * <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/>
     * <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/>
     * <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/>
     * <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/>
     * <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/>
     * <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/>
     * <p/>