timertask.java.svn-base

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package net.java.sipmack.common.scheduler;

/**
 * A task that can be scheduled for one-time or repeated execution by a Timer.
 *
 * @author Josh Bloch
 * @version 1.8, 01/23/03
 * @see Timer
 * @since 1.3
 */

public abstract class TimerTask implements Runnable {
    /**
     * This object is used to control access to the TimerTask internals.
     */
    final Object lock = new Object();

    /**
     * The state of this task, chosen from the constants below.
     */
    int state = VIRGIN;

    /**
     * This task has not yet been scheduled.
     */
    static final int VIRGIN = 0;

    /**
     * This task is scheduled for execution. If it is a non-repeating task, it
     * has not yet been executed.
     */
    static final int SCHEDULED = 1;

    /**
     * This non-repeating task has already executed (or is currently executing)
     * and has not been cancelled.
     */
    static final int EXECUTED = 2;

    /**
     * This task has been cancelled (with a call to TimerTask.cancel).
     */
    static final int CANCELLED = 3;

    /**
     * Next execution time for this task in the format returned by
     * System.currentTimeMillis, assuming this task is schedule for execution.
     * For repeating tasks, this field is updated prior to each task execution.
     */
    long nextExecutionTime;

    /**
     * Period in milliseconds for repeating tasks. A positive value indicates
     * fixed-rate execution. A negative value indicates fixed-delay execution. A
     * value of 0 indicates a non-repeating task.
     */
    long period = 0;

    /**
     * Creates a new timer task.
     */
    protected TimerTask() {
    }

    /**
     * The action to be performed by this timer task.
     */
    public abstract void run();

    /**
     * Cancels this timer task. If the task has been scheduled for one-time
     * execution and has not yet run, or has not yet been scheduled, it will
     * never run. If the task has been scheduled for repeated execution, it will
     * never run again. (If the task is running when this call occurs, the task
     * will run to completion, but will never run again.)
     * <p/>
     * <p/>
     * Note that calling this method from within the <tt>run</tt> method of a
     * repeating timer task absolutely guarantees that the timer task will not
     * run again.
     * <p/>
     * <p/>
     * This method may be called repeatedly; the second and subsequent calls
     * have no effect.
     *
     * @return true if this task is scheduled for one-time execution and has not
     *         yet run, or this task is scheduled for repeated execution.
     *         Returns false if the task was scheduled for one-time execution
     *         and has already run, or if the task was never scheduled, or if
     *         the task was already cancelled. (Loosely speaking, this method
     *         returns <tt>true</tt> if it prevents one or more scheduled
     *         executions from taking place.)
     */
    public boolean cancel() {
        synchronized (lock) {
            boolean result = (state == SCHEDULED);
            state = CANCELLED;
            return result;
        }
    }

    /**
     * Returns the <i>scheduled</i> execution time of the most recent <i>actual</i>
     * execution of this task. (If this method is invoked while task execution
     * is in progress, the return value is the scheduled execution time of the
     * ongoing task execution.)
     * <p/>
     * <p/>
     * This method is typically invoked from within a task's run method, to
     * determine whether the current execution of the task is sufficiently
     * timely to warrant performing the scheduled activity:
     * <p/>
     * <pre>
     * public void run() {
     * 	if (System.currentTimeMillis() - scheduledExecutionTime() &gt;= MAX_TARDINESS)
     * 		return; // Too late; skip this execution.
     * 	// Perform the task
     * }
     * </pre>
     * <p/>
     * This method is typically <i>not</i> used in conjunction with
     * <i>fixed-delay execution</i> repeating tasks, as their scheduled
     * execution times are allowed to drift over time, and so are not terribly
     * significant.
     *
     * @return the time at which the most recent execution of this task was
     *         scheduled to occur, in the format returned by Date.getTime(). The
     *         return value is undefined if the task has yet to commence its
     *         first execution.
     * @see Date#getTime()
     */
    public long scheduledExecutionTime() {
        synchronized (lock) {
            return (period < 0 ? nextExecutionTime + period : nextExecutionTime
                    - period);
        }
	}
}