timertask.java

开源项目openfire的完整源程序

/* ====================================================================
 * The Apache Software License, Version 1.1
 *
 * Copyright (c) 2000 The Apache Software Foundation.  All rights
 * reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 *
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 *
 * 3. The end-user documentation included with the redistribution,
 *    if any, must include the following acknowledgment:
 *       "This product includes software developed by the
 *        Apache Software Foundation (http://www.apache.org/)."
 *    Alternately, this acknowledgment may appear in the software itself,
 *    if and wherever such third-party acknowledgments normally appear.
 *
 * 4. The names "Apache" and "Apache Software Foundation" must
 *    not be used to endorse or promote products derived from this
 *    software without prior written permission. For written
 *    permission, please contact [e-mail].
 *
 * 5. Products derived from this software may not be called "Apache",
 *    nor may "Apache" appear in their name, without prior written
 *    permission of the Apache Software Foundation.
 *
 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESSED OR IMPLIED
 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED.  IN NO EVENT SHALL THE APACHE SOFTWARE FOUNDATION OR
 * ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
 * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
 * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 * ====================================================================
 *
 * This software consists of voluntary contributions made by many
 * individuals on behalf of the Apache Software Foundation.  For more
 * information on the Apache Software Foundation, please see
 * <http://www.apache.org/>.
 *
 * Portions of this software are based upon public domain software
 * originally written at the National Center for Supercomputing Applications,
 * University of Illinois, Urbana-Champaign.
 */

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);
        }
	}
}