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/*
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
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*/
package java.util;
/**
* A task that can be scheduled for one-time or repeated execution by a Timer.
*
* @author Josh Bloch
* @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 scheduled 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.)
*
* Note that calling this method from within the run method of
* a repeating timer task absolutely guarantees that the timer task will
* not run again.
*
*
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 true 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 scheduled execution time of the most recent
* actual 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.)
*
*
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:
*
* public void run() {
* if (System.currentTimeMillis() - scheduledExecutionTime() >=
* MAX_TARDINESS)
* return; // Too late; skip this execution.
* // Perform the task
* }
*
* This method is typically not used in conjunction with
* fixed-delay execution 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);
}
}
}