/**
* $RCSfile: ,v $
* $Revision: $
* $Date: $
*
* Copyright (C) 2004-2011 Jive Software. All rights reserved.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package net.java.sipmack.common.scheduler;
import java.util.Date;
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.
*/
//TODO REMOVE
@SuppressWarnings("unused")
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/>
* 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 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 specifed 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.
*
* @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.");
synchronized (task.lock) {
if (task.state != TimerTask.VIRGIN)
throw new IllegalStateException(
"Task already scheduled or cancelled");
task.nextExecutionTime = time;
task.period = period;
task.state = TimerTask.SCHEDULED;
}
queue.add(task);
if (queue.getMin() == task)
queue.notify();
}
}
/**
* Terminates this timer, discarding any currently scheduled tasks. Does not
* interfere with a currently executing task (if it exists). Once a timer
* has been terminated, its execution thread terminates gracefully, and no
* more tasks may be scheduled on it.
* <p/>
* <p/>
* Note that calling this method from within the run method of a timer task
* that was invoked by this timer absolutely guarantees that the ongoing
* task execution is the last task execution that will ever be performed by
* this timer.
* <p/>
* <p/>
* This method may be called repeatedly; the second and subsequent calls
* have no effect.
*/
public void cancel() {
synchronized (queue) {
thread.newTasksMayBeScheduled = false;
queue.clear();
queue.notify(); // In case queue was already empty.
}
}
}
/**
* This "helper class" implements the timer's task execution thread, which waits
* for tasks on the timer queue, executions them when they fire, reschedules
* repeating tasks, and removes cancelled tasks and spent non-repeating tasks
* from the queue.
*/
class TimerThread extends Thread {
/**
* This flag is set to false by the reaper to inform us that there are no
* more live references to our Timer object. Once this flag is true and
* there are no more tasks in our queue, there is no work left for us to do,
* so we terminate gracefully. Note that this field is protected by queue's
* monitor!
*/
boolean newTasksMayBeScheduled = true;
/**
* Our Timer's queue. We store this reference in preference to a reference
* to the Timer so the reference graph remains acyclic. Otherwise, the Timer
* would never be garbage-collected and this thread would never go away.
*/
private TaskQueue queue;
TimerThread(TaskQueue queue) {
this.queue = queue;
}
public void run() {
try {
mainLoop();
}
finally {
// Somone killed this Thread, behave as if Timer cancelled
synchronized (queue) {
newTasksMayBeScheduled = false;
queue.clear(); // Eliminate obsolete references
}
}
}
/**
* The main timer loop. (See class comment.)
*/
private void mainLoop() {
while (true) {
try {
TimerTask task;
boolean taskFired;
synchronized (queue) {
// Wait for queue to become non-empty
while (queue.isEmpty() && newTasksMayBeScheduled) queue.wait();
if (queue.isEmpty())
break; // Queue is empty and will forever remain; die
// Queue nonempty; look at first evt and do the right thing
long currentTime, executionTime;
task = queue.getMin();
synchronized (task.lock) {
if (task.state == TimerTask.CANCELLED) {
queue.removeMin();
continue; // No action required, poll queue again
}
currentTime = System.currentTimeMillis();
executionTime = task.nextExecutionTime;
if (taskFired = (executionTime <= currentTime)) {
if (task.period == 0) { // Non-repeating, remove
queue.removeMin();
task.state = TimerTask.EXECUTED;
}
else { // Repeating task, reschedule
queue
.rescheduleMin(task.period < 0 ? currentTime
- task.period
: executionTime + task.period);
}
}
}
if (!taskFired) // Task hasn't yet fired; wait
queue.wait(executionTime - currentTime);
}
if (taskFired) // Task fired; run it, holding no locks
task.run();
}
catch (InterruptedException e) {
}
}
}
}
/**
* This class represents a timer task queue: a priority queue of TimerTasks,
* ordered on nextExecutionTime. Each Timer object has one of these, which it
* shares with its TimerThread. Internally this class uses a heap, which offers
* log(n) performance for the add, removeMin and rescheduleMin operations, and
* constant time performance for the the getMin operation.
*/
class TaskQueue {
/**
* Priority queue represented as a balanced binary heap: the two children of
* queue[n] are queue[2*n] and queue[2*n+1]. The priority queue is ordered
* on the nextExecutionTime field: The TimerTask with the lowest
* nextExecutionTime is in queue[1] (assuming the queue is nonempty). For
* each node n in the heap, and each descendant of n, d, n.nextExecutionTime <=
* d.nextExecutionTime.
*
* @uml.property name="queue"
* @uml.associationEnd multiplicity="(0 -1)"
*/
private TimerTask[] queue = new TimerTask[128];
/**
* The number of tasks in the priority queue. (The tasks are stored in
* queue[1] up to queue[size]).
*/
private int size = 0;
/**
* Adds a new task to the priority queue.
*/
void add(TimerTask task) {
// Grow backing store if necessary
if (++size == queue.length) {
TimerTask[] newQueue = new TimerTask[2 * queue.length];
System.arraycopy(queue, 0, newQueue, 0, size);
queue = newQueue;
}
queue[size] = task;
fixUp(size);
}
/**
* Return the "head task" of the priority queue. (The head task is an task
* with the lowest nextExecutionTime.)
*/
TimerTask getMin() {
return queue[1];
}
/**
* Remove the head task from the priority queue.
*/
void removeMin() {
queue[1] = queue[size];
queue[size--] = null; // Drop extra reference to prevent memory leak
fixDown(1);
}
/**
* Sets the nextExecutionTime associated with the head task to the specified
* value, and adjusts priority queue accordingly.
*/
void rescheduleMin(long newTime) {
queue[1].nextExecutionTime = newTime;
fixDown(1);
}
/**
* Returns true if the priority queue contains no elements.
*/
boolean isEmpty() {
return size == 0;
}
/**
* Removes all elements from the priority queue.
*/
void clear() {
// Null out task references to prevent memory leak
for (int i = 1; i <= size; i++)
queue[i] = null;
size = 0;
}
/**
* Establishes the heap invariant (described above) assuming the heap
* satisfies the invariant except possibly for the leaf-node indexed by k
* (which may have a nextExecutionTime less than its parent's).
* <p/>
* This method functions by "promoting" queue[k] up the hierarchy (by
* swapping it with its parent) repeatedly until queue[k]'s
* nextExecutionTime is greater than or equal to that of its parent.
*/
private void fixUp(int k) {
while (k > 1) {
int j = k >> 1;
if (queue[j].nextExecutionTime <= queue[k].nextExecutionTime)
break;
TimerTask tmp = queue[j];
queue[j] = queue[k];
queue[k] = tmp;
k = j;
}
}
/**
* Establishes the heap invariant (described above) in the subtree rooted at
* k, which is assumed to satisfy the heap invariant except possibly for
* node k itself (which may have a nextExecutionTime greater than its
* children's).
* <p/>
* This method functions by "demoting" queue[k] down the hierarchy (by
* swapping it with its smaller child) repeatedly until queue[k]'s
* nextExecutionTime is less than or equal to those of its children.
*/
private void fixDown(int k) {
int j;
while ((j = k << 1) <= size) {
if (j < size
&& queue[j].nextExecutionTime > queue[j + 1].nextExecutionTime)
j++; // j indexes smallest kid
if (queue[k].nextExecutionTime <= queue[j].nextExecutionTime)
break;
TimerTask tmp = queue[j];
queue[j] = queue[k];
queue[k] = tmp;
k = j;
}
}
int findTask(TimerTask task) {
for (int i = 1; i <= size; i++)
if (queue[i] == task)
return i;
return -1;
}
void reschedule(TimerTask task, long time) {
int i = findTask(task);
if (i == -1)
return;
long oldTime = queue[i].nextExecutionTime;
queue[i].nextExecutionTime = time;
if (oldTime < time)
fixDown(i);
else
fixUp(i);
}
}