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 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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package java.util;
import java.util.Date;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * 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 {@code Timer} 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 {@code Timer} 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 timer's {@code cancel} method.
 *
 * <p>If the timer's task execution thread terminates unexpectedly, for
 * example, because its {@code stop} method is invoked, any further
 * attempt to schedule a task on the timer will result in an
 * {@code IllegalStateException}, as if the timer's {@code cancel}
 * method had been invoked.
 *
 * <p>This class is thread-safe: multiple threads can share a single
 * {@code Timer} object without the need for external synchronization.
 *
 * <p>This class does <i>not</i> offer real-time guarantees: it schedules
 * tasks using the {@code Object.wait(long)} method.
 *
 * <p>Java 5.0 introduced the {@code java.util.concurrent} package and
 * one of the concurrency utilities therein is the {@link
 * java.util.concurrent.ScheduledThreadPoolExecutor
 * ScheduledThreadPoolExecutor} which is a thread pool for repeatedly
 * executing tasks at a given rate or delay. It is effectively a more
 * versatile replacement for the {@code Timer}/{@code TimerTask}
 * combination, as it allows multiple service threads, accepts various
 * time units, and doesn't require subclassing {@code TimerTask} (just
 * implement {@code Runnable}). Configuring {@code
 * ScheduledThreadPoolExecutor} with one thread makes it equivalent to
 * {@code Timer}.
 *
 * <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.
 *
 * <p>Implementation note: All constructors start a timer thread.
 *
 * @author Josh Bloch
 * @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 final TaskQueue queue = new TaskQueue();

 /**
 * The timer thread.
 */
 private final TimerThread thread = new TimerThread(queue);

 /**
 * 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 final Object threadReaper = new Object() {
 @SuppressWarnings("deprecation")
 protected void finalize() throws Throwable {
 synchronized(queue) {
 thread.newTasksMayBeScheduled = false;
 queue.notify(); // In case queue is empty.
 }
 }
 };

 /**
 * This ID is used to generate thread names.
 */
 private static final AtomicInteger nextSerialNumber = new AtomicInteger(0);
 private static int serialNumber() {
 return nextSerialNumber.getAndIncrement();
 }

 /**
 * Creates a new timer. The associated thread does <i>not</i>
 * {@linkplain Thread#setDaemon run as a daemon}.
 */
 public Timer() {
 this("Timer-" + serialNumber());
 }

 /**
 * Creates a new timer whose associated thread may be specified to
 * {@linkplain Thread#setDaemon run as a daemon}.
 * A daemon 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.
 */
 public Timer(boolean isDaemon) {
 this("Timer-" + serialNumber(), isDaemon);
 }

 /**
 * Creates a new timer whose associated thread has the specified name.
 * The associated thread does <i>not</i>
 * {@linkplain Thread#setDaemon run as a daemon}.
 *
 * @param name the name of the associated thread
 * @throws NullPointerException if {@code name} is null
 * @since 1.5
 */
 public Timer(String name) {
 thread.setName(name);
 thread.start();
 }

 /**
 * Creates a new timer whose associated thread has the specified name,
 * and may be specified to
 * {@linkplain Thread#setDaemon run as a daemon}.
 *
 * @param name the name of the associated thread
 * @param isDaemon true if the associated thread should run as a daemon
 * @throws NullPointerException if {@code name} is null
 * @since 1.5
 */
 public Timer(String name, boolean isDaemon) {
 thread.setName(name);
 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 {@code delay} is negative, or
 * {@code delay + System.currentTimeMillis()} is negative.
 * @throws IllegalStateException if task was already scheduled or
 * cancelled, timer was cancelled, or timer thread terminated.
 * @throws NullPointerException if {@code task} is null
 */
 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 {@code time.getTime()} is negative.
 * @throws IllegalStateException if task was already scheduled or
 * cancelled, timer was cancelled, or timer thread terminated.
 * @throws NullPointerException if {@code task} or {@code time} is null
 */
 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>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 {@code Object.wait(long)} is accurate).
 *
 * <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 {@code delay < 0}, or
 * {@code delay + System.currentTimeMillis() < 0}, or
 * {@code period <= 0}
 * @throws IllegalStateException if task was already scheduled or
 * cancelled, timer was cancelled, or timer thread terminated.
 * @throws NullPointerException if {@code task} is null
 */
 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>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 {@code Object.wait(long)} is accurate). As a
 * consequence of the above, if the scheduled first time is in the past,
 * it is scheduled for immediate execution.
 *
 * <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 {@code firstTime.getTime() < 0}, or
 * {@code period <= 0}
 * @throws IllegalStateException if task was already scheduled or
 * cancelled, timer was cancelled, or timer thread terminated.
 * @throws NullPointerException if {@code task} or {@code firstTime} is null
 */
 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>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 {@code Object.wait(long)} is accurate).
 *
 * <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 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 {@code delay < 0}, or
 * {@code delay + System.currentTimeMillis() < 0}, or
 * {@code period <= 0}
 * @throws IllegalStateException if task was already scheduled or
 * cancelled, timer was cancelled, or timer thread terminated.
 * @throws NullPointerException if {@code task} is null
 */
 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>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 {@code Object.wait(long)} is accurate). As a
 * consequence of the above, if the scheduled first time is in the past,
 * then any "missed" executions will be scheduled for immediate "catch up"
 * execution.
 *
 * <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 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 {@code firstTime.getTime() < 0} or
 * {@code period <= 0}
 * @throws IllegalStateException if task was already scheduled or
 * cancelled, timer was cancelled, or timer thread terminated.
 * @throws NullPointerException if {@code task} or {@code firstTime} is null
 */
 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 specified 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 {@code time} is negative.
 * @throws IllegalStateException if task was already scheduled or
 * cancelled, timer was cancelled, or timer thread terminated.
 * @throws NullPointerException if {@code task} is null
 */
 private void sched(TimerTask task, long time, long period) {
 if (time < 0)
 throw new IllegalArgumentException("Illegal execution time.");

 // Constrain value of period sufficiently to prevent numeric
 // overflow while still being effectively infinitely large.
 if (Math.abs(period) > (Long.MAX_VALUE >> 1))
 period >>= 1;

 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>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>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.
 }
 }

 /**
 * Removes all cancelled tasks from this timer's task queue. <i>Calling
 * this method has no effect on the behavior of the timer</i>, but
 * eliminates the references to the cancelled tasks from the queue.
 * If there are no external references to these tasks, they become
 * eligible for garbage collection.
 *
 * <p>Most programs will have no need to call this method.
 * It is designed for use by the rare application that cancels a large
 * number of tasks. Calling this method trades time for space: the
 * runtime of the method may be proportional to n + c log n, where n
 * is the number of tasks in the queue and c is the number of cancelled
 * tasks.
 *
 * <p>Note that it is permissible to call this method from within
 * a task scheduled on this timer.
 *
 * @return the number of tasks removed from the queue.
 * @since 1.5
 */
 public int purge() {
 int result = 0;

 synchronized(queue) {
 for (int i = queue.size(); i > 0; i--) {
 if (queue.get(i).state == TimerTask.CANCELLED) {
 queue.quickRemove(i);
 result++;
 }
 }

 if (result != 0)
 queue.heapify();
 }

 return result;
 }
}

/**
 * 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 {
 // Someone 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 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.
 */
 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;

 /**
 * Returns the number of tasks currently on the queue.
 */
 int size() {
 return size;
 }

 /**
 * Adds a new task to the priority queue.
 */
 void add(TimerTask task) {
 // Grow backing store if necessary
 if (size + 1 == queue.length)
 queue = Arrays.copyOf(queue, 2*queue.length);

 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];
 }

 /**
 * Return the ith task in the priority queue, where i ranges from 1 (the
 * head task, which is returned by getMin) to the number of tasks on the
 * queue, inclusive.
 */
 TimerTask get(int i) {
 return queue[i];
 }

 /**
 * 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);
 }

 /**
 * Removes the ith element from queue without regard for maintaining
 * the heap invariant. Recall that queue is one-based, so
 * 1 <= i <= size.
 */
 void quickRemove(int i) {
 assert i <= size;

 queue[i] = queue[size];
 queue[size--] = null; // Drop extra ref to prevent memory leak
 }

 /**
 * 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).
 *
 * 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).
 *
 * 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 && j > 0) {
 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;
 }
 }

 /**
 * Establishes the heap invariant (described above) in the entire tree,
 * assuming nothing about the order of the elements prior to the call.
 */
 void heapify() {
 for (int i = size/2; i >= 1; i--)
 fixDown(i);
 }
}