timer.java

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     * @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>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.
        }
    }
}

/**
 * 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. 
     */
    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).
     *
     * 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) {
            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;
        }
    }
}