* The normal scheduling will result in a more or less even delay in time * between successive executions, but the executions could drift in time if * the task (or other tasks) takes a long time to execute. Fixed delay * scheduling guarantees more or less that the task will be executed at a * specific time, but if there is ever a delay in execution then the period * between successive executions will be shorter. The first method of * repeated scheduling is prefered for repeated tasks in response to user * interaction, the second method of repeated scheduling is prefered for tasks * that act like alarms. *
* The Timer keeps a binary heap as a task priority queue which means that * scheduling and serving of a task in a queue of n tasks costs O(log n). * * @see TimerTask * @since 1.3 * @author Mark Wielaard (mark@klomp.org) */ public class Timer { /** * Priority Task Queue. * TimerTasks are kept in a binary heap. * The scheduler calls sleep() on the queue when it has nothing to do or * has to wait. A sleeping scheduler can be notified by calling interrupt() * which is automatically called by the enqueue(), cancel() and * timerFinalized() methods. */ private static final class TaskQueue { /** Default size of this queue */ private final int DEFAULT_SIZE = 32; /** Wheter to return null when there is nothing in the queue */ private boolean nullOnEmpty; /** * The heap containing all the scheduled TimerTasks * sorted by the TimerTask.scheduled field. * Null when the stop() method has been called. */ private TimerTask heap[]; /** * The actual number of elements in the heap * Can be less then heap.length. * Note that heap[0] is used as a sentinel. */ private int elements; /** * Creates a TaskQueue of default size without any elements in it. */ public TaskQueue() { heap = new TimerTask[DEFAULT_SIZE]; elements = 0; nullOnEmpty = false; } /** * Adds a TimerTask at the end of the heap. * Grows the heap if necessary by doubling the heap in size. */ private void add(TimerTask task) { elements++; if (elements == heap.length) { TimerTask new_heap[] = new TimerTask[heap.length * 2]; System.arraycopy(heap, 0, new_heap, 0, heap.length); heap = new_heap; } heap[elements] = task; } /** * Removes the last element from the heap. * Shrinks the heap in half if * elements+DEFAULT_SIZE/2 <= heap.length/4. */ private void remove() { // clear the entry first heap[elements] = null; elements--; if (elements + DEFAULT_SIZE / 2 <= (heap.length / 4)) { TimerTask new_heap[] = new TimerTask[heap.length / 2]; System.arraycopy(heap, 0, new_heap, 0, elements + 1); heap = new_heap; } } /** * Adds a task to the queue and puts it at the correct place * in the heap. */ public synchronized void enqueue(TimerTask task) { // Check if it is legal to add another element if (heap == null) { throw new IllegalStateException ("cannot enqueue when stop() has been called on queue"); } heap[0] = task; // sentinel add(task); // put the new task at the end // Now push the task up in the heap until it has reached its place int child = elements; int parent = child / 2; while (heap[parent].scheduled > task.scheduled) { heap[child] = heap[parent]; child = parent; parent = child / 2; } // This is the correct place for the new task heap[child] = task; heap[0] = null; // clear sentinel // Maybe sched() is waiting for a new element this.notify(); } /** * Returns the top element of the queue. * Can return null when no task is in the queue. */ private TimerTask top() { if (elements == 0) { return null; } else { return heap[1]; } } /** * Returns the top task in the Queue. * Removes the element from the heap and reorders the heap first. * Can return null when there is nothing in the queue. */ public synchronized TimerTask serve() { // The task to return TimerTask task = null; while (task == null) { // Get the next task task = top(); // return null when asked to stop // or if asked to return null when the queue is empty if ((heap == null) || (task == null && nullOnEmpty)) { return null; } // Do we have a task? if (task != null) { // The time to wait until the task should be served long time = task.scheduled - System.currentTimeMillis(); if (time > 0) { // This task should not yet be served // So wait until this task is ready // or something else happens to the queue task = null; // set to null to make sure we call top() try { this.wait(time); } catch (InterruptedException _) { } } } else { // wait until a task is added // or something else happens to the queue try { this.wait(); } catch (InterruptedException _) { } } } // reconstruct the heap TimerTask lastTask = heap[elements]; remove(); // drop lastTask at the beginning and move it down the heap int parent = 1; int child = 2; heap[1] = lastTask; while (child <= elements) { if (child < elements) { if (heap[child].scheduled > heap[child + 1].scheduled) { child++; } } if (lastTask.scheduled <= heap[child].scheduled) break; // found the correct place (the parent) - done heap[parent] = heap[child]; parent = child; child = parent * 2; } // this is the correct new place for the lastTask heap[parent] = lastTask; // return the task return task; } /** * When nullOnEmpty is true the serve() method will return null when * there are no tasks in the queue, otherwise it will wait until * a new element is added to the queue. It is used to indicate to * the scheduler that no new tasks will ever be added to the queue. */ public synchronized void setNullOnEmpty(boolean nullOnEmpty) { this.nullOnEmpty = nullOnEmpty; this.notify(); } /** * When this method is called the current and all future calls to * serve() will return null. It is used to indicate to the Scheduler * that it should stop executing since no more tasks will come. */ public synchronized void stop() { this.heap = null; this.notify(); } } // TaskQueue /** * The scheduler that executes all the tasks on a particular TaskQueue, * reschedules any repeating tasks and that waits when no task has to be * executed immediatly. Stops running when canceled or when the parent * Timer has been finalized and no more tasks have to be executed. */ private static final class Scheduler implements Runnable { // The priority queue containing all the TimerTasks. private TaskQueue queue; /** * Creates a new Scheduler that will schedule the tasks on the * given TaskQueue. */ public Scheduler(TaskQueue queue) { this.queue = queue; } public void run() { TimerTask task; while ((task = queue.serve()) != null) { // If this task has not been canceled if (task.scheduled >= 0) { // Mark execution time task.lastExecutionTime = task.scheduled; // Repeatable task? if (task.period < 0) { // Last time this task is executed task.scheduled = -1; } // Run the task try { task.run(); } catch (Throwable t) { /* ignore all errors */ } } // Calculate next time and possibly re-enqueue if (task.scheduled >= 0) { if (task.fixed) { task.scheduled += task.period; } else { task.scheduled = task.period + System.currentTimeMillis(); } queue.enqueue(task); } } } } // Scheduler // Number of Timers created. // Used for creating nice Thread names. private static int nr = 0; // The queue that all the tasks are put in. // Given to the scheduler private TaskQueue queue; // The Scheduler that does all the real work private Scheduler scheduler; // Used to run the scheduler. // Also used to checked if the Thread is still running by calling // thread.isAlive(). Sometimes a Thread is suddenly killed by the system // (if it belonged to an Applet). private Thread thread; // When cancelled we don't accept any more TimerTasks. private boolean canceled; /** * Creates a new Timer with a non deamon Thread as Scheduler, with normal * priority and a default name. */ public Timer() { this(false); } /** * Creates a new Timer with a deamon Thread as scheduler if deamon is true, * with normal priority and a default name. */ public Timer(boolean daemon) { this(daemon, Thread.NORM_PRIORITY); } /** * Creates a new Timer with a deamon Thread as scheduler if deamon is true, * with the priority given and a default name. */ private Timer(boolean daemon, int priority) { this(daemon, priority, "Timer-" + (++nr)); } /** * Creates a new Timer with a deamon Thread as scheduler if deamon is true, * with the priority and name given.E */ private Timer(boolean daemon, int priority, String name) { canceled = false; queue = new TaskQueue(); scheduler = new Scheduler(queue); thread = new Thread(scheduler, name); thread.setDaemon(daemon); thread.setPriority(priority); thread.start(); } /** * Cancels the execution of the scheduler. If a task is executing it will * normally finish execution, but no other tasks will be executed and no * more tasks can be scheduled. */ public void cancel() { canceled = true; queue.stop(); } /** * Schedules the task at Time time, repeating every period * milliseconds if period is positive and at a fixed rate if fixed is true. * * @exception IllegalArgumentException if time is negative * @exception IllegalStateException if the task was already scheduled or * canceled or this Timer is canceled or the scheduler thread has died */ private void schedule(TimerTask task, long time, long period, boolean fixed) { if (time < 0) throw new IllegalArgumentException("negative time"); if (task.scheduled == 0 && task.lastExecutionTime == -1) { task.scheduled = time; task.period = period; task.fixed = fixed; } else { throw new IllegalStateException ("task was already scheduled or canceled"); } if (!this.canceled && this.thread != null) { queue.enqueue(task); } else { throw new IllegalStateException ("timer was canceled or scheduler thread has died"); } } private static void positiveDelay(long delay) { if (delay < 0) { throw new IllegalArgumentException("delay is negative"); } } private static void positivePeriod(long period) { if (period < 0) { throw new IllegalArgumentException("period is negative"); } } /** * Schedules the task at the specified data for one time execution. * * @exception IllegalArgumentException if date.getTime() is negative * @exception IllegalStateException if the task was already scheduled or * canceled or this Timer is canceled or the scheduler thread has died */ public void schedule(TimerTask task, Date date) { long time = date.getTime(); schedule(task, time, -1, false); } /** * Schedules the task at the specified date and reschedules the task every * period milliseconds after the last execution of the task finishes until * this timer or the task is canceled. * * @exception IllegalArgumentException if period or date.getTime() is * negative * @exception IllegalStateException if the task was already scheduled or * canceled or this Timer is canceled or the scheduler thread has died */ public void schedule(TimerTask task, Date date, long period) { positivePeriod(period); long time = date.getTime(); schedule(task, time, period, false); } /** * Schedules the task after the specified delay milliseconds for one time * execution. * * @exception IllegalArgumentException if delay or * System.currentTimeMillis + delay is negative * @exception IllegalStateException if the task was already scheduled or * canceled or this Timer is canceled or the scheduler thread has died */ public void schedule(TimerTask task, long delay) { positiveDelay(delay); long time = System.currentTimeMillis() + delay; schedule(task, time, -1, false); } /** * Schedules the task after the delay milliseconds and reschedules the * task every period milliseconds after the last execution of the task * finishes until this timer or the task is canceled. * * @exception IllegalArgumentException if delay or period is negative * @exception IllegalStateException if the task was already scheduled or * canceled or this Timer is canceled or the scheduler thread has died */ public void schedule(TimerTask task, long delay, long period) { positiveDelay(delay); positivePeriod(period); long time = System.currentTimeMillis() + delay; schedule(task, time, period, false); } /** * Schedules the task at the specified date and reschedules the task at a * fixed rate every period milliseconds until this timer or the task is * canceled. * * @exception IllegalArgumentException if period or date.getTime() is * negative * @exception IllegalStateException if the task was already scheduled or * canceled or this Timer is canceled or the scheduler thread has died */ public void scheduleAtFixedRate(TimerTask task, Date date, long period) { positivePeriod(period); long time = date.getTime(); schedule(task, time, period, true); } /** * Schedules the task after the delay milliseconds and reschedules the task * at a fixed rate every period milliseconds until this timer or the task * is canceled. * * @exception IllegalArgumentException if delay or * System.currentTimeMillis + delay is negative * @exception IllegalStateException if the task was already scheduled or * canceled or this Timer is canceled or the scheduler thread has died */ public void scheduleAtFixedRate(TimerTask task, long delay, long period) { positiveDelay(delay); positivePeriod(period); long time = System.currentTimeMillis() + delay; schedule(task, time, period, true); } /** * Tells the scheduler that the Timer task died * so there will be no more new tasks scheduled. */ protected void finalize() { queue.setNullOnEmpty(true); } }