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Timer.javaAPI DocphoneME MR2 API (J2ME)25481Wed May 02 18:00:10 BST 2007java.util

Timer

public class Timer extends Object
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.

Corresponding to each 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.

After the last live reference to a Timer object goes away and 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 daemon thread, 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 cancel method.

If the timer's task execution thread terminates unexpectedly, for example, because its stop method is invoked, any further attempt to schedule a task on the timer will result in an IllegalStateException, as if the timer's cancel method had been invoked.

This class is thread-safe: multiple threads can share a single Timer object without the need for external synchronization.

This class does not offer real-time guarantees: it schedules tasks using the Object.wait(long) method.

Timers function only within a single VM and are cancelled when the VM exits. When the VM is started no timers exist, they are created only by application request.

see
TimerTask
see
Object#wait(long)
since
1.3

Fields Summary
private TaskQueue
queue
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 TimerThread
thread
The timer thread.
Constructors Summary
public Timer()
Creates a new timer. The associated thread does not run as a daemon thread, which may prevent an application from terminating.

see
Thread
see
#cancel()


                                   
      
    
Methods Summary
public voidcancel()
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.

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.

This method may be called repeatedly; the second and subsequent calls have no effect.

        synchronized (queue) {
            queue.newTasksMayBeScheduled = false;
            queue.clear();
            queue.notify();  // In case queue was already empty.
        }
    
private voidsched(java.util.TimerTask task, long time, long 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.

param
task task to be scheduled.
param
time time at which task is to be executed.
param
period time in milliseconds between successive task executions.
throws
IllegalArgumentException if time() is negative.
throws
IllegalStateException if task was already scheduled or cancelled, timer was cancelled, or timer thread terminated.

        if (time < 0)
            throw new IllegalArgumentException("Illegal execution time.");
        
        synchronized (queue) {
            if (!queue.newTasksMayBeScheduled) {
                throw new IllegalStateException("Timer already cancelled.");
            }

	    /*
	     * If the TimerThread has exited without an error
	     * it is restarted. See the commentary in TimerThread.run.
	     */
	    if (thread == null || !thread.isAlive()) {
		thread = new TimerThread(queue);
		thread.start();
	    }

            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();
        }
    
public voidschedule(java.util.TimerTask task, long delay)
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 delay is negative, or delay + System.currentTimeMillis() is negative.
throws
IllegalStateException if task was already scheduled or cancelled, or timer was cancelled.

        if (delay < 0)
            throw new IllegalArgumentException("Negative delay.");
        sched(task, System.currentTimeMillis()+delay, 0);
    
public voidschedule(java.util.TimerTask task, java.util.Date time)
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 time.getTime() is negative.
throws
IllegalStateException if task was already scheduled or cancelled, timer was cancelled, or timer thread terminated.

        sched(task, time.getTime(), 0);
    
public voidschedule(java.util.TimerTask task, long delay, long period)
Schedules the specified task for repeated fixed-delay execution, beginning after the specified delay. Subsequent executions take place at approximately regular intervals separated by the specified period.

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 Object.wait(long) is accurate).

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 delay is negative, or delay + System.currentTimeMillis() is negative.
throws
IllegalStateException if task was already scheduled or cancelled, timer was cancelled, or timer thread terminated.

        if (delay < 0)
            throw new IllegalArgumentException("Negative delay.");
        if (period <= 0)
            throw new IllegalArgumentException("Non-positive period.");
        sched(task, System.currentTimeMillis()+delay, -period);
    
public voidschedule(java.util.TimerTask task, java.util.Date firstTime, long period)
Schedules the specified task for repeated fixed-delay execution, beginning at the specified time. Subsequent executions take place at approximately regular intervals, separated by the specified period.

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 Object.wait(long) is accurate).

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 time.getTime() is negative.
throws
IllegalStateException if task was already scheduled or cancelled, timer was cancelled, or timer thread terminated.

        if (period <= 0)
            throw new IllegalArgumentException("Non-positive period.");
        sched(task, firstTime.getTime(), -period);
    
public voidscheduleAtFixedRate(java.util.TimerTask task, long delay, long period)
Schedules the specified task for repeated fixed-rate execution, beginning after the specified delay. Subsequent executions take place at approximately regular intervals, separated by the specified period.

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 Object.wait(long) is accurate).

Fixed-rate execution is appropriate for recurring activities that are sensitive to absolute 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 delay is negative, or delay + System.currentTimeMillis() is negative.
throws
IllegalStateException if task was already scheduled or cancelled, timer was cancelled, or timer thread terminated.

        if (delay < 0)
            throw new IllegalArgumentException("Negative delay.");
        if (period <= 0)
            throw new IllegalArgumentException("Non-positive period.");
        sched(task, System.currentTimeMillis()+delay, period);
    
public voidscheduleAtFixedRate(java.util.TimerTask task, java.util.Date firstTime, long period)
Schedules the specified task for repeated fixed-rate execution, beginning at the specified time. Subsequent executions take place at approximately regular intervals, separated by the specified period.

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 Object.wait(long) is accurate).

Fixed-rate execution is appropriate for recurring activities that are sensitive to absolute 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 time.getTime() is negative.
throws
IllegalStateException if task was already scheduled or cancelled, timer was cancelled, or timer thread terminated.

        if (period <= 0)
            throw new IllegalArgumentException("Non-positive period.");
        sched(task, firstTime.getTime(), period);