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.
- API Note:
- Java 5.0 introduced the
java.util.concurrent
package and one of the concurrency utilities therein is theScheduledThreadPoolExecutor
which is a thread pool for repeatedly executing tasks at a given rate or delay. It is effectively a more versatile replacement for theTimer
/TimerTask
combination, as it allows multiple service threads, accepts various time units, and doesn't require subclassingTimerTask
(just implementRunnable
). ConfiguringScheduledThreadPoolExecutor
with one thread makes it equivalent toTimer
. - 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.
All constructors start a timer thread.
- Since:
- 1.3
- See Also:
-
Constructor Summary
ConstructorDescriptionTimer()
Creates a new timer.Timer
(boolean isDaemon) Creates a new timer whose associated thread may be specified to run as a daemon.Creates a new timer whose associated thread has the specified name.Creates a new timer whose associated thread has the specified name, and may be specified to run as a daemon. -
Method Summary
Modifier and TypeMethodDescriptionvoid
cancel()
Terminates this timer, discarding any currently scheduled tasks.int
purge()
Removes all cancelled tasks from this timer's task queue.void
Schedules the specified task for execution after the specified delay.void
Schedules the specified task for repeated fixed-delay execution, beginning after the specified delay.void
Schedules the specified task for execution at the specified time.void
Schedules the specified task for repeated fixed-delay execution, beginning at the specified time.void
scheduleAtFixedRate
(TimerTask task, long delay, long period) Schedules the specified task for repeated fixed-rate execution, beginning after the specified delay.void
scheduleAtFixedRate
(TimerTask task, Date firstTime, long period) Schedules the specified task for repeated fixed-rate execution, beginning at the specified time.
-
Constructor Details
-
Timer
public Timer()Creates a new timer. The associated thread does not run as a daemon. -
Timer
public Timer(boolean isDaemon) Creates a new timer whose associated thread may be specified to 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.- Parameters:
isDaemon
- true if the associated thread should run as a daemon.
-
Timer
Creates a new timer whose associated thread has the specified name. The associated thread does not run as a daemon.- Parameters:
name
- the name of the associated thread- Throws:
NullPointerException
- ifname
is null- Since:
- 1.5
-
Timer
Creates a new timer whose associated thread has the specified name, and may be specified to run as a daemon.- Parameters:
name
- the name of the associated threadisDaemon
- true if the associated thread should run as a daemon- Throws:
NullPointerException
- ifname
is null- Since:
- 1.5
-
-
Method Details
-
schedule
Schedules the specified task for execution after the specified delay.- Parameters:
task
- task to be scheduled.delay
- delay in milliseconds before task is to be executed.- Throws:
IllegalArgumentException
- ifdelay
is negative, ordelay + System.currentTimeMillis()
is negative.IllegalStateException
- if task was already scheduled or cancelled, timer was cancelled, or timer thread terminated.NullPointerException
- iftask
is null
-
schedule
Schedules the specified task for execution at the specified time. If the time is in the past, the task is scheduled for immediate execution.- Parameters:
task
- task to be scheduled.time
- time at which task is to be executed.- Throws:
IllegalArgumentException
- iftime.getTime()
is negative.IllegalStateException
- if task was already scheduled or cancelled, timer was cancelled, or timer thread terminated.NullPointerException
- iftask
ortime
is null
-
schedule
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.
- Parameters:
task
- task to be scheduled.delay
- delay in milliseconds before task is to be executed.period
- time in milliseconds between successive task executions.- Throws:
IllegalArgumentException
- ifdelay < 0
, ordelay + System.currentTimeMillis() < 0
, orperiod <= 0
IllegalStateException
- if task was already scheduled or cancelled, timer was cancelled, or timer thread terminated.NullPointerException
- iftask
is null
-
schedule
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). As a consequence of the above, if the scheduled first time is in the past, it is scheduled for immediate execution.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.
- Parameters:
task
- task to be scheduled.firstTime
- First time at which task is to be executed.period
- time in milliseconds between successive task executions.- Throws:
IllegalArgumentException
- iffirstTime.getTime() < 0
, orperiod <= 0
IllegalStateException
- if task was already scheduled or cancelled, timer was cancelled, or timer thread terminated.NullPointerException
- iftask
orfirstTime
is null
-
scheduleAtFixedRate
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 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.
- Parameters:
task
- task to be scheduled.delay
- delay in milliseconds before task is to be executed.period
- time in milliseconds between successive task executions.- Throws:
IllegalArgumentException
- ifdelay < 0
, ordelay + System.currentTimeMillis() < 0
, orperiod <= 0
IllegalStateException
- if task was already scheduled or cancelled, timer was cancelled, or timer thread terminated.NullPointerException
- iftask
is null
-
scheduleAtFixedRate
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). 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.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 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.
- Parameters:
task
- task to be scheduled.firstTime
- First time at which task is to be executed.period
- time in milliseconds between successive task executions.- Throws:
IllegalArgumentException
- iffirstTime.getTime() < 0
orperiod <= 0
IllegalStateException
- if task was already scheduled or cancelled, timer was cancelled, or timer thread terminated.NullPointerException
- iftask
orfirstTime
is null
-
cancel
public void cancel()Terminates this timer, discarding any currently scheduled tasks. It should be noted that this method does not cancel the scheduled tasks. For a task to be considered cancelled, the task itself should invokeTimerTask.cancel()
.This method 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.
- See Also:
-
purge
public int purge()Removes all cancelled tasks from this timer's task queue. Calling this method has no effect on the behavior of the timer, 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.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
, wheren
is the number of tasks in the queue andc
is the number of cancelled tasks.Note that it is permissible to call this method from within a task scheduled on this timer.
- Returns:
- the number of tasks removed from the queue.
- Since:
- 1.5
- See Also:
-