GeezerVison is showing docs.oracle.com — Timer (Java SE 25 & JDK 25)
Class 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.
- API Note:
- Java 5.0 introduced the
java.util.concurrent package and
one of the concurrency utilities therein is the 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 Timer/TimerTask
combination, as it allows multiple service threads, accepts various
time units, and doesn't require subclassing TimerTask (just
implement Runnable). Configuring
ScheduledThreadPoolExecutor with one thread makes it equivalent to
Timer.
- 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:
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Constructor Summary
Constructors
Constructor
Description
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.
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Method Summary
Modifier and Type
Method
Description
void
Terminates this timer, discarding any currently scheduled tasks.
int
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
Schedules the specified task for repeated fixed-rate execution,
beginning after the specified delay.
void
Schedules the specified task for repeated fixed-rate execution,
beginning at the specified time.
Methods declared in class Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
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Constructor Details
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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.
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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 - if name is null- Since:
- 1.5
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Timer
public Timer(String name,
boolean isDaemon)
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 - if name is null- Since:
- 1.5
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Method Details
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schedule
public void schedule(TimerTask task,
long delay)
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 - if delay is negative, or
delay + System.currentTimeMillis() is negative.IllegalStateException - if task was already scheduled or
cancelled, timer was cancelled, or timer thread terminated.NullPointerException - if task is null
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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 - if time.getTime() is negative.IllegalStateException - if task was already scheduled or
cancelled, timer was cancelled, or timer thread terminated.NullPointerException - if task or time is null
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schedule
public void schedule(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.
- 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 - if delay < 0, or
delay + System.currentTimeMillis() < 0, or
period <= 0IllegalStateException - if task was already scheduled or
cancelled, timer was cancelled, or timer thread terminated.NullPointerException - if task is null
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schedule
public void schedule(TimerTask task,
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). 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 - if firstTime.getTime() < 0, or
period <= 0IllegalStateException - if task was already scheduled or
cancelled, timer was cancelled, or timer thread terminated.NullPointerException - if task or firstTime is null
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scheduleAtFixedRate
public void scheduleAtFixedRate(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 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 - if delay < 0, or
delay + System.currentTimeMillis() < 0, or
period <= 0IllegalStateException - if task was already scheduled or
cancelled, timer was cancelled, or timer thread terminated.NullPointerException - if task is null
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scheduleAtFixedRate
public void scheduleAtFixedRate(TimerTask task,
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). 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 - if firstTime.getTime() < 0 or
period <= 0IllegalStateException - if task was already scheduled or
cancelled, timer was cancelled, or timer thread terminated.NullPointerException - if task or firstTime is null
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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
invoke TimerTask.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:
-
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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, where
n is the number of tasks in the queue and c 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:
-
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