Top Description Inners Fields Constructors Methods
java.util

public Class Timer

Additional top-level classes in compilation unit: TimerThread, TaskQueue.

extends Object
Class Inheritance
Imports
java.util.Date, java.util.concurrent.atomic.AtomicInteger, java.lang.ref.Cleaner.Cleanable, jdk.internal.ref.CleanerFactory
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.

Author
Josh Bloch
Since
1.3
See Also
TimerTask, Object#wait(long)

Nested and Inner Type Summary

Modifier and TypeClass and Description
private static class
Timer.ThreadReaper

An object of this class is registered with a Cleaner as the cleanup handler for this Timer object.

Field Summary

Modifier and TypeField and Description
private final Cleaner.Cleanable
cleanup

private static final AtomicInteger
nextSerialNumber

This ID is used to generate thread names.

private final TaskQueue
queue

The timer task queue.

private final TimerThread
thread

The timer thread.

Constructor Summary

AccessConstructor and Description
public
Timer()

Creates a new timer.

public
Timer(boolean
true if the associated thread should run as a daemon.
isDaemon)

Creates a new timer whose associated thread may be specified to run as a daemon.

public
Timer(String
the name of the associated thread
name)

Creates a new timer whose associated thread has the specified name.

public
Timer(String
the name of the associated thread
name, boolean
true if the associated thread should run as a daemon
isDaemon)

Creates a new timer whose associated thread has the specified name, and may be specified to run as a daemon.

Method Summary

Modifier and TypeMethod and Description
public void
cancel()

Terminates this timer, discarding any currently scheduled tasks.

public int

Returns:

the number of tasks removed from the queue.
purge()

Removes all cancelled tasks from this timer's task queue.

private void
sched(TimerTask task, long time, long period)

Schedule the specified timer task for execution at the specified time with the specified period, in milliseconds.

public void
schedule(TimerTask
task to be scheduled.
task, long
delay in milliseconds before task is to be executed.
delay)

Schedules the specified task for execution after the specified delay.

public void
schedule(TimerTask
task to be scheduled.
task, Date
time at which task is to be executed.
time)

Schedules the specified task for execution at the specified time.

public void
schedule(TimerTask
task to be scheduled.
task, long
delay in milliseconds before task is to be executed.
delay, long
time in milliseconds between successive task executions.
period)

Schedules the specified task for repeated fixed-delay execution, beginning after the specified delay.

public void
schedule(TimerTask
task to be scheduled.
task, Date
First time at which task is to be executed.
firstTime, long
time in milliseconds between successive task executions.
period)

Schedules the specified task for repeated fixed-delay execution, beginning at the specified time.

public void
scheduleAtFixedRate(TimerTask
task to be scheduled.
task, long
delay in milliseconds before task is to be executed.
delay, long
time in milliseconds between successive task executions.
period)

Schedules the specified task for repeated fixed-rate execution, beginning after the specified delay.

public void
scheduleAtFixedRate(TimerTask
task to be scheduled.
task, Date
First time at which task is to be executed.
firstTime, long
time in milliseconds between successive task executions.
period)

Schedules the specified task for repeated fixed-rate execution, beginning at the specified time.

private static int
serialNumber()

Inherited from java.lang.Object:
cloneequalsfinalizegetClasshashCodenotifynotifyAlltoStringwaitwaitwait

Field Detail

cleanupback to summary
private final Cleaner.Cleanable cleanup
nextSerialNumberback to summary
private static final AtomicInteger nextSerialNumber

This ID is used to generate thread names.

queueback to summary
private final 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.

threadback to summary
private final TimerThread thread

The timer thread.

Constructor Detail

Timerback to summary
public Timer()

Creates a new timer. The associated thread does not run as a daemon.

Timerback to summary
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:boolean

true if the associated thread should run as a daemon.

Timerback to summary
public Timer(String name)

Creates a new timer whose associated thread has the specified name. The associated thread does not run as a daemon.

Parameters
name:String

the name of the associated thread

Exceptions
NullPointerException:
if name is null
Since
1.5
Timerback to summary
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:String

the name of the associated thread

isDaemon:boolean

true if the associated thread should run as a daemon

Exceptions
NullPointerException:
if name is null
Since
1.5

Method Detail

cancelback to summary
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
TimerTask#cancel()
purgeback to summary
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:int

the number of tasks removed from the queue.

Since
1.5
See Also
cancel(), TimerTask#cancel()
schedback to summary
private void sched(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.

Exceptions
IllegalArgumentException:
if time is negative.
IllegalStateException:
if task was already scheduled or cancelled, timer was cancelled, or timer thread terminated.
NullPointerException:
if task is null
scheduleback to summary
public void schedule(TimerTask task, long delay)

Schedules the specified task for execution after the specified delay.

Parameters
task:TimerTask

task to be scheduled.

delay:long

delay in milliseconds before task is to be executed.

Exceptions
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
scheduleback to summary
public void schedule(TimerTask task, 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.

Parameters
task:TimerTask

task to be scheduled.

time:Date

time at which task is to be executed.

Exceptions
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
scheduleback to summary
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:TimerTask

task to be scheduled.

delay:long

delay in milliseconds before task is to be executed.

period:long

time in milliseconds between successive task executions.

Exceptions
IllegalArgumentException:
if delay < 0, or delay + System.currentTimeMillis() < 0, or period <= 0
IllegalStateException:
if task was already scheduled or cancelled, timer was cancelled, or timer thread terminated.
NullPointerException:
if task is null
scheduleback to summary
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:TimerTask

task to be scheduled.

firstTime:Date

First time at which task is to be executed.

period:long

time in milliseconds between successive task executions.

Exceptions
IllegalArgumentException:
if firstTime.getTime() < 0, or period <= 0
IllegalStateException:
if task was already scheduled or cancelled, timer was cancelled, or timer thread terminated.
NullPointerException:
if task or firstTime is null
scheduleAtFixedRateback to summary
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:TimerTask

task to be scheduled.

delay:long

delay in milliseconds before task is to be executed.

period:long

time in milliseconds between successive task executions.

Exceptions
IllegalArgumentException:
if delay < 0, or delay + System.currentTimeMillis() < 0, or period <= 0
IllegalStateException:
if task was already scheduled or cancelled, timer was cancelled, or timer thread terminated.
NullPointerException:
if task is null
scheduleAtFixedRateback to summary
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:TimerTask

task to be scheduled.

firstTime:Date

First time at which task is to be executed.

period:long

time in milliseconds between successive task executions.

Exceptions
IllegalArgumentException:
if firstTime.getTime() < 0 or period <= 0
IllegalStateException:
if task was already scheduled or cancelled, timer was cancelled, or timer thread terminated.
NullPointerException:
if task or firstTime is null
serialNumberback to summary
private static int serialNumber()
java.util back to summary

private Class Timer.ThreadReaper

extends Object
implements Runnable
Class Inheritance
All Implemented Interfaces
java.lang.Runnable
An object of this class is registered with a Cleaner as the cleanup handler for this Timer object. This causes the execution thread to exit gracefully when there are no live references to the Timer object and no tasks in the timer queue.

Field Summary

Modifier and TypeField and Description
private final TaskQueue
queue

private final TimerThread
thread

Constructor Summary

AccessConstructor and Description
pack-priv
ThreadReaper(TaskQueue queue, TimerThread thread)

Method Summary

Modifier and TypeMethod and Description
public void
run()

Implements java.lang.Runnable.run.

Runs this operation.

Inherited from java.lang.Object:
cloneequalsfinalizegetClasshashCodenotifynotifyAlltoStringwaitwaitwait

Field Detail

queueback to summary
private final TaskQueue queue
threadback to summary
private final TimerThread thread

Constructor Detail

ThreadReaperback to summary
pack-priv ThreadReaper(TaskQueue queue, TimerThread thread)

Method Detail

runback to summary
public void run()

Implements java.lang.Runnable.run.

Doc from java.lang.Runnable.run.

Runs this operation.