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.

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.

Implementation note: All constructors start a timer thread.

Public Constructor Summary

Timer()
Creates a new timer.
Timer(boolean isDaemon)
Creates a new timer whose associated thread may be specified to {@linkplain Thread#setDaemon run as a daemon}.
Timer(String name)
Creates a new timer whose associated thread has the specified name.
Timer(String name, boolean isDaemon)
Creates a new timer whose associated thread has the specified name, and may be specified to {@linkplain Thread#setDaemon run as a daemon}.

Public Method Summary

void
cancel()
Terminates this timer, discarding any currently scheduled tasks.
int
purge()
Removes all cancelled tasks from this timer's task queue.
void
schedule(TimerTask task, long delay, long period)
Schedules the specified task for repeated fixed-delay execution, beginning after the specified delay.
void
schedule(TimerTask task, Date time)
Schedules the specified task for execution at the specified time.
void
schedule(TimerTask task, Date firstTime, long period)
Schedules the specified task for repeated fixed-delay execution, beginning at the specified time.
void
schedule(TimerTask task, long delay)
Schedules the specified task for execution after the specified delay.
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.

Inherited Method Summary

Public Constructors

public Timer ()

Creates a new timer. The associated thread does not {@linkplain Thread#setDaemon run as a daemon}.

public Timer (boolean isDaemon)

Creates a new timer whose associated thread may be specified to {@linkplain Thread#setDaemon 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.

public Timer (String name)

Creates a new timer whose associated thread has the specified name. The associated thread does not {@linkplain Thread#setDaemon run as a daemon}.

Parameters
name the name of the associated thread
Throws
NullPointerException if name is null

public Timer (String name, boolean isDaemon)

Creates a new timer whose associated thread has the specified name, and may be specified to {@linkplain Thread#setDaemon run as a daemon}.

Parameters
name the name of the associated thread
isDaemon true if the associated thread should run as a daemon
Throws
NullPointerException if name is null

Public Methods

public void cancel ()

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.

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 a task scheduled on this timer.

Returns
  • the number of tasks removed from the queue.

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 f