public class SimpleTimer extends Object
Timer
to work on top of the InternalClock
interface.
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.
TimerTask
,
Object.wait(long)
Constructor and Description |
---|
SimpleTimer(InternalClock c,
String name,
boolean isDaemon)
Creates a new timer whose associated thread has the specified name,
and may be specified to
run as a daemon.
|
Modifier and Type | Method and Description |
---|---|
void |
cancel()
Terminates this timer, discarding any currently scheduled tasks.
|
int |
purge()
Removes all cancelled tasks from this timer's task queue.
|
void |
schedule(SimpleTask task,
Date time)
Schedules the specified task for execution at the specified time.
|
public SimpleTimer(InternalClock c, String name, boolean isDaemon)
name
- the name of the associated threadisDaemon
- true if the associated thread should run as a daemonNullPointerException
- if name
is nullpublic void schedule(SimpleTask task, Date time)
task
- task to be scheduled.time
- time at which task is to be executed.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 nullpublic void cancel()
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()
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.
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