The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.
A Swing timer (an instance of
javax.swing.Timer
) fires one or more action events after a specified delay. Do not confuse Swing timers with the general-purpose timer facility in the java.util
package. This page describes only Swing timers.
In general, we recommend using Swing timers rather than general-purpose timers for GUI-related tasks because Swing timers all share the same, pre-existing timer thread and the GUI-related task automatically executes on the event-dispatch thread. However, you might use a general-purpose timer if you don't plan on touching the GUI from the timer, or need to perform lengthy processing.
You can use Swing timers in two ways:
Swing timers are very easy to use. When you create the timer, you specify an action listener to be notified when the timer "goes off". The actionPerformed
method in this listener should contain the code for whatever task you need to be performed. When you create the timer, you also specify the number of milliseconds between timer firings. If you want the timer to go off only once, you can invoke setRepeats(false)
on the timer. To start the timer, call its start
method. To suspend it, call stop
.
Note that the Swing timer's task is performed in the event dispatch thread. This means that the task can safely manipulate components, but it also means that the task should execute quickly. If the task might take a while to execute, then consider using a SwingWorker
instead of or in addition to the timer. See
Concurrency in Swing for instructions about using the SwingWorker
class and information on using Swing components in multi-threaded programs.
Let's look at an example of using a timer to periodically update a component. The
applet uses a timer to update its display at regular intervals. (To see this applet running, go to
How to Make Applets. This applet begins by creating and starting a timer:TumbleItem
timer = new Timer(speed, this); timer.setInitialDelay(pause); timer.start();
The speed
and pause
variables represent applet parameters; as configured on the other page, these are 100 and 1900 respectively, so that the first timer event will occur in approximately 1.9 seconds, and recur every 0.1 seconds. By specifying this
as the second argument to the Timer
constructor, TumbleItem
specifies that it is the action listener for timer events.
After starting the timer, TumbleItem
begins loading a series of images in a background thread. Meanwhile, the timer events begin to occur, causing the actionPerformed
method to execute:
public void actionPerformed(ActionEvent e) { //If still loading, can't animate. if (!worker.isDone()) { return; } loopslot++; if (loopslot >= nimgs) { loopslot = 0; off += offset; if (off < 0) { off = width - maxWidth; } else if (off + maxWidth > width) { off = 0; } } animator.repaint(); if (loopslot == nimgs - 1) { timer.restart(); } }
Until the images are loaded, worker.isDone
returns false
, so timer events are effectively ignored. The first part of the event handling code simply sets values that are employed in the animation control's paintComponent
method: loopslot
(the index of the next graphic in the animation) and off
(the horizontal offset of the next graphic).
Eventually, loopslot
will reach the end of the image array and start over. When this happens, the code at the end of actionPerformed
restarts the timer. Doing this causes a short delay before the animation sequence begins again.