Event dispatching thread: Difference between revisions

Revision as of 15:18, 5 January 2013

Revision as of 15:46, 5 January 2013

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The '''event dispatching thread''' (EDT) is a background [[Thread (computer science)|thread]] used in [[Java (programming language)|Java]] to process events from the [[Abstract Window Toolkit]] (AWT) [[graphical user interface]] [[event queue]]. These events are primarily update events that cause user interface [[Software componentry|components]] to redraw themselves, or input events from [[input device]]s such as the mouse or keyboard. The AWT uses a single-threaded painting [[Model (abstract)|model]] in which all screen updates must be performed from a single thread. The event dispatching thread is the only valid thread to update the visual state of visible user interface components. Updating visible components from other threads is the source of many common [[Software bug|bugs]] in Java [[Computer program|programs]] that use [[Swing (Java)|Swing]].<ref>This problem is not specific to Java [[Swing (Java)|Swing]]. There is the same issue in most [[Widget toolkit]]s, as for example [[Windows Forms]], where the [[BackgroundWorker]] class performs the same purpose as [[SwingWorker]] in Java.</ref> The event dispatching thread is called the '''primordial worker''' in [[Adobe Flash]] and the '''UI thread''' in [[.NET Framework]] and [[Android (operating system)|Android]].

== Swing and thread safety ==

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A popular third-party [[Pluggable look and feel|Look and Feel]] named [http://java.net/projects/substance/ Substance] goes as far as to refuse to instantiate any Swing component when not running within the Event Dispatch Thread,<ref>http://www.pushing-pixels.org/?p=368</ref> to prevent such a coding mistake from occurring.

== Executing code in the EDTevent dispatching thread ==

Other application threads can have code executed in the event dispatching thread by defining the code in a {{Javadoc:SE|java/lang|Runnable}} object and pass it to the {{Javadoc:SE|javax/swing|SwingUtilities}} helper class or to the {{Javadoc:SE|java/awt|EventQueue}}. Two methods of these classes allow:

* synchronous code execution ({{Javadoc:SE|member=invokeAndWait(Runnable)|javax/swing|SwingUtilities|invokeAndWait(java.lang.Runnable)}} or {{Javadoc:SE|member=invokeAndWait(Runnable)|java/awt|EventQueue|invokeAndWait(java.lang.Runnable)}})

* and asynchronous code execution ({{Javadoc:SE|member=invokeLater(Runnable)|javax/swing|SwingUtilities|invokeLater(java.lang.Runnable)}} or {{Javadoc:SE|member=invokeLater(Runnable)|java/awt|EventQueue|invokeLater(java.lang.Runnable)}})

from the EDTevent dispatching thread.

The method <code>invokeAndWait()</code> should never be called from the event dispatching thread&mdash;it will throw an [[Exception handling|exception]]. The method {{Javadoc:SE|javax/swing|SwingUtilities|isEventDispatchThread()}} or {{Javadoc:SE|java/awt|EventQueue|isDispatchThread()}} can be called to determine if the current thread is the event dispatching thread.

== Worker design pattern ==

Another solution for executing code in the EDTevent dispatching thread is using the ''[[SwingWorker|worker design pattern]]''. The <code>[[SwingWorker|javax.swing.SwingWorker]]</code> class, developed by [[Sun Microsystems]], is an implementation of the worker design pattern, and as of Java 6 is part of standard Swing distribution. The open source project [http://foxtrot.sourceforge.net/ Foxtrot] provides another synchronous execution solution similar to <code>SwingWorker</code>.

Equivalents

* <code>System.ComponentModel.BackgroundWorker</code> - [[.NET Framework]]

* <code>flash.system.Worker</code> - [[Adobe Flash]]

* <code>android.os.AsyncTask</code> - [[Android (operating system)|Android]]

== Timer ==

You can also run the code in the event dispatching thread using <code>javax.swing.Timer</code>.

Equivalents

* <code>System.Windows.Forms.Timer</code> - [[.NET Framework]]

* <code>flash.utils.Timer</code> - [[Adobe Flash]]

== See also ==

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* [[Swing (Java)]]

* [[SwingWorker]]

* [[BackgroundWorker]], an equivalent [[.NET Framework]] class for ''SwingWorker''

==References==