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miércoles, 16 de abril de 2008

javax.swing (Java Platform SE 6)

javax.swing (Java Platform SE 6):

Swing's Threading Policy

In general Swing is not thread safe. All Swing components and related classes, unless otherwise documented, must be accessed on the event dispatching thread.

Typical Swing applications do processing in response to an event generated from a user gesture. For example, clicking on a JButton notifies all ActionListeners added to the JButton. As all events generated from a user gesture are dispatched on the event dispatching thread, most developers are not impacted by the restriction.

Where the impact lies, however, is in constructing and showing a Swing application. Calls to an application's main method, or methods in Applet, are not invoked on the event dispatching thread. As such, care must be taken to transfer control to the event dispatching thread when constructing and showing an application or applet. The preferred way to transfer control and begin working with Swing is to use invokeLater. The invokeLater method schedules a Runnable to be processed on the event dispatching thread. The following two examples work equally well for transferring control and starting up a Swing application:

public class MyApp implements Runnable {
public void run() {
// Invoked on the event dispatching thread.
// Construct and show GUI.
}

public static void main(String[] args) {
SwingUtilities.invokeLater(new MyApp(args));
}
}
Or:
public class MyApp {
MyApp(String[] args) {
// Invoked on the event dispatching thread. Do any initialization
// here.
}

public void show() {
// Show the UI.
}

public static void main(final String[] args) {
// Schedule a job for the event-dispatching thread:
// creating and showing this application's GUI.
SwingUtilities.invokeLater(new Runnable() {
public void run() {
new MyApp(args).show();
}
});
}
}
This restriction also applies to models attached to Swing components. For example, if a TableModel is attached to a JTable, the TableModel should only be modified on the event dispatching thread. If you modify the model on a separate thread you run the risk of exceptions and possible display corruption.

As all events are delivered on the event dispatching thread, care must be taken in event processing. In particular, a long running task, such as network io or computational intensive processing, executed on the event dispatching thread blocks the event dispatching thread from dispatching any other events. While the event dispatching thread is blocked the application is completely unresponsive to user input. Refer to SwingWorker for the preferred way to do such processing when working with Swing.

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