Thread-safe problem of the java GUI creation

Question

In the following sample code,

/*
 * Copyright (c) 1995, 2008, Oracle and/or its affiliates. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 *   - Redistributions of source code must retain the above copyright
 *     notice, this list of conditions and the following disclaimer.
 *
 *   - Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution.
 *
 *   - Neither the name of Oracle or the names of its
 *     contributors may be used to endorse or promote products derived
 *     from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
 * IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
 * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR
 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */ 

package components;

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

/* TopLevelDemo.java requires no other files. */
public class TopLevelDemo {
    /**
     * Create the GUI and show it.  For thread safety,
     * this method should be invoked from the
     * event-dispatching thread.
     */
    private static void createAndShowGUI() {
        //Create and set up the window.
        JFrame frame = new JFrame("TopLevelDemo");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        //Create the menu bar.  Make it have a green background.
        JMenuBar greenMenuBar = new JMenuBar();
        greenMenuBar.setOpaque(true);
        greenMenuBar.setBackground(new Color(154, 165, 127));
        greenMenuBar.setPreferredSize(new Dimension(200, 20));

        //Create a yellow label to put in the content pane.
        JLabel yellowLabel = new JLabel();
        yellowLabel.setOpaque(true);
        yellowLabel.setBackground(new Color(248, 213, 131));
        yellowLabel.setPreferredSize(new Dimension(200, 180));

        //Set the menu bar and add the label to the content pane.
        frame.setJMenuBar(greenMenuBar);
        frame.getContentPane().add(yellowLabel, BorderLayout.CENTER);

        //Display the window.
        frame.pack();
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        //Schedule a job for the event-dispatching thread:
        //creating and showing this application's GUI.
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
            }
        });
    }
}

The comment above the function createAndShowGUI(), "For thread safety, this method should be invoked from the event-dispatching thread.".

Could you explain why this method should be invoked from the event-dispatching thread. I have tried to put this method in the public void main method, but it does not matter.

It may not matter 98% of the time, but it will matter that one time when you need to demo something in front of the boss or customers. As noted in the answers, you should take care to make Swing calls on the EDT. — Hovercraft Full Of Eels, Dec 26 '10 at 17:45
By the way, isn't that copyrighted code that you're publishing in this forum? — Hovercraft Full Of Eels, Dec 26 '10 at 17:46
@Hovercraft Full Of Eels: apparently you were able to spot the *Copyright* word... Maybe now you can try to read the paragraph below that *Copyright* word that you were able to spot? — SyntaxT3rr0r, Dec 26 '10 at 17:53

score 5 · Accepted Answer · edited May 23 '17 at 12:04

It may work for you, but that doesn't mean that it will work everywhere. (Almost) all access to Swing classes should be performed from EDT, as documented in Javadocs. This includes initial creation of JFrame and components:

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

...

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.

In the past, this 'initial creation' rule was not there, but some threading problems were found when using this approach. If I find relevant discussion, I will link it here.

Update: I've found this answer on Stack Overflow discussing Swing Single Thread Policy rule and its change to disallow 'not yet realized' components to be accessed from other threads.

Thread-safe problem of the java GUI creation

1 Answers1