java GUI创建中的线程安全问题

Question

在下面的sample code中，

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 *     from this software without specific prior written permission.
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 * 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,
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 * 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();
            }
        });
    }
}

函数createAndShowGUI()上面的注释“为了线程安全，应该从事件分派线程调用此方法。”

你能解释一下为什么要从事件调度线程调用这个方法吗？我曾尝试将此方法放入public void main方法中，但这无关紧要。

Answer 1

它可能对你有效，但这并不意味着它在所有地方都有效。(几乎)所有对Swing类的访问都应该从EDT执行，如Javadocs中所述。这包括JFrame和组件的初始创建：

一般来说，Swing中的

不是线程安全的。除非另有说明，否则必须在事件调度线程上访问所有Swing组件和相关类。

..。

然而，其影响在于构造和显示Swing应用程序。对应用程序的main方法或Applet中的方法的调用不会在事件调度线程上调用。因此，在构造和显示应用程序或小程序时，必须注意将控制转移到事件分派线程。

在过去，这个“初始创建”规则是不存在的，但在使用这种方法时发现了一些线程问题。如果我找到相关的讨论，我会链接到这里。

更新:我发现this answer on Stack Overflow正在讨论Swing单线程策略规则及其更改，以禁止“尚未实现”的组件从其他线程访问。