为什么JComponent中的图像在加载时未正确绘制？

Question

这个程序运行时，为什么initializeGameField()函数不能一致加载图像？有时它不显示绑定到JLabel的图像，有时它会正确显示所有9个图像。 我已打印图像路径以验证它是否正确。如何将拖放功能实现到添加的图像？

package game;  
import java.awt.Color; 
import java.awt.Cursor; 
import java.awt.Dimension; 
import java.awt.Panel; 
import java.awt.Point; 
import java.awt.Toolkit; 
import java.awt.event.ActionEvent; 
import java.awt.event.ActionListener; 
import java.awt.event.MouseAdapter; 
import java.awt.event.MouseEvent; 
import java.net.URL;  
import javax.swing.ImageIcon; 
import javax.swing.JButton; 
import javax.swing.JDialog; 
import javax.swing.JFrame; 
import javax.swing.JLabel; 
import javax.swing.JLayeredPane; 
import javax.swing.JOptionPane; 
import javax.swing.JPanel; 
import javax.swing.JRadioButton; 
import javax.swing.JScrollPane; 
import javax.swing.JTextArea; 
import javax.swing.JTextField; 
import javax.swing.SwingConstants; 
import javax.swing.event.MouseInputAdapter; 

public class NMMPanel {  
    private JFrame frame; 
    private int appWidth = 728; 
    private int appHeigth = 828; 

    private JPanel topButtonPanel, topLeftPanel, topRightPanel; // main 
    private JLayeredPane centerPanel; 
    private JTextArea txtLogArea; 
    private JButton btnNewButton, btnQuitButton; 

    private JPanel user1Panel, user2Panel; // sub panels for user1 and user2 
    private JLabel chessBoardLabel, stateLabel; 

    private JTextField name1TextField, name2TextField; 
    private JDialog newGameDialog; 
    private boolean turnOfStarter = true; 

    private JLabel[] blacks = null; 
    private JLabel[] whites = null; 

    private Node[] nodes = new Node[24]; 

    /** 
    * Constructor: components and variables setup. 
    */ 
    public NMMPanel() { 
     initializeWindow(); 
     initializeGameField(); 
    } 

    private void initializeWindow() { 
     Dimension dim = Toolkit.getDefaultToolkit().getScreenSize(); 
     frame = new JFrame(); 
     frame.setResizable(false); 
     frame.setBounds((dim.width/2 - (appWidth/2)), 
       (dim.height/2 - (appHeigth/2)), appWidth, appHeigth); 
     frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
     frame.getContentPane().setLayout(null); 

     frame.setTitle("9MM - Nine Men's Morris"); 

     topButtonPanel = new JPanel(); 
     topButtonPanel.setBounds(6, 0, 710, 36); 
     btnNewButton = new JButton("New"); 
     btnQuitButton = new JButton("Quit"); 
     topButtonPanel.add(btnNewButton); 
     topButtonPanel.add(btnQuitButton); 
     frame.getContentPane().add(topButtonPanel); 

     // Add listeners to buttons. 
     ButtonListener listenToButton = new ButtonListener(); 
     btnNewButton.addActionListener(listenToButton); 
     btnQuitButton.addActionListener(listenToButton); 

     topLeftPanel = new JPanel(); 
     topLeftPanel.setForeground(new Color(0, 0, 0)); 
     topLeftPanel.setBackground(new Color(255, 255, 204)); 
     topLeftPanel.setBounds(6, 35, 352, 100); 
     frame.getContentPane().add(topLeftPanel); 
     topLeftPanel.setLayout(null); 

     topRightPanel = new JPanel(); 
     topRightPanel.setBackground(Color.DARK_GRAY); 
     topRightPanel.setBounds(364, 35, 350, 100); 
     frame.getContentPane().add(topRightPanel); 
     topRightPanel.setLayout(null); 

     centerPanel = new JLayeredPane(); 
     centerPanel.setBackground(Color.ORANGE); 
     centerPanel.setBounds(118, 140, 500, 500); 
     frame.getContentPane().add(centerPanel); 
     centerPanel.setLayout(null); 

     ImageIcon field = createImageIcon("/resources/Spielfeld_roundedCorners.png"); 
     JLabel feld = new JLabel(field); 
     feld.setBounds(0, 0, 500, 500); 
     centerPanel.add(feld, 2); 

     ImageIcon texture = createImageIcon("/resources/Wood.jpg"); 
     JLabel textureCenter = new JLabel(texture); 
     textureCenter.setBounds(0, 0, 500, 500); 
     centerPanel.add(textureCenter, 3); 

     setupEventFields(24); 

     JScrollPane scrollPane = new JScrollPane(); 
     scrollPane.setBounds(6, 645, 710, 149); 
     frame.getContentPane().add(scrollPane); 

     txtLogArea = new JTextArea(); 
     txtLogArea.setEditable(false); 
     scrollPane.setViewportView(txtLogArea); 

     if (turnOfStarter) { 
      txtLogArea.append("Whites turn!\n"); 
     } else { 
      txtLogArea.append("Blacks turn!\n"); 
     } 
     frame.setVisible(true); 
    } 

    private void setupEventFields(int counter) { 
     initNodes(); 
     for (int i = 0; i < counter; i++) { 
      final JLabel interactionFields = new JLabel("FFFF"); 
      interactionFields.setHorizontalAlignment(SwingConstants.CENTER); 
      interactionFields.setBounds(nodes[i].location.x, 
        nodes[i].location.y, 50, 50); 
      interactionFields.setCursor(Cursor 
        .getPredefinedCursor(Cursor.HAND_CURSOR)); 

      // TODO : Remove later 
      interactionFields.setForeground(new Color(255, 255, 255)); 

      interactionFields.addMouseListener(new MouseAdapter() { 
       @Override 
       public void mouseEntered(MouseEvent e) { 
        System.out.println("entered here"); 
        System.out.println(interactionFields.getLocation()); 
       } 

       @Override 
       public void mouseClicked(MouseEvent e) { 
        System.out.println("Clicked here"); 

       } 

      }); 
      centerPanel.add(interactionFields, 0); 
     } 
    } 

    private void initNodes() { 
     int x = 10; 
     int y = 10; 
     nodes[0] = new Node("O_0", x, y); 

     x = 225; 
     y = 10; 
     nodes[1] = new Node("O_1", x, y); 

     x = 440; 
     y = 10; 
     nodes[2] = new Node("O_2", x, y); 

     x = 74; 
     y = 74; 
     nodes[3] = new Node("M_0", x, y); 

     x = 225; 
     y = 72; 
     nodes[4] = new Node("M_1", x, y); 

     x = 376; 
     y = 74; 
     nodes[5] = new Node("M_2", x, y); 

     x = 137; 
     y = 135; 
     nodes[6] = new Node("I_0", x, y); 

     x = 225; 
     y = 135; 
     nodes[7] = new Node("I_1", x, y); 

     x = 314; 
     y = 136; 
     nodes[8] = new Node("I_2", x, y); 

     x = 10; 
     y = 225; 
     nodes[9] = new Node("O_3", x, y); 

     x = 72; 
     y = 225; 
     nodes[10] = new Node("M_3", x, y); 

     x = 135; 
     y = 225; 
     nodes[11] = new Node("I_3", x, y); 

     x = 316; 
     y = 225; 
     nodes[12] = new Node("I_4", x, y); 

     x = 378; 
     y = 225; 
     nodes[13] = new Node("M_4", x, y); 

     x = 440; 
     y = 225; 
     nodes[14] = new Node("O_4", x, y); 

     x = 137; 
     y = 313; 
     nodes[15] = new Node("I_5", x, y); 

     x = 225; 
     y = 316; 
     nodes[16] = new Node("I_6", x, y); 

     x = 314; 
     y = 314; 
     nodes[17] = new Node("I_7", x, y); 

     x = 74; 
     y = 375; 
     nodes[18] = new Node("M_5", x, y); 

     x = 225; 
     y = 377; 
     nodes[19] = new Node("M_6", x, y); 

     x = 376; 
     y = 376; 
     nodes[20] = new Node("M_7", x, y); 

     x = 10; 
     y = 440; 
     nodes[21] = new Node("O_5", x, y); 

     x = 225; 
     y = 440; 
     nodes[22] = new Node("O_6", x, y); 

     x = 440; 
     y = 440; 
     nodes[23] = new Node("O_7", x, y); 
    } 

    protected Node getNode(Point location) { 
     for (int i = 0; i < nodes.length; i++) { 
      if ((nodes[i].location.x == location.x) 
        && (nodes[i].location.y == location.y)) { 
       return nodes[i]; 
      } 
     } 
     return null; 
    } 

    /** Returns an ImageIcon, or null if the path was invalid. */ 
    protected ImageIcon createImageIcon(String path) { 
     URL imgURL = getClass().getResource(path); 
     if (imgURL != null) { 
      return new ImageIcon(imgURL); 
     } else { 
      System.err.println("Couldn't find file: " + path); 
      return null; 
     } 
    } 

    private void initializeGameField() { 
     ImageIcon iconWhite = createImageIcon("/resources/White_Stone.png"); 
     int space = 10; 
     blacks = new JLabel[9]; 
     whites = new JLabel[9]; 

     JLabel lblPlayer1 = new JLabel("PLAYER 1"); 
     lblPlayer1.setBounds(150, 80, 61, 16); 
     topLeftPanel.add(lblPlayer1); 

     for (int i = 0; i < 9; i++) { 
      final JLabel lblWhite = new JLabel(iconWhite); 
      lblWhite.addMouseListener(new MouseAdapter() { 

       @Override 
       public void mouseClicked(MouseEvent e) { 
        System.out.println("Clicked"); 
       } 

       @Override 
       public void mouseDragged(MouseEvent e) { 
        System.out.println("Dragged"); 
       } 

       @Override 
       public void mouseMoved(MouseEvent e) { 
        System.out.println("Moved"); 
       } 

       @Override 
       public void mousePressed(MouseEvent e) { 
        System.out.println("Pressed"); 
       } 

       @Override 
       public void mouseReleased(MouseEvent e) { 
        System.out.println("Released"); 

       } 
      }); 

      lblWhite.setBounds(space, 25, 50, 50); 
      topLeftPanel.add(lblWhite); 
      space += 35; 
      whites[i] = lblWhite; 
     } 

     JLabel lblPlayer2 = new JLabel("PLAYER 2"); 
     lblPlayer2.setForeground(new Color(255, 255, 255)); 
     lblPlayer2.setBounds(150, 80, 61, 16); 
     topRightPanel.add(lblPlayer2); 

     ImageIcon iconBlack = createImageIcon("/resources/Black_Stone.png"); 

     space = 10; 
     for (int j = 0; j < 9; j++) { 
      final JLabel lblBlack = new JLabel(iconBlack); 
      lblBlack.addMouseListener(new MouseAdapter() { 
       @Override 
       public void mouseClicked(MouseEvent e) { 

       } 

       @Override 
       public void mouseDragged(MouseEvent e) { 
        System.out.println("Dragged"); 
       } 

       @Override 
       public void mouseMoved(MouseEvent e) { 
        System.out.println("Moved"); 
       } 

       @Override 
       public void mousePressed(MouseEvent e) { 
        System.out.println("Pressed"); 
       } 

       @Override 
       public void mouseReleased(MouseEvent e) { 
        System.out.println("Released"); 
        super.mouseReleased(e); 
       } 
      }); 

      lblBlack.setBounds(space, 25, 50, 50); 
      topRightPanel.add(lblBlack); 
      space += 35; 
      blacks[j] = lblBlack; 
      System.out.println(lblBlack); 
     } 
    } 
} 

Answer 1

问题的间歇性表明了线程问题。

您是否正在从Swing事件派发线程调用构造函数new NMMPanel();？否则，您可能正在创建不同线程中的所有组件。 由于Swing方法不是线程安全的，因此您可能会在图像加载完成时创建标签（带图标）。

通常情况下，JLabel将自己设置为ImageIcon的ImageObserver，以便在加载图像后可以收到重新绘制消息的消息。如果修改不在事件线程上，则可能不会发生。

---

编辑：从另一个类

public static void main(String[] args) { // Set up the frame and add the panel. 
    SwingUtilities.invokeLater(new Runnable() { 
     public void run() { 
     NMMPanel panel = new NMMPanel(); 
     } 
    } 
}