关于paint背景的问题

Question

public class Frame1 extends JFrame implements ActionListener {
 JPanel jPanel3 = new JPanel(){
       //绘制背景
        public void paintComponent(Graphics g) {        if(image!=null){            g.drawImage(image,0,0,this.getWidth(),this.getHeight(),null);
        }else{
            g.setColor(new Color(100, 155, 100));
            g.fillRect(0, 0, this.getWidth(), this.getHeight());
            g.setColor(new Color(80, 135, 80));
            g.setFont(new Font("黑体", 1, 25));
            g.drawString("无背景图片", this.getWidth()/2, 30);
        }
        }    };
。 /**
   *加载背景图
   * @return boolean
   */
  public boolean loadBgPic(){
      imageIcon=new ImageIcon(this.bgPicname);
      image=imageIcon.getImage();
      image = image.getScaledInstance(jPanel3.getWidth(), jPanel3.getHeight(), Image.SCALE_SMOOTH);     jPanel3.validate();
     jPanel3.repaint();      return true;
  }

}
上面的程序中，我在jPanel3里加了一幅背景图片，然后，在jFram1里定义loadBgPic用来生成image
现在的问题是，调用后，jPanel3显示的是jframe1的背景色，这时如果jPanel3被遮挡重绘时，就会显示部分背景图片，但为何第一次 jPanel3.validate();
     jPanel3.repaint();不显示呢？

Answer 1

有没有调试啊?
关键是你的loadBgPic是什么时候调用的恶，如果在jpanel3初始化之前调用，那肯定是可以的，如果在jpanel3初始化后调用，第一次肯定是不会出来的，而是默认的颜色，类似于灰色。
建议把源码都贴出来看看

Answer 2

在paintComponent的第一行加上super.paintComponent(g);

Answer 3

加不加super.paintComponent(g);
结果都一样to diggywang(Miner Lover！) 
loadBgPic是我点击按钮时候，选择背景图片文件，然后调用
所以肯定是jpanel3初始化以后
但是，现在我调用后，jPanel3.validate();
     jPanel3.repaint();
我也看到输出了，的确调用了jPanel3的paintComponent方法
但问题是，随后，jPanel3有被frame1的背景色覆盖了
如果拖动程序，让jPanel3部分被窗口遮挡，然后再移动，系统会调用paintComponent将被遮挡部分绘制上我所设置的图片。

Answer 4

你把JPanel的背景涂成红色，看看只是图片没出来，还是整个JPanel没出来

Answer 5

package gktsys.frame;import java.awt.BorderLayout;import javax.swing.JFrame;
import javax.swing.JButton;
import javax.swing.JPanel;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.Dimension;
import java.awt.event.MouseEvent;
import java.awt.event.*;
import javax.swing.BorderFactory;
import java.awt.FlowLayout;
import java.awt.Color;
import java.awt.SystemColor;
import javax.swing.JLabel;
import java.awt.Point;
import java.io.*;
import java.awt.Component;
import java.awt.Frame;
import java.awt.FileDialog;
import java.awt.Image;
import java.awt.Graphics;
import javax.swing.ImageIcon;
import java.awt.Font;public class Frame1 extends JFrame implements ActionListener {
    //背景图片
      private ImageIcon imageIcon = null;
     transient   Image image = null;
    JPanel jPanel1 = new JPanel();
    JButton jButton2 = new JButton();
    FlowLayout flowLayout2 = new FlowLayout();
    JPanel jPanel2 = new JPanel();
    //主编辑panel
    JPanel jPanel3 = new JPanel(){
       //绘制背景
        public void paintComponent(Graphics g) {        if(image!=null){
            g.drawImage(image,0,0,this.getWidth(),this.getHeight(),null);
        }else{
            g.setColor(new Color(100, 155, 100));
            g.fillRect(0, 0, this.getWidth(), this.getHeight());
            g.setColor(new Color(80, 135, 80));
            g.setFont(new Font("黑体", 1, 25));
            g.drawString("无背景图片", this.getWidth()/2, 30);
        }
        }
    };
    int x=0;
    int y=0;
    JButton jButton3 = new JButton();
    int START=50;
    BorderLayout borderLayout1 = new BorderLayout();
    //重载
    public void actionPerformed(ActionEvent e)
    {
        if(e.getSource().getClass().getName().equals("javax.swing.JButton"))//getSource判断哪个按钮被按下了。
        {
           //用来选择图片的
           FileDialog filedlg=new FileDialog(this,"选择背景图片");           filedlg.setVisible(true);
           String picpath=filedlg.getDirectory();
           String  picfile=filedlg.getFile();
           System.out.println(picpath+picfile);
           imageIcon=new ImageIcon(picpath+picfile);
           image=imageIcon.getImage();
           image = image.getScaledInstance(jPanel3.getWidth(), jPanel3.getHeight(), Image.SCALE_SMOOTH);
           jPanel3.validate();
           jPanel3.updateUI();
                
        }
    }    public Frame1() {
        try {
            jbInit();
        } catch (Exception exception) {
            exception.printStackTrace();
        }
    }
    private void jbInit() throws Exception {
        getContentPane().setLayout(borderLayout1);
        jButton2.setText("添加标签");
        jButton2.addMouseListener(new Frame1_jButton2_mouseAdapter(this));
        jButton2.addActionListener(new Frame1_jButton2_actionAdapter(this));
        jPanel1.setBorder(null);
        jPanel1.setMaximumSize(new Dimension(7670, 7670));
        jPanel1.setLayout(flowLayout2);
        this.getContentPane().setBackground(SystemColor.controlText);
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setTitle("test");
        this.setVisible(true);
        jPanel2.setBorder(null);
        jPanel2.setPreferredSize(new Dimension(100, 10));
        jPanel3.setBounds(0,0,800,600);
        jPanel3.setBackground(SystemColor.activeCaptionBorder);
        jPanel3.setBorder(BorderFactory.createLoweredBevelBorder());
        jPanel3.setOpaque(false);
        jPanel3.setInputVerifier(null);
        jPanel3.setLayout(null);
        jPanel3.addMouseListener(new Frame1_jPanel3_mouseAdapter(this));
        jPanel3.addMouseMotionListener(new Frame1_jPanel3_mouseMotionAdapter(this));
        jButton3.setText("加载图像");
        jButton3.addActionListener(this);
        jPanel1.add(jButton3);
        jPanel1.add(jButton2, null);
        this.getContentPane().add(jPanel2, java.awt.BorderLayout.EAST);
        this.getContentPane().add(jPanel3, java.awt.BorderLayout.CENTER);
        this.getContentPane().add(jPanel1, java.awt.BorderLayout.NORTH);
    }    public static void main(String[] args) {
        Frame1 frame1 = new Frame1();
        frame1.setSize(800,600);
        frame1.setVisible(true);
    }    public void jButton2_actionPerformed(ActionEvent e) {    }//增加测点
    public void jButton2_mouseClicked(MouseEvent e) {        System.out.println("clicked");
        Point p=new Point(50+START,100);
         JLabel jcedian = new JLabel();
             jcedian.setText("xin cedian");
             jcedian.setBounds(50+START,100,48,15);
              jPanel3.add(jcedian);
               START+=50;
             jPanel3.validate();
              jPanel3.repaint();
    }
}
class Frame1_jPanel3_mouseAdapter extends MouseAdapter {
    private Frame1 adaptee;
    Frame1_jPanel3_mouseAdapter(Frame1 adaptee) {
        this.adaptee = adaptee;
    }
}class Frame1_jPanel3_mouseMotionAdapter extends MouseMotionAdapter {
    private Frame1 adaptee;
    Frame1_jPanel3_mouseMotionAdapter(Frame1 adaptee) {
        this.adaptee = adaptee;
    }
}class Frame1_jButton2_actionAdapter implements ActionListener {
    private Frame1 adaptee;
    Frame1_jButton2_actionAdapter(Frame1 adaptee) {
        this.adaptee = adaptee;
    }    public void actionPerformed(ActionEvent e) {
        adaptee.jButton2_actionPerformed(e);
    }
}
class Frame1_jButton2_mouseAdapter extends MouseAdapter {
    private Frame1 adaptee;
    Frame1_jButton2_mouseAdapter(Frame1 adaptee) {
        this.adaptee = adaptee;
    }    public void mouseClicked(MouseEvent e) {
        adaptee.jButton2_mouseClicked(e);
    }
}
这是源程序，运行后，加载一jpg格式的图片，会出现黑色背景将jPanel3覆盖

Answer 6

黑色背景，会不会是imageBuffer没装载图片阿。

Answer 7

哦  没用imageBuffer啊，反正用到哪个放的图片，调试一下看图片放进去没，否则应该不会出黑色背景的。

Answer 8

单步时候，jPanel3.paintComponent被执行了

Answer 9

等等g.drawImage(image,0,0,this.getWidth(),this.getHeight(),null);
改为
g.drawImage(image,0,0,this.getWidth(),this.getHeight(),this);ImageObserver怎么设置成null了？要用绘制面板才可以

Answer 10

来晚一步啊，重载的方法drawImage（）传null参数，哎，太不小心了，这些个情况下传null是最容易出错的

Answer 11

package gktsys.frame;import java.awt.BorderLayout;import javax.swing.JFrame;
import javax.swing.JButton;
import javax.swing.JPanel;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.Dimension;
import java.awt.event.MouseEvent;
import java.awt.event.*;
import javax.swing.BorderFactory;
import java.awt.FlowLayout;
import java.awt.Color;
import java.awt.SystemColor;
import javax.swing.JLabel;
import java.awt.Point;
import java.io.*;
import java.awt.Component;
import java.awt.Frame;
import java.awt.FileDialog;
import java.awt.Image;
import java.awt.Graphics;
import javax.swing.ImageIcon;
import java.awt.Font;
import java.awt.*;public class Frame1
    extends JFrame
    implements ActionListener {
  //背景图片
  private ImageIcon imageIcon = null;
  transient Image image = null;
  JPanel jPanel1 = new JPanel();
  JButton jButton2 = new JButton();
  FlowLayout flowLayout2 = new FlowLayout();
  JPanel jPanel2 = new JPanel();
  //主编辑panel
  JPanel jPanel3 = new JPanel() {
    //绘制背景
    public void paintComponent(Graphics g) {
      super.paintComponent(g);
      g.setColor(new Color(100, 155, 100));
      g.fillRect(0, 0, this.getWidth(), this.getHeight());
      if (image != null) {
        g.drawImage(image, 0, 0, this.getWidth(), this.getHeight(), this);
      }
      else {
        g.setColor(new Color(80, 135, 80));
        g.setFont(new Font("黑体", 1, 25));
        g.drawString("无背景图片", this.getWidth() / 2, 30);
      }
    }
  };
  int x = 0;
  int y = 0;
  JButton jButton3 = new JButton();
  int START = 50;
  BorderLayout borderLayout1 = new BorderLayout();
  //重载
  public void actionPerformed(ActionEvent e) {
    if (e.getSource().getClass().getName().equals("javax.swing.JButton")) { //getSource判断哪个按钮被按下了。
      //用来选择图片的
      FileDialog filedlg = new FileDialog(this, "选择背景图片");      filedlg.setVisible(true);
      String picpath = filedlg.getDirectory();
      String picfile = filedlg.getFile();
      System.out.println(picpath + picfile);
      imageIcon = new ImageIcon(picpath + picfile);
      image = imageIcon.getImage();
      image = image.getScaledInstance(jPanel3.getWidth(), jPanel3.getHeight(),
                                      Image.SCALE_SMOOTH);
      MediaTracker mt=new MediaTracker(this);
      mt.addImage(image,0);
      try {
        mt.waitForID(0);
      }
      catch (InterruptedException ex) {
      }
      jPanel3.validate();
      jPanel3.updateUI();    }
  }  public Frame1() {
    try {
      jbInit();
    }
    catch (Exception exception) {
      exception.printStackTrace();
    }
  }  private void jbInit() throws Exception {
    getContentPane().setLayout(borderLayout1);
    jButton2.setText("添加标签");
    jButton2.addMouseListener(new Frame1_jButton2_mouseAdapter(this));
    jButton2.addActionListener(new Frame1_jButton2_actionAdapter(this));
    jPanel1.setBorder(null);
    jPanel1.setMaximumSize(new Dimension(7670, 7670));
    jPanel1.setLayout(flowLayout2);
    this.getContentPane().setBackground(SystemColor.controlText);
    this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    this.setTitle("test");
    this.setVisible(true);
    jPanel2.setBorder(null);
    jPanel2.setPreferredSize(new Dimension(100, 10));
    jPanel3.setBounds(0, 0, 800, 600);
    jPanel3.setBackground(SystemColor.activeCaptionBorder);
    jPanel3.setBorder(BorderFactory.createLoweredBevelBorder());
    jPanel3.setOpaque(false);
    jPanel3.setInputVerifier(null);
    jPanel3.setLayout(null);
    jPanel3.addMouseListener(new Frame1_jPanel3_mouseAdapter(this));
    jPanel3.addMouseMotionListener(new Frame1_jPanel3_mouseMotionAdapter(this));
    jButton3.setText("加载图像");
    jButton3.addActionListener(this);
    jPanel1.add(jButton3);
    jPanel1.add(jButton2, null);
    this.getContentPane().add(jPanel2, java.awt.BorderLayout.EAST);
    this.getContentPane().add(jPanel3, java.awt.BorderLayout.CENTER);
    this.getContentPane().add(jPanel1, java.awt.BorderLayout.NORTH);
  }  public static void main(String[] args) {
    Frame1 frame1 = new Frame1();
    frame1.setSize(800, 600);
    frame1.setVisible(true);
  }  public void jButton2_actionPerformed(ActionEvent e) {  }//增加测点
  public void jButton2_mouseClicked(MouseEvent e) {    System.out.println("clicked");
    Point p = new Point(50 + START, 100);
    JLabel jcedian = new JLabel();
    jcedian.setText("xin cedian");
    jcedian.setBounds(50 + START, 100, 48, 15);
    jPanel3.add(jcedian);
    START += 50;
    jPanel3.validate();
    jPanel3.repaint();
  }
}class Frame1_jPanel3_mouseAdapter
    extends MouseAdapter {
  private Frame1 adaptee;
  Frame1_jPanel3_mouseAdapter(Frame1 adaptee) {
    this.adaptee = adaptee;
  }
}class Frame1_jPanel3_mouseMotionAdapter
    extends MouseMotionAdapter {
  private Frame1 adaptee;
  Frame1_jPanel3_mouseMotionAdapter(Frame1 adaptee) {
    this.adaptee = adaptee;
  }
}class Frame1_jButton2_actionAdapter
    implements ActionListener {
  private Frame1 adaptee;
  Frame1_jButton2_actionAdapter(Frame1 adaptee) {
    this.adaptee = adaptee;
  }  public void actionPerformed(ActionEvent e) {
    adaptee.jButton2_actionPerformed(e);
  }
}class Frame1_jButton2_mouseAdapter
    extends MouseAdapter {
  private Frame1 adaptee;
  Frame1_jButton2_mouseAdapter(Frame1 adaptee) {
    this.adaptee = adaptee;
  }  public void mouseClicked(MouseEvent e) {
    adaptee.jButton2_mouseClicked(e);
  }
}好了我修改了两个地方
一个是paintComponent方法，这个不碍事，不是原因所在，不过最好注意一下
还有一个是Image加载的地方，要用MediaTracker来等待，等到Image加载完毕为止。应为new Image（）虽然方法调用完毕，但图片加载还没完毕。
建议加载图片的方法放到Thread里面写哈哈，那个帖子没看到哦

Answer 12

to sxnucseven(*) 
什么人？一句玩笑而已，他给分我没看到那个帖子而已。你有能耐去解决问题呀。

Answer 13

http://community.csdn.net/Expert/topic/5223/5223973.xml?temp=.6551172赫赫，多谢了