Swing JLabel：强制重绘（）

Question

I am trying to repaint a JLabel dynamically and I can't for the life of me figure out how to do it. The code below will do as expected once I resize the screen but will not execute the code by itself.

The JLabel has a little circle to the left which is drawn in the border region to the left of the text. The color of the circle should change as a function of the health of an FTP connection (not shown). The thread monitoring the FTP connection calls the setStatus(int) method when the health changes.

The circle is painted during initialization of the JLabel, and I am trying to re-execute this code using repaint() .

EDIT: I have also tried playing with revalidate() , invalidate() , and validate() to no avail.

EDIT: Thanks for pointing that out, I started by using paintComponent() and changed to paint() when that didn't work. So no glory for giving that as an answer (sorry, take it up with the Oracle)

import java.awt.Color;
import java.awt.Component;
import java.awt.Font;
import java.awt.Graphics;

import javax.swing.BorderFactory;
import javax.swing.JLabel;

import com.my.package.io.ftp.FTPConnectionListenable;

class StatusLabel extends JLabel implements FTPConnectionListenable {

    private Integer status;

    // Constructor
    StatusLabel(final String text) {
        super(text);
        setFont(new Font("Dialog", Font.PLAIN, 10));
        setBorder(BorderFactory.createEmptyBorder(0, 10, 0, 0));
    }

    @Override
    public void paintComponent(final Graphics g) {
        super.paintComponent(g);
        g.setColor(getColor());
        g.fillArc(0, this.getHeight()/4, 8, 8, 0, 360);
    }

    @Override
    public void setStatus (final int status) {
        this.status = status;
        if (status !=0) 
        repaint(); // Doesn't work :(
    }

    private Color getColor () {
        switch (status) {
        case FTPConnectionListenable.STATUS_OK:
            return Color.GREEN;
        case FTPConnectionListenable.STATUS_WARNING:
            return Color.ORANGE;
        case FTPConnectionListenable.STATUS_ERROR:
            return Color.RED;
        default: 
            return Color.PINK;
        }
    }
}

Answer 1

Klong, your code works for me, so likely you have a bug elsewhere. How I tested it,

import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.*;


class StatusLabel extends JLabel implements FTPConnectionListenable {

    private Integer status;

    // Constructor
    StatusLabel(final String text) {
        super(text);
        setFont(new Font("Dialog", Font.PLAIN, 10));
        setBorder(BorderFactory.createEmptyBorder(0, 10, 0, 0));
    }

    @Override
    public void paintComponent(final Graphics g) {
        super.paintComponent(g);
        g.setColor(getColor());
        g.fillArc(0, this.getHeight()/4, 8, 8, 0, 360);
    }

    @Override
    public void setStatus (final int status) {
        this.status = status;
        if (status !=0) 
        repaint(); 
    }

    private Color getColor () {
        switch (status) {
        case FTPConnectionListenable.STATUS_OK:
            return Color.GREEN;
        case FTPConnectionListenable.STATUS_WARNING:
            return Color.ORANGE;
        case FTPConnectionListenable.STATUS_ERROR:
            return Color.RED;
        default: 
            return Color.PINK;
        }
    }

    public static void main(String[] args) {
      final StatusLabel statusLabel = new StatusLabel("Foo");
      statusLabel.setStatus(FTPConnectionListenable.STATUS_OK);
      new Timer(1000, new ActionListener() {
         int counter = 0;
         @Override
         public void actionPerformed(ActionEvent e) {
            counter++;
            counter %= 4;
            statusLabel.setStatus(counter);
         }
      }).start();

      JOptionPane.showMessageDialog(null, statusLabel);
   }
}

interface FTPConnectionListenable {

   static final int STATUS_ERROR = 0;
   static final int STATUS_WARNING = 1;
   static final int STATUS_OK = 2;
   void setStatus(int status);

}

By the way, that's one small arc.

Answer 2

You should override paintComponent and call super method before doing anything else:

@Override
public void paintComponent(Graphics g) {
    super.paintComponent(g);
    g.setColor(getColor());
    g.fillArc(0, this.getHeight()/4, 8, 8, 0, 360);

}

Answer 3

我注意到的第一件事是你应该重写paintComponent而不是paint 。

Answer 4

Rather thanoverriding paint() , you probably just want to override paintComponent() .

Also, the call to super.paint(g) should be done at the beginning of your method, otherwise any painting you perform may be overwritten by the JLabel own painting.

Answer 5

If it doesn't repaint then the problem is likely that you are doing your FTP connection on the Event Dispatch Thread which is blocking and preventing the GUI from repainting.

You need to use a separate Thread for the FTP connection. Read the section from the Swing tutorial on Concurrency for more information and a solution using a SwingWorker .

Answer 6

状态需要是volatile，以确保Event Dispatch Thread看到ftp线程设置的新值。