java - 创建一个线程来重复捕获屏幕

Question

package demo;
import java.awt.*;
import java.awt.image.*;
import java.util.*;
import javax.swing.*;

public class ScreenCapturingThread extends Thread{
    public ScreenCapturingThread(Vector<BufferedImage> screenShots,
            int frameRate,
            Icon cursor,
            Rectangle recordingArea){
        this.screenShots = screenShots;
        this.frameRate = frameRate;
        this.cursor = cursor;
        this.recordingArea = recordingArea;

        try{
            bot = new Robot();
        }catch(Exception e){
            System.out.println(e);
        }
        calculateSleepTime();
    }
    @Override
    public void run(){
        while(keepCapturing == true){
            try{
                screenShots.add(takeScreenShot());
                sleep(sleepTime);

                keepCapturing = false; //take only one shot

                System.out.println("here");
                JFrame frame = new JFrame();
                frame.setSize(recordingArea.width,recordingArea.height);
                frame.getGraphics().drawImage(screenShots.firstElement(), 0, 0,frame);
                frame.repaint();
                frame.setVisible(true);

            }catch(InterruptedException e){
                e.printStackTrace();
            }
        }
    }
    public BufferedImage takeScreenShot(){
        p = m.getPointerInfo();
        Point location = p.getLocation();
        image = bot.createScreenCapture(recordingArea);
        if(cursor!=null){
            Graphics g = image.getGraphics();
            g.drawImage(((ImageIcon)cursor).getImage(), location.x,location.y,null);
        }

        return image;
    }
    public void stopIt(){
        keepCapturing = false;
    }
    public void calculateSleepTime(){
        sleepTime = 1/frameRate;
    }

    public static void main(String[] args) {
        Vector<BufferedImage> bufferedImages = new Vector<>(100);
        int frameRate = 10;
        Icon cursor = (Icon) new ImageIcon("src/images/blackCursor.png");
        Rectangle r = new Rectangle(1280,800);
        ScreenCapturingThread sc = new ScreenCapturingThread(bufferedImages,frameRate,cursor,r);

        sc.start();
    }
    Vector<BufferedImage> screenShots;
    int frameRate;
    long sleepTime;
    boolean keepCapturing = true;
    Icon cursor;
    Rectangle recordingArea;
    Robot bot;
    MouseInfo m;
    PointerInfo p;
    BufferedImage image;
}

说明

我设计了与我的屏幕录像机配合使用的线程，但我决定先测试它。这就是它应该做的:

通过传递适当的参数创建一个新的线程对象

拍摄仅一个屏幕截图(仅在此测试期间)，将其存储在 vector 中，并在 run() 结束之前，将其绘制在 JFrame 上，以便我可以查看已捕获的内容。

问题

我不断收到 NullPointerException

frame.getGraphics().drawImage(screenShots.firstElement(), 0, 0,frame);

我不知道出了什么问题。
如果可以请找出错误吗？

更新:

现在，虽然 NullPointerException 消失了，但框架却是空白的，而它不应该是空白的

为什么？

Answer 1

JFrame 在显示之前不会为您提供任何图形。

如果您想在显示 JFrame 之前进行绘制，您永远不应该这样做:

frame.pack();
frame.setSize(recordingArea.width, recordingArea.height);
Graphics g = frame.getContentPane().getGraphics();
g.drawImage(screenShots.firstElement(), 0, 0, frame);

因为如Andrew Thompson has correctly written :

Do not use Component.getGraphics(). Instead, subclass and override the paint() (AWT), or paintComponent() (Swing) method.

Component.getGraphics() simply can't work. Java uses a callback mechanism for drawing graphics. You are not supposed to "push" graphics information into a component using getGraphics(). Instead you are supposed to wait until Java calls your paint()/paintComponent() method. At that moment you are supposed to provide the Component with the drawings you would like to do.

相反，你最好这样做:

BufferedImage img = new BufferedImage(recordingArea.width, recordingArea.height,
                    BufferedImage.TYPE_3BYTE_BGR);
            Graphics g = img.createGraphics();
            g.drawImage(screenShots.firstElement(), 0, 0, frame);
            JLabel l = new JLabel(new ImageIcon(img));
            frame.getContentPane().add(l);