在绘制图像时，jFrame GetGraphics在Java中为null [英] jFrame GetGraphics is null in java While Drawing image

问题描述

在将图像绘制到jframe时，我得到null异常错误。
i调试代码并检查图像和帧是否为空，但仍然在绘制图像到帧时抛出NULL异常。

请看一看： / p>

  public void run（）{
 try {
 
 ObjectInputStream objVideoIn = new ObjectInputStream（serVideoIn ）; 
 byte [] imgbytes = null; 
 ByteArrayInputStream barrin = null; 
 
 
 JFrame jf = new JFrame（）; 
图形ga = jf.getGraphics（）; //获取null异常
 //Thread.sleep（10000）; 
 jf.setVisible（true）; 
 jf.setSize（400，400）; 
 
 
 while（true）{
 int index = 0; 
 //Thread.sleep(300）; 
 int size =（int）objVideoIn.readObject（）; 
 imgbytes = new byte [size]; 
 barrin = new ByteArrayInputStream（imgbytes）; 
 System.out.println（image size+ size）; 
 //Thread.sleep(200）; 
 while（index< size）
 {
 System.out.println（reading image）; 
 int bytesread = objVideoIn.read（imgbytes，index，size-index）; 
 if（bytesread <0）{
 System.out.println（接收字节时发生错误yar）; 
} 
 index + = bytesread; 
} 
 //barrin.read（imgbytes，0，imgbytes.length）; 
 barrin = new ByteArrayInputStream（imgbytes）; 
 
 buffImg = ImageIO.read（barrin）; 
 
 if（buffImg == null）
 {
 System.out.println（null received）; 
} 
 else {
 System.out.println（image received）; 
 
 ** ga.drawImage（buffImg，0，0，null）; ** 
 
 
 
 
 
）
} 
 catch（Exception ex）
 {
 System.out.println（error reading video+ ex.getMessage（））; 
} 
 
} 
  

解决方案

NPE很可能来自这里：

 图形ga = jf.getGraphics（）; 
  

按 docs ：

为这个组件创建一个图形上下文。如果此组件目前无法显示，则此方法将返回
null。

1）不要使用组件＃getGraphics 作为其不良实践/不持久，并且将返回 null ，除非组件可见。

2）而是使用 JPanel 并覆盖 paintComponent（Graphics g）不要忘记调用 super.paintComponent（g）; 作为overriden paintComponent 中的第一个调用。
$ b <3>覆盖 getPreferredSize（）并返回正确的维 s以适合正在绘制的图像。

4）将 JPanel 添加到框架中， 。

或者你也可以使用 JLabel ，它只需要一个 setIcon （..）调用并添加到 JFrame 。

这里有一些我的例子：

使用 JPanel ：

• 加载图片从C盘启动Java代码



=https://stackoverflow.com/questions/12996501/how-to-display-an-image-in-a-frame/129967 18＃12996718>如何在框架中显示图像？
将JPanel转换为JScrollPane中的图像

使用 JLabel ：

• 调整图像大小并在JPanel或JLabel中显示而不会损失质量

I'm getting null exception error while drawing image to jframe. i debug the code and check the image and frame is not null but still it is throwing NULL exception while drawing image to frame.

Please have a look :

public void run(){
    try{

        ObjectInputStream objVideoIn = new ObjectInputStream(serVideoIn);
        byte[] imgbytes=null;
        ByteArrayInputStream barrin=null;


        JFrame jf = new JFrame();
        Graphics ga=jf.getGraphics(); //Getting null exception 
                //Thread.sleep(10000);
        jf.setVisible(true);
        jf.setSize(400, 400);


        while(true){
                    int index=0;
                    //Thread.sleep(300);
                    int size= (int)objVideoIn.readObject();
                    imgbytes = new byte[size];
                    barrin = new ByteArrayInputStream(imgbytes);
                    System.out.println("image size" + size);
                    //Thread.sleep(200);
                    while(index<size)
                    {
                        System.out.println("reading image");
                        int bytesread = objVideoIn.read(imgbytes, index, size-index);
                        if(bytesread<0){
                            System.out.println("error in receiving bytes yar");
                        }
                        index+=bytesread;
                    }
                    //barrin.read(imgbytes, 0, imgbytes.length);
                    barrin = new ByteArrayInputStream(imgbytes);

                    buffImg = ImageIO.read(barrin);

                        if(buffImg==null)
                        {
                            System.out.println("null received");
                        }
                        else {
                            System.out.println("image received");

                        **ga.drawImage(buffImg, 0, 0, null);**


                        }
                    }

            }
    }
    catch(Exception ex)
    {
        System.out.println("error reading video" +ex.getMessage());
    }

}

解决方案

The NPE is likely coming from here:

Graphics ga=jf.getGraphics();

as per docs:

Creates a graphics context for this component. This method will return null if this component is currently not displayable.

1) Dont use Component#getGraphics as its bad pratice/not persistent and will return null unless component is visible.

2) Rather use JPanel and override paintComponent(Graphics g) dont forget to call super.paintComponent(g); as first call in overriden paintComponent.

3) Override getPreferredSize() and return correct Dimensions to fit image being drawn.

4) add JPanel to the frame for image to be visible of course.

Alternatively You could also use a JLabel which would require nothing more than a setIcon(..) call and be added added to JFrame.

Here are some of my examples:

Using JPanel:

• Loading image in Java Code from C drive

• Image Drawing Works for JFrame, But Not JPanel

• How to display an image in a frame?

• Convert a JPanel to an image in a JScrollPane

Using JLabel:

• Image resizing and displaying in a JPanel or a JLabel without loss of quality

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