Canvas - null pointer exception

问题:

Sometimes, I am getting NullPointerException in the line. Please let me know how to correct this. At the time of error, I have checked the possibility for the null value in variable backgoundImage. The variable backgoundImage is not null.

  canvas.drawBitmap(backgoundImage, 0, 0    , null);

Code:

    @Override
    public void run() {
        // TODO Auto-generated method stub
        ourHolder = getHolder();
        while (isRunning) { 
            if (!ourHolder.getSurface().isValid()){
                continue;
            } 
            canvas = ourHolder.lockCanvas();    
            screenCenterX = dWidth / 2; 
            screenCenterY = dHeight / 2;  

            //-----------------------------------------------------------------------------------
            if(backgoundImage == null){ 
                try { 
                    Log.i("DragDropCheck", "----------------backgoundImage is null--------");
                    backgoundImage = getAssetImage(getContext(),"backgroundhomepage");
                    canvas.drawBitmap(backgoundImage, 0, 0  , null); 
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                } 
            }else{ 
                canvas.drawBitmap(backgoundImage, 0, 0  , null);
            } 
            //-----------------------------------------------------------------------------------

            if (imagePublishDone) {
                if(!welcomeDone){                       
                 welcomeDone=true;
                }
                moveImageInEllipticalPath();
            } else {
                initialImagePublish();
            } 
            centreReached = false;
            ourHolder.unlockCanvasAndPost(canvas);
        }
    } 

logcat:

07-09 22:40:18.689: E/AndroidRuntime(8794): FATAL EXCEPTION: Thread-621
07-09 22:40:18.689: E/AndroidRuntime(8794): java.lang.NullPointerEception
07-09 22:40:18.689: E/AndroidRuntime(8794): at com.eample.funandlearn.DragDrop$MyBringBackSurface.run(DragDrop.java:645)
07-09 22:40:18.689: E/AndroidRuntime(8794): at java.lang.Thread.run(Thread.java:856)

回答1:

Probably, the problem is that the canvas variable is null. You should manage that specific case to avoid executing code when the ourHolder.lockCanvas() call returns null (also, you should check that your ourHolder variable isn't null before executing the while loop).