Android动态Wallpaer崩溃（空指针异常，画布是一个空对象），但我找不到在哪里

Question

04-14 12:04:30.971 1639-1938/system_process I/WindowManager: Destroying surface Surface(name=Media:com.android.wallpaper.livepicker/com.android.wallpaper.livepicker.LiveWallpaperChange) called by com.android.server.wm.WindowStateAnimator.destroySurface:2014 com.android.server.wm.WindowStateAnimator.destroySurfaceLocked:881 com.android.server.wm.WindowState.removeLocked:1449 com.android.server.wm.WindowManagerService.removeWindowInnerLocked:2478 com.android.server.wm.WindowManagerService.removeWindowLocked:2436 com.android.server.wm.WindowManagerService.removeWindowLocked:2305 com.android.server.wm.WindowManagerService.removeWindow:2300 com.android.server.wm.Session.remove:193 
04-14 12:04:30.990 3852-3852/tianranwang.livewallpaper D/MainService: dispose 
04-14 12:04:31.018 1639-1990/system_process I/WindowManager: Destroying surface Surface(name=tianranwang.livewallpaper.MainService) called by com.android.server.wm.WindowStateAnimator.destroySurface:2014 com.android.server.wm.WindowStateAnimator.destroySurfaceLocked:881 com.android.server.wm.WindowState.removeLocked:1449 com.android.server.wm.WindowManagerService.removeWindowInnerLocked:2478 com.android.server.wm.WindowManagerService.removeWindowLocked:2436 com.android.server.wm.WindowManagerService.removeWindowLocked:2305 com.android.server.wm.WindowManagerService.removeWindow:2300 com.android.server.wm.Session.remove:193 

      --------- beginning of crash 
04-14 12:04:31.020 3852-4240/tianranwang.livewallpaper E/AndroidRuntime: FATAL EXCEPTION: Animation Thread 
      Process: tianranwang.livewallpaper, PID: 3852 
      java.lang.NullPointerException: Attempt to invoke interface method 'void android.view.SurfaceHolder.unlockCanvasAndPost(android.graphics.Canvas)' on a null object reference 
       at processing.a2d.PGraphicsAndroid2D.endDraw(Unknown Source) 
       at processing.core.PApplet.handleDraw(Unknown Source) 
       at processing.core.PSurfaceNone.callDraw(Unknown Source) 
       at processing.core.PSurfaceNone$AnimationThread.run(Unknown Source) 

在界面中的“设置壁纸”，我旋转屏幕和草图（绘画动态壁纸）重新启动，应用程序坠毁（并不总是崩溃），和素描（动态壁纸）保持在背景上运行..

有人可以给我一个提示吗？提前致谢。

---

的源代码，即我已经在processing.a2d.PGraphicsAndroid2D.endDraw（）

从https://github.com/processing/processing-android/tree/master/core/src/processing

public void endDraw() { 
    if (bitmap == null) return; 

    // hm, mark pixels as changed, because this will instantly do a full 
    // copy of all the pixels to the surface.. so that's kind of a mess. 
    //updatePixels(); 

// if (primaryGraphics) { 
//  if (canvas != null) { 
//  parent.getSurfaceHolder().unlockCanvasAndPost(canvas); 
//  } 
// } 

    if (primaryGraphics) { 
     SurfaceHolder holder = parent.getSurface().getSurfaceHolder(); 
     if (holder != null) { 
     Canvas screen = null; 
     try { 
      screen = holder.lockCanvas(null); 
      if (screen != null) { 
      screen.drawBitmap(bitmap, new Matrix(), null); 
      } 
     } finally { 
      if (screen != null) { 
      try { 
       holder.unlockCanvasAndPost(screen); 
      } catch (IllegalStateException ex) { 
      } 
      } 
     } 
     } 
    } else { 
     // TODO this is probably overkill for most tasks... 
     loadPixels(); 
    } 

    // Marking as modified, and then calling updatePixels() in 
    // the super class, which just sets the mx1, my1, mx2, my2 
    // coordinates of the modified area. This avoids doing the 
    // full copy of the pixels to the surface in this.updatePixels(). 
    setModified(); 
    super.updatePixels(); 
    } 

发现上面的代码可能会导致此的NullPointerException。

---

我现在没有这个问题，因为我更新处理库的新版本..而上面的代码是从新版本的lib。 （我看不到旧版本的代码）

Answer 1

问题很明显，您指向的是空对象引用，即'void android.view.SurfaceHolder.unlockCanvasAndPost(android.graphics.Canvas)'的实例尚未创建，但您正试图获取它，因此引发错误。

java.lang.NullPointerException: Attempt to invoke interface method 'void android.view.SurfaceHolder.unlockCanvasAndPost(android.graphics.Canvas)' on a null object reference 

可以回溯哪里该类的实例被创建，以获得坠毁

Answer 2

什么是一个空指针？

空指针是Java中非常常见的异常。当您尝试拨打尚未启动的实例时会发生这种情况。这适用于：

Canvas c; 
c.drawRect(...); 

这基本上就是你在做什么。所有虽然你的代码可能会有所不同（这可能也是涉及到线程

之前您发掘并发布你必须锁定画布就像这样：。

Canvas canvas = sf.lockCanvas(); 

画布说本身，而SF是SurfaceView 。SurfaceView可以与任何视图（View，SUrfaceView等），因此，如果您添加上面的行更换。

，你会做画布是一个参考，而不是空的，异常会被解决

---

更改此：

screen = holder.lockCanvas(null); 

这个

screen = holder.lockCanvas();