Android動態Wallpaer崩潰（空指針異常，畫布是一個空對象），但我找不到在哪裏

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「設置壁紙」，我旋轉屏幕和草圖（繪畫動態壁紙）重新啓動，應用程序墜毀（並不總是崩潰），和素描（動態壁紙）保持在背景上運行..

有人可以給我一個提示嗎？提前致謝。

---

的源代碼，即我已經在processing.a2d.PGraphicsAndroid2D.endDraw（）

從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(); 
    } 

發現上面的代碼可能會導致此的NullPointerException。

---

我現在沒有這個問題，因爲我更新處理庫的新版本..而上面的代碼是從新版本的lib。 （我看不到舊版本的代碼）

Answer 1

問題很明顯，您指向的是空對象引用，即'void android.view.SurfaceHolder.unlockCanvasAndPost(android.graphics.Canvas)'的實例尚未創建，但您正試圖獲取它，因此引發錯誤。

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

可以回溯哪裏該類的實例被創建，以獲得墜毀

Answer 2

什麼是一個空指針？

空指針是Java中非常常見的異常。當您嘗試撥打尚未啓動的實例時會發生這種情況。這適用於：

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

這基本上就是你在做什麼。所有雖然你的代碼可能會有所不同（這可能也是涉及到線程

之前您發掘併發布你必須鎖定畫布就像這樣：。

Canvas canvas = sf.lockCanvas(); 

畫布說本身，而SF是SurfaceView 。SurfaceView可以與任何視圖（View，SUrfaceView等），因此，如果您添加上面的行更換。

，你會做畫布是一個參考，而不是空的，異常會被解決

---

更改此：

screen = holder.lockCanvas(null); 

這個

screen = holder.lockCanvas();