Android Live Wallpaer 崩溃（NullPointerException, canvas is a null object）但我找不到在哪里[重复]

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“设置壁纸”界面，我旋转了屏幕，重新启动了草图（绘制动态壁纸），应用程序崩溃（并非总是崩溃），而草图（动态壁纸）一直在后台运行。

有人可以给我一个提示吗？提前致谢。

---

我在 processing.a2d.PGraphicsAndroid2D.endDraw() 中找到的源代码

来自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();
  }

上面的代码可能会导致这个 NullPointerException。

---

我现在没有这个问题，因为我更新了新版本的处理库..上面的代码来自新版本的库。 （我看不到旧版本的代码）

Answer 1

问题很明显，您指向的是一个空对象引用，即 'void android.view.SurfaceHolder.unlockCanvasAndPost(android.graphics.Canvas)' 的实例尚未创建，但您正在尝试获取它，因此它会引发错误。

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

您可以回溯创建该类的实例的位置以获取崩溃