layout使用继承TextView，Activity未使用，报java.lang.NullPointerException一例

最新推荐文章于 2021-05-27 10:51:32 发布

最新推荐文章于 2021-05-27 10:51:32 发布 · 188 阅读

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本文探讨了在Android开发过程中遇到的一个常见问题：当移除Activity中的特定组件时，如果该组件在布局文件中仍有配置，可能会导致布局无法正确绘制，进而引发NullPointerException。通过分析LogCat报错信息，本文详细解释了问题发生的根本原因，并提供了解决方案，即在自定义组件类中初始化关键对象，以避免此类异常。

Android 2.3.3 Eclipse Version: 3.7.0 LogCat

AutoScrollTextView继承TextView，用于文字特殊效果显示，代码如下：

public class AutoScrollTextView extends TextView implements OnClickListener { …… public void onDraw(Canvas canvas) { …… canvas.drawText(text, temp_view_plus_text_length - step, y, paint); …… } …… }

Activity中使用：

taobaoTextView = (AutoScrollTextView) findViewById(R.id.text);

Activity对应的layout ：

<com.taobao.util.AutoScrollTextView android:id="@+id/text" android:text="text" android:layout_height="fill_parent" android:layout_width="40px" android:textColor="#999" android:layout_gravity="right" android:background="#333"> </com.taobao.util.AutoScrollTextView>

当Activity中取消使用AutoScrollTextView，而layout 未删除有关AutoScrollTextView的配置时，
LogCat 报错信息：

02-07 12:06:58.728: DEBUG/AndroidRuntime(1034): Shutting down VM 02-07 12:06:58.728: WARN/dalvikvm(1034): threadid=1: thread exiting with uncaught exception (group=0x40015560) 02-07 12:06:58.747: ERROR/AndroidRuntime(1034): FATAL EXCEPTION: main 02-07 12:06:58.747: ERROR/AndroidRuntime(1034): java.lang.NullPointerException 02-07 12:06:58.747: ERROR/AndroidRuntime(1034): at android.graphics.Canvas.drawText(Native Method) 02-07 12:06:58.747: ERROR/AndroidRuntime(1034): at com.taobao.util.AutoScrollTextView.onDraw(AutoScrollTextView.java:181) 02-07 12:06:58.747: ERROR/AndroidRuntime(1034): at android.view.View.draw(View.java:6880) 02-07 12:06:58.747: ERROR/AndroidRuntime(1034): at android.view.ViewGroup.drawChild(ViewGroup.java:1646) 02-07 12:06:58.747: ERROR/AndroidRuntime(1034): at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1373) 02-07 12:06:58.747: ERROR/AndroidRuntime(1034): at android.view.ViewGroup.drawChild(ViewGroup.java:1644) 02-07 12:06:58.747: ERROR/AndroidRuntime(1034): at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1373) 02-07 12:06:58.747: ERROR/AndroidRuntime(1034): at android.view.ViewGroup.drawChild(ViewGroup.java:1644) 02-07 12:06:58.747: ERROR/AndroidRuntime(1034): at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1373) 02-07 12:06:58.747: ERROR/AndroidRuntime(1034): at android.view.ViewGroup.drawChild(ViewGroup.java:1644) 02-07 12:06:58.747: ERROR/AndroidRuntime(1034): at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1373) 02-07 12:06:58.747: ERROR/AndroidRuntime(1034): at android.view.View.draw(View.java:6883) 02-07 12:06:58.747: ERROR/AndroidRuntime(1034): at android.widget.FrameLayout.draw(FrameLayout.java:357) 02-07 12:06:58.747: ERROR/AndroidRuntime(1034): at com.android.internal.policy.impl.PhoneWindow$DecorView.draw(PhoneWindow.java:1862) 02-07 12:06:58.747: ERROR/AndroidRuntime(1034): at android.view.ViewRoot.draw(ViewRoot.java:1522) 02-07 12:06:58.747: ERROR/AndroidRuntime(1034): at android.view.ViewRoot.performTraversals(ViewRoot.java:1258) 02-07 12:06:58.747: ERROR/AndroidRuntime(1034): at android.view.ViewRoot.handleMessage(ViewRoot.java:1859) 02-07 12:06:58.747: ERROR/AndroidRuntime(1034): at android.os.Handler.dispatchMessage(Handler.java:99) 02-07 12:06:58.747: ERROR/AndroidRuntime(1034): at android.os.Looper.loop(Looper.java:123) 02-07 12:06:58.747: ERROR/AndroidRuntime(1034): at android.app.ActivityThread.main(ActivityThread.java:3683) 02-07 12:06:58.747: ERROR/AndroidRuntime(1034): at java.lang.reflect.Method.invokeNative(Native Method) 02-07 12:06:58.747: ERROR/AndroidRuntime(1034): at java.lang.reflect.Method.invoke(Method.java:507) 02-07 12:06:58.747: ERROR/AndroidRuntime(1034): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839) 02-07 12:06:58.747: ERROR/AndroidRuntime(1034): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597) 02-07 12:06:58.747: ERROR/AndroidRuntime(1034): at dalvik.system.NativeStart.main(Native Method) 02-07 12:06:58.797: WARN/ActivityManager(61): Force finishing activity com.taobao.tmall/.PlayAd 02-07 12:06:58.807: WARN/ActivityManager(61): Force finishing activity com.taobao.tmall/.WakeUp

发生错误原因分析：

canvas.drawText(text, temp_view_plus_text_length - step, y, paint);

此处的paint为null，报错。

解决办法：在AutoScrollTextView.java中初始化paint，为其赋值。

paint = getPaint();