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

最新推荐文章于 2020-07-10 17:22:24 发布

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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();