Android 12 源码编译Apk 新建工程启动报NullPointerException

Android 12兼容问题：新手MainActivity崩溃与AppCompat主题的使用指南

最新推荐文章于 2024-12-26 06:36:29 发布

原创最新推荐文章于 2024-12-26 06:36:29 发布 · 1.4k 阅读

0 ·

CC 4.0 BY-SA版权

文章标签：

#android #java #android studio

本文讲述了在Android 12环境下，新创建的MainActivity遇到NullPointerException的问题，解决过程包括检查兼容性问题、尝试AppCompatActivity和正确设置Theme.AppCompat。最后，作者分享了解决方案和必要的主题配置技巧。

Activity代码如下，就是一个纯净的新建MainActivity，继承自Activity。

布局文件如下

Android.mk编写如下，

然而启动后会报如下NullPointerException，就很奇怪，查了很久，原来是因为在Android12的版本比较高了，出了兼容问题。

11-04 03:45:57.786  4985  4985 E test    : onCreate:App 
11-04 03:45:57.805  4985  4985 E AndroidRuntime: FATAL EXCEPTION: main
11-04 03:45:57.805  4985  4985 E AndroidRuntime: Process: com.example.myapplication, PID: 4985
11-04 03:45:57.805  4985  4985 E AndroidRuntime: java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.myapplication/com.example.myapplication.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Class java.lang.Object.getClass()' on a null object reference
11-04 03:45:57.805  4985  4985 E AndroidRuntime: 	at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3707)
11-04 03:45:57.805  4985  4985 E AndroidRuntime: 	at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3864)
11-04 03:45:57.805  4985  4985 E AndroidRuntime: 	at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:103)
11-04 03:45:57.805  4985  4985 E AndroidRuntime: 	at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
11-04 03:45:57.805  4985  4985 E AndroidRuntime: 	at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
11-04 03:45:57.805  4985  4985 E AndroidRuntime: 	at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2253)
11-04 03:45:57.805  4985  4985 E AndroidRuntime: 	at android.os.Handler.dispatchMessage(Handler.java:106)
11-04 03:45:57.805  4985  4985 E AndroidRuntime: 	at android.os.Looper.loopOnce(Looper.java:201)
11-04 03:45:57.805  4985  4985 E AndroidRuntime: 	at android.os.Looper.loop(Looper.java:288)
11-04 03:45:57.805  4985  4985 E AndroidRuntime: 	at android.app.ActivityThread.main(ActivityThread.java:7870)
11-04 03:45:57.805  4985  4985 E AndroidRuntime: 	at java.lang.reflect.Method.invoke(Native Method)
11-04 03:45:57.805  4985  4985 E AndroidRuntime: 	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:548)
11-04 03:45:57.805  4985  4985 E AndroidRuntime: 	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1003)
11-04 03:45:57.805  4985  4985 E AndroidRuntime: Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Class java.lang.Object.getClass()' on a null object reference
11-04 03:45:57.805  4985  4985 E AndroidRuntime: 	at com.android.internal.app.WindowDecorActionBar.getDecorToolbar(WindowDecorActionBar.java:247)
11-04 03:45:57.805  4985  4985 E AndroidRuntime: 	at com.android.internal.app.WindowDecorActionBar.init(WindowDecorActionBar.java:199)
11-04 03:45:57.805  4985  4985 E AndroidRuntime: 	at com.android.internal.app.WindowDecorActionBar.<init>(WindowDecorActionBar.java:173)
11-04 03:45:57.805  4985  4985 E AndroidRuntime: 	at android.app.Activity.initWindowDecorActionBar(Activity.java:3509)
11-04 03:45:57.805  4985  4985 E AndroidRuntime: 	at android.app.Activity.setContentView(Activity.java:3527)
11-04 03:45:57.805  4985  4985 E AndroidRuntime: 	at com.example.myapplication.MainActivity.onCreate(MainActivity.java:11)
11-04 03:45:57.805  4985  4985 E AndroidRuntime: 	at android.app.Activity.performCreate(Activity.java:8059)
11-04 03:45:57.805  4985  4985 E AndroidRuntime: 	at android.app.Activity.performCreate(Activity.java:8039)
11-04 03:45:57.805  4985  4985 E AndroidRuntime: 	at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1341)
11-04 03:45:57.805  4985  4985 E AndroidRuntime: 	at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3688)
11-04 03:45:57.805  4985  4985 E AndroidRuntime: 	... 12 more

检查了半天，没有发现哪里有错啊，这就搞不懂了，于是就尝试着就尝试着继承一下比较新的AppCompatActivity

再依赖一下所在的包androidx.appcompat_appcompat

进行编译却报了如下错，java.lang.IllegalStateException: You need to use a Theme.AppCompat theme

11-04 03:46:07.195  5129  5129 E test    : onCreate:App 
11-04 03:46:07.228  5129  5129 E ThemeUtils: View class androidx.appcompat.widget.AppCompatImageView is an AppCompat widget that can only be used with a Theme.AppCompat theme (or descendant).
11-04 03:46:07.233  5129  5129 E AndroidRuntime: FATAL EXCEPTION: main
11-04 03:46:07.233  5129  5129 E AndroidRuntime: Process: com.example.myapplication, PID: 5129
11-04 03:46:07.233  5129  5129 E AndroidRuntime: java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.myapplication/com.example.myapplication.MainActivity}: java.lang.IllegalStateException: You need to use a Theme.AppCompat theme (or descendant) with this activity.
11-04 03:46:07.233  5129  5129 E AndroidRuntime: 	at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3707)
11-04 03:46:07.233  5129  5129 E AndroidRuntime: 	at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3864)
11-04 03:46:07.233  5129  5129 E AndroidRuntime: 	at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:103)
11-04 03:46:07.233  5129  5129 E AndroidRuntime: 	at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
11-04 03:46:07.233  5129  5129 E AndroidRuntime: 	at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
11-04 03:46:07.233  5129  5129 E AndroidRuntime: 	at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2253)
11-04 03:46:07.233  5129  5129 E AndroidRuntime: 	at android.os.Handler.dispatchMessage(Handler.java:106)
11-04 03:46:07.233  5129  5129 E AndroidRuntime: 	at android.os.Looper.loopOnce(Looper.java:201)
11-04 03:46:07.233  5129  5129 E AndroidRuntime: 	at android.os.Looper.loop(Looper.java:288)
11-04 03:46:07.233  5129  5129 E AndroidRuntime: 	at android.app.ActivityThread.main(ActivityThread.java:7870)
11-04 03:46:07.233  5129  5129 E AndroidRuntime: 	at java.lang.reflect.Method.invoke(Native Method)
11-04 03:46:07.233  5129  5129 E AndroidRuntime: 	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:548)
11-04 03:46:07.233  5129  5129 E AndroidRuntime: 	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1003)
11-04 03:46:07.233  5129  5129 E AndroidRuntime: Caused by: java.lang.IllegalStateException: You need to use a Theme.AppCompat theme (or descendant) with this activity.
11-04 03:46:07.233  5129  5129 E AndroidRuntime: 	at androidx.appcompat.app.AppCompatDelegateImpl.createSubDecor(AppCompatDelegateImpl.java:845)
11-04 03:46:07.233  5129  5129 E AndroidRuntime: 	at androidx.appcompat.app.AppCompatDelegateImpl.ensureSubDecor(AppCompatDelegateImpl.java:808)
11-04 03:46:07.233  5129  5129 E AndroidRuntime: 	at androidx.appcompat.app.AppCompatDelegateImpl.setContentView(AppCompatDelegateImpl.java:695)
11-04 03:46:07.233  5129  5129 E AndroidRuntime: 	at androidx.appcompat.app.AppCompatActivity.setContentView(AppCompatActivity.java:195)
11-04 03:46:07.233  5129  5129 E AndroidRuntime: 	at com.example.myapplication.MainActivity.onCreate(MainActivity.java:39)
11-04 03:46:07.233  5129  5129 E AndroidRuntime: 	at android.app.Activity.performCreate(Activity.java:8059)
11-04 03:46:07.233  5129  5129 E AndroidRuntime: 	at android.app.Activity.performCreate(Activity.java:8039)
11-04 03:46:07.233  5129  5129 E AndroidRuntime: 	at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1341)
11-04 03:46:07.233  5129  5129 E AndroidRuntime: 	at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3688)
11-04 03:46:07.233  5129  5129 E AndroidRuntime: 	... 12 more

在AndroidManifest.xml中添加一个Theme.AppCompat类型的theme，如下是一个白天黑夜模式的主题。

android:theme="@style/Theme.AppCompat.DayNight"

再次进行编译，终于编译通过了，启动也不会报错了。