java.lang.ClassNotFoundException

用AS运行的时候抛出异常：

 java.lang.RuntimeException: Unable to get provider com.heytap.msp.sdk.MspFileProvider: java.lang.ClassNotFoundException: Didn't find class "com.heytap.msp.sdk.MspFileProvider" on path: DexPathList[[zip file "/data/app/包名-2/base.apk"],nativeLibraryDirectories=[/data/app/包名-2/lib/arm, /data/app/包名-2/base.apk!/lib/armeabi-v7a, /system/lib, /vendor/lib]]
        at android.app.ActivityThread.installProvider(ActivityThread.java:5858)
        at android.app.ActivityThread.installContentProviders(ActivityThread.java:5447)
        at android.app.ActivityThread.handleBindApplication(ActivityThread.java:5386)
        at android.app.ActivityThread.-wrap2(ActivityThread.java)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1546)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:154)
        at android.app.ActivityThread.main(ActivityThread.java:6121)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:889)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:779)
     Caused by: java.lang.ClassNotFoundException: Didn't find class "com.heytap.msp.sdk.MspFileProvider" on path: DexPathList[[zip file "/data/app/包名-2/base.apk"],nativeLibraryDirectories=[/data/app/包名-2/lib/arm, /data/app/包名-2/base.apk!/lib/armeabi-v7a, /system/lib, /vendor/lib]]
        at dalvik.system.BaseDexClassLoader.findClass(BaseDexClassLoader.java:56)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:380)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:312)
        at android.app.ActivityThread.installProvider(ActivityThread.java:5843)
        at android.app.ActivityThread.installContentProviders(ActivityThread.java:5447) 
        at android.app.ActivityThread.handleBindApplication(ActivityThread.java:5386) 
        at android.app.ActivityThread.-wrap2(ActivityThread.java) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1546) 
        at android.os.Handler.dispatchMessage(Handler.java:102) 
        at android.os.Looper.loop(Looper.java:154) 
        at android.app.ActivityThread.main(ActivityThread.java:6121) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:889) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:779) 
        Suppressed: java.lang.NoClassDefFoundError: Failed resolution of: Landroidx/core/content/FileProvider;
        at java.lang.VMClassLoader.findLoadedClass(Native Method)
        at java.lang.ClassLoader.findLoadedClass(ClassLoader.java:742)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:362)
                ... 12 more
     Caused by: java.lang.ClassNotFoundException: Didn't find class "androidx.core.content.FileProvider" on path: DexPathList[[zip file "/data/app/包名-2/base.apk"],nativeLibraryDirectories=[/data/app/包名-2/lib/arm, /data/app/包名-2/base.apk!/lib/armeabi-v7a, /system/lib, /vendor/lib]]
        at dalvik.system.BaseDexClassLoader.findClass(BaseDexClassLoader.java:56)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:380)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:312)
                ... 15 more

解决办法：
1.在gradle.properties(Global Proerties)文件里头添加

android.useAndroidX=true
android.enableJetifier=true

2.在gradle.properties(Project Proerties)文件里头同样添加

android.useAndroidX=true
android.enableJetifier=true

3.在AndroidManifest.xml文件中若有FileProvider，查看是否为androidx.core.content.FileProvider，若不是则修改为

<provider
            android:name="androidx.core.content.FileProvider"
            android:authorities="${applicationId}.fileprovider"
            android:exported="false"
            android:grantUriPermissions="true">
            <meta-data
                android:name="android.support.FILE_PROVIDER_PATHS"
                android:resource="@xml/filepaths" />
        </provider>

同时项目里报红的地方alt+enter重新导入
4.Sync now——>Debug，就可以看到程序正常跑起来了，模拟器界面不会出现“xxx已停止运行”的提示，AS控制台也不会报错

建议跳至另一篇文章瞅一瞅Androidx初尝及其新旧包对照表