ClassNotFoundException和NoClassDefFoundError的解决办法

程序在其他手机都没有问题，但在刷到版本稍微较低的平板或手机上时就会直接闪退，并报出以下异常： 

java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.simple/com.simple.activity.LoadingActivity}:
java.lang.ClassNotFoundException: Didn't find class "com.simple.activity.LoadingActivity" on path: DexPathList[[zip file "/data/app/com.winstars.petclient-2.apk"],nativeLibraryDirectories=[/data/app-lib/com.simple-2, /vendor/lib, /system/lib]]
                                                       at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2124)
                                                       at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2247)
                                                       at android.app.ActivityThread.access$800(ActivityThread.java:139)
                                                       at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1210)
                                                       at android.os.Handler.dispatchMessage(Handler.java:102)
                                                       at android.os.Looper.loop(Looper.java:136)
                                                       at android.app.ActivityThread.main(ActivityThread.java:5028)
                                                       at java.lang.reflect.Method.invokeNative(Native Method)
                                                       at java.lang.reflect.Method.invoke(Method.java:515)
                                                       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:791)
                                                       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:607)
                                                       at dalvik.system.NativeStart.main(Native Method)
                                                    Caused by: java.lang.ClassNotFoundException: Didn't find class "com.simple.activity.LoadingActivity" on path: DexPathList[[zip file "/data/app/com.winstars.petclient-2.apk"],nativeLibraryDirectories=[/data/app-lib/com.winstars.petclient-2, /vendor/lib, /system/lib]]
                                                       at dalvik.system.BaseDexClassLoader.findClass(BaseDexClassLoader.java:56)
                                                       at java.lang.ClassLoader.loadClass(ClassLoader.java:497)
                                                       at java.lang.ClassLoader.loadClass(ClassLoader.java:457)
                                                       at android.app.Instrumentation.newActivity(Instrumentation.java:1061)
                                                       at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2115)
                                                       at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2247) 
                                                       at android.app.ActivityThread.access$800(ActivityThread.java:139) 
                                                       at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1210) 
                                                       at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                       at android.os.Looper.loop(Looper.java:136) 
                                                       at android.app.ActivityThread.main(ActivityThread.java:5028) 
                                                       at java.lang.reflect.Method.invokeNative(Native Method) 
                                                       at java.lang.reflect.Method.invoke(Method.java:515)

...

java.lang.NoClassDefFoundError: com.simple.service.SimpleService$1
                                                       at com.simple.service.SimpleService.<init>(SimpleService.java:29)
                                                       at java.lang.Class.newInstanceImpl(Native Method)
                                                       at java.lang.Class.newInstance(Class.java:1208)
                                                       at android.app.ActivityThread.handleCreateService(ActivityThread.java:2554)
                                                       at android.app.ActivityThread.access$1800(ActivityThread.java:139)
                                                       at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1292)
                                                       at android.os.Handler.dispatchMessage(Handler.java:102)
                                                       at android.os.Looper.loop(Looper.java:136)
                                                       at android.app.ActivityThread.main(ActivityThread.java:5028)
                                                       at java.lang.reflect.Method.invokeNative(Native Method)
                                                       at java.lang.reflect.Method.invoke(Method.java:515)
                                                       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:791)
                                                       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:607)
                                                       at dalvik.system.NativeStart.main(Native Method)

试过网上的一些改包名，check jar包的导入等，没反应。

我自己的解决办法是使用MultiDex——

如果你build.gradle中设置的minSdk >= 21，则只需在build.gradle中配置：

android {
    defaultConfig {
        ...
        minSdkVersion 21 
        targetSdkVersion 26
        multiDexEnabled true
    }
    ...
}

如果你build.gradle中设置的minSdk <= 20，就要使用multidex support library：

android {
    defaultConfig {
        ...
        minSdkVersion 20 
        targetSdkVersion 26
        multiDexEnabled true
    }
    ...
}

dependencies {
  compile 'com.android.support:multidex:1.0.1'
}

注意还没完，还需要配置一下Application：

如果没有重写Application，在AndroidManifest.xml中进行如下配置：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.simple.app">
    <application
            android:name="android.support.multidex.MultiDexApplication" >
        ...
    </application>
</manifest>

如果重写了Application，你可以直接用你的Application类继承MultiDexApplication，具体如下所示：

public class MyApplication extends MultiDexApplication { ... }

或者不继承MultiDexApplication，直接在你的Application类的初始化中对MultiDex进行初始化

public class MyApplication extends Application {
  @Override
  protected void attachBaseContext(Context context) {         
      super.attachBaseContext(context); 
      Multidex.install(this); 
  }
}

参考链接：https://blog.csdn.net/k1457047898/article/details/53467238