[Android/안드로이드] java.lang.RuntimeException: Unable to instantiate activity

[Problem Log]

01-05 09:42:37.923: E/AndroidRuntime(2092): FATAL EXCEPTION: main 01-05 09:42:37.923: E/AndroidRuntime(2092): Process: com.victor.devicemanager_new, PID: 2092 01-05 09:42:37.923: E/AndroidRuntime(2092): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.victor.devicemanager_new/com.victor.devicemanager_new.MainActivity}: java.lang.ClassNotFoundException: Didn't find class "com.victor.devicemanager_new.MainActivity" on path: DexPathList[[zip file "/data/app/com.victor.devicemanager_new-1/base.apk"],nativeLibraryDirectories=[/vendor/lib, /system/lib]] 01-05 09:42:37.923: E/AndroidRuntime(2092): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2209) 01-05 09:42:37.923: E/AndroidRuntime(2092): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360) 01-05 09:42:37.923: E/AndroidRuntime(2092): at android.app.ActivityThread.access$800(ActivityThread.java:144) 01-05 09:42:37.923: E/AndroidRuntime(2092): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278) 01-05 09:42:37.923: E/AndroidRuntime(2092): at android.os.Handler.dispatchMessage(Handler.java:102) 01-05 09:42:37.923: E/AndroidRuntime(2092): at android.os.Looper.loop(Looper.java:135) 01-05 09:42:37.923: E/AndroidRuntime(2092): at android.app.ActivityThread.main(ActivityThread.java:5221) 01-05 09:42:37.923: E/AndroidRuntime(2092): at java.lang.reflect.Method.invoke(Native Method) 01-05 09:42:37.923: E/AndroidRuntime(2092): at java.lang.reflect.Method.invoke(Method.java:372) 01-05 09:42:37.923: E/AndroidRuntime(2092): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899) 01-05 09:42:37.923: E/AndroidRuntime(2092): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694) 01-05 09:42:37.923: E/AndroidRuntime(2092): Caused by: java.lang.ClassNotFoundException: Didn't find class "com.victor.devicemanager_new.MainActivity" on path: DexPathList[[zip file "/data/app/com.victor.devicemanager_new-1/base.apk"],nativeLibraryDirectories=[/vendor/lib, /system/lib]] 01-05 09:42:37.923: E/AndroidRuntime(2092): at dalvik.system.BaseDexClassLoader.findClass(BaseDexClassLoader.java:56) 01-05 09:42:37.923: E/AndroidRuntime(2092): at java.lang.ClassLoader.loadClass(ClassLoader.java:511) 01-05 09:42:37.923: E/AndroidRuntime(2092): at java.lang.ClassLoader.loadClass(ClassLoader.java:469) 01-05 09:42:37.923: E/AndroidRuntime(2092): at android.app.Instrumentation.newActivity(Instrumentation.java:1065) 01-05 09:42:37.923: E/AndroidRuntime(2092): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2199) 01-05 09:42:37.923: E/AndroidRuntime(2092): ... 10 more 01-05 09:42:37.923: E/AndroidRuntime(2092): Suppressed: java.lang.NoClassDefFoundError: com.victor.devicemanager_new.MainActivity 01-05 09:42:37.923: E/AndroidRuntime(2092): at dalvik.system.DexFile.defineClassNative(Native Method) 01-05 09:42:37.923: E/AndroidRuntime(2092): at dalvik.system.DexFile.defineClass(DexFile.java:226) 01-05 09:42:37.923: E/AndroidRuntime(2092): at dalvik.system.DexFile.loadClassBinaryName(DexFile.java:219) 01-05 09:42:37.923: E/AndroidRuntime(2092): at dalvik.system.DexPathList.findClass(DexPathList.java:321) 01-05 09:42:37.923: E/AndroidRuntime(2092): at dalvik.system.BaseDexClassLoader.findClass(BaseDexClassLoader.java:54) 01-05 09:42:37.923: E/AndroidRuntime(2092): ... 14 more 01-05 09:42:37.923: E/AndroidRuntime(2092): Suppressed: java.lang.ClassNotFoundException: com.victor.devicemanager_new.MainActivity 01-05 09:42:37.923: E/AndroidRuntime(2092): at java.lang.Class.classForName(Native Method) 01-05 09:42:37.923: E/AndroidRuntime(2092): at java.lang.BootClassLoader.findClass(ClassLoader.java:781) 01-05 09:42:37.923: E/AndroidRuntime(2092): at java.lang.BootClassLoader.loadClass(ClassLoader.java:841) 01-05 09:42:37.923: E/AndroidRuntime(2092): at java.lang.ClassLoader.loadClass(ClassLoader.java:504) 01-05 09:42:37.923: E/AndroidRuntime(2092): ... 13 more 01-05 09:42:37.923: E/AndroidRuntime(2092): Caused by: java.lang.NoClassDefFoundError: Class not found using the boot class loader; no stack available

[Solution]

Go the "Java Build Path" -> Order and export -> check all checkboxes -> clean -> build -> run