无法创建 ViewModel 类的实例

Question

I'm doing this inside my onCreate method of MainActivity

journalViewModel = new ViewModelProvider(this).get(JournalViewModel.class);

and my ViewModel is as following:

public class JournalViewModel extends AndroidViewModel {

    public JournalViewModel(@NonNull Application application) {
        super(application);
        repository = new JournalRepository(application);
        journals = repository.getAllJournals();
    }
}

However, I'm getting the error for this line,

journalViewModel = new ViewModelProvider(this).get(JournalViewModel.class);

saying,

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.diaryapp/com.example.diaryapp.MainActivity}: java.lang.RuntimeException: Cannot create an instance of class com.example.diaryapp.JournalViewModel

As per my knowledge the constructor inside ViewModel shouldn't have any other parameter except the one in my code and both constructor and class should be public.

However, further inside my error logs, I get the error:

java.lang.InstantiationException: java.lang.Class<com.example.diaryapp.JournalViewModel> has no zero argument constructor

Here's the complete error log:

2020-03-23 21:37:18.986 8632-8632/com.example.diaryapp E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.diaryapp, PID: 8632
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.diaryapp/com.example.diaryapp.MainActivity}: java.lang.RuntimeException: Cannot create an instance of class com.example.diaryapp.JournalViewModel
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2946)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3081)
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1831)
    at android.os.Handler.dispatchMessage(Handler.java:106)
    at android.os.Looper.loop(Looper.java:201)
    at android.app.ActivityThread.main(ActivityThread.java:6823)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:873)
 Caused by: java.lang.RuntimeException: Cannot create an instance of class com.example.diaryapp.JournalViewModel
    at androidx.lifecycle.ViewModelProvider$NewInstanceFactory.create(ViewModelProvider.java:221)
    at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:187)
    at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:150)
    at com.example.diaryapp.MainActivity.onCreate(MainActivity.java:40)
    at android.app.Activity.performCreate(Activity.java:7224)
    at android.app.Activity.performCreate(Activity.java:7213)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1272)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2926)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3081) 
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) 
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108) 
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1831) 
    at android.os.Handler.dispatchMessage(Handler.java:106) 
    at android.os.Looper.loop(Looper.java:201) 
    at android.app.ActivityThread.main(ActivityThread.java:6823) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:873) 
 Caused by: java.lang.InstantiationException: java.lang.Class<com.example.diaryapp.JournalViewModel> has no zero argument constructor
    at java.lang.Class.newInstance(Native Method)
    at androidx.lifecycle.ViewModelProvider$NewInstanceFactory.create(ViewModelProvider.java:219)
    at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:187) 
    at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:150) 
    at com.example.diaryapp.MainActivity.onCreate(MainActivity.java:40) 
    at android.app.Activity.performCreate(Activity.java:7224) 
    at android.app.Activity.performCreate(Activity.java:7213) 
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1272) 
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2926) 
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3081) 
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) 
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108) 
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1831) 
    at android.os.Handler.dispatchMessage(Handler.java:106) 
    at android.os.Looper.loop(Looper.java:201) 
    at android.app.ActivityThread.main(ActivityThread.java:6823) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:873) 

Answer 1

You should obtain JournalViewModel like:

journalViewModel = new ViewModelProvider(this, ViewModelProvider.AndroidViewModelFactory.getInstance(this.getApplication())).get(JournalViewModel.class);  

instead of

journalViewModel = new ViewModelProvider(this).get(JournalViewModel.class);

Answer 2

Not sure about Java, but this worked for me in Kotlin.

You need to define a ViewModelProvidor.Factory:

class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)
    viewModelFactory.setApplication(application)
    val journalViewModel = ViewModelProvider(this, viewModelFactory).get(JournalViewModel::class.java)
    }
}

object viewModelFactory : ViewModelProvider.Factory {
    lateinit var app : Application

    fun setApplication(application: Application) {
        app = application
    }
    override fun <T : ViewModel?> create(modelClass: Class<T>): T {
        return JournalViewModel(app) as T
    }

}

Answer 3

I got it working with the help of this answer.

Here's what I did in,

JournalFactory.java

public class JournalFactory extends ViewModelProvider.NewInstanceFactory {
    private Application application;

    public JournalFactory(@NonNull Application application) {
        this.application = application;
    }

    @NonNull
    @Override
    public <T extends ViewModel> T create(@NonNull Class<T> modelClass) {
        if (modelClass == JournalViewModel.class)
            return (T) new JournalViewModel(application);
        return null;
    }
}

onCreate() method of MainActivity.java

journalViewModel = new ViewModelProvider(this, new JournalFactory(getApplication())).get(JournalViewModel.class);