无法创建ViewModel类的实例

Question

我在MainActivity 的onCreate方法中执行此操作

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

我的ViewModel如下：

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

然而，我收到了这条线的错误，

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

说

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

据我所知，ViewModel中的构造函数不应该有任何其他参数，除了我代码中的那个参数，构造函数和类都应该是公共的。

然而，在我的错误日志中，我得到了错误：

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

以下是完整的错误日志：

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

您应该获得JournalViewModel，如：

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

而不是

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

Answer 2

不确定Java，但这对我在Kotlin的工作很有效。

您需要定义一个ViewModelProvider。工厂：

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

我在这个答案的帮助下完成了它。

这是我在做的

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;
}
}

MainActivity.java的onCreate((方法

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