无法获取NoteViewModel实例致Scribble笔记应用崩溃

问题概况 #

应用名称：Scribble
功能：基础笔记记录应用
问题：应用启动时崩溃，提示无法创建NoteViewModel实例，原因是该类没有无参构造函数。

主活动代码 #

class MainActivity : AppCompatActivity(), ClickListeners {

lateinit var recyclerView : RecyclerView
lateinit var addButton : FloatingActionButton
lateinit var viewModel : NoteViewModel
lateinit var adapter : NoteViewAdapter


override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)

    recyclerView = findViewById(R.id.NotesRV)
    addButton = findViewById(R.id.addButton)
    viewModel = ViewModelProvider.AndroidViewModelFactory.getInstance(application).create(NoteViewModel::class.java)

    addButton.setOnClickListener{
         val intent : Intent = Intent(this,AddNoteActivity::class.java)
         startActivity(intent)
    }
    showAllNotes()

}

override fun onNoteClick(note: Note) {
    TODO("Not yet implemented")
}

override fun onDeleteButtonClick(note: Note) {
    TODO("Not yet implemented")
}

override fun onNoteLongClick(note: Note) {
    TODO("Not yet implemented")
}

fun showAllNotes(){
    viewModel.allNotes.observe(this, Observer {list ->
        adapter = NoteViewAdapter(list as ArrayList<Note>,this, this)
        recyclerView.adapter = adapter
        recyclerView.layoutManager = GridLayoutManager(this,2,GridLayoutManager.VERTICAL,false)
    })
}

问题代码行 #

ViewModelProvider.AndroidViewModelFactory.getInstance(application).create(NoteViewModel::class.java)

ViewModel类代码 #

class NoteViewModel (application: Application) : ViewModel() {

 val allNotes: LiveData<List<Note>>
 val repository: NoteRepository

init {

    val dao = NoteDatabase.getDatabase(application).getNotesDao()

    repository = NoteRepository(dao)

    allNotes = repository.allNotes
}

fun insertNote(note: Note) {
 
    viewModelScope.launch(Dispatchers.IO){
        
        repository.insertNote(note)
    }
}

fun deleteNote(note: Note) = viewModelScope.launch(Dispatchers.IO){repository.deleteNote(note)}

fun updateNote(note: Note) = viewModelScope.launch(Dispatchers.IO){repository.updateNote(note)}

Logcat日志 #

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.scribble/com.example.scribble.MainActivity}: java.lang.RuntimeException: Cannot create an instance of class com.example.scribble.model.NoteViewModel
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3635)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3792)
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:103)
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2210)
    at android.os.Handler.dispatchMessage(Handler.java:106)
    at android.os.Looper.loopOnce(Looper.java:201)
    at android.os.Looper.loop(Looper.java:288)
    at android.app.ActivityThread.main(ActivityThread.java:7839)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:548)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1003)
 Caused by: java.lang.RuntimeException: Cannot create an instance of class com.example.scribble.model.NoteViewModel
    at androidx.lifecycle.ViewModelProvider$NewInstanceFactory.create(ViewModelProvider.kt:188)
    at androidx.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.kt:238)
    at com.example.scribble.MainActivity.onCreate(MainActivity.kt:31)
    at android.app.Activity.performCreate(Activity.java:8051)
    at android.app.Activity.performCreate(Activity.java:8031)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1329)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3608)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3792) 
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:103) 
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2210) 
    at android.os.Handler.dispatchMessage(Handler.java:106) 
    at android.os.Looper.loopOnce(Looper.java:201) 
    at android.os.Looper.loop(Looper.java:288) 
    at android.app.ActivityThread.main(ActivityThread.java:7839) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:548) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1003) 
 Caused by: java.lang.InstantiationException: java.lang.Class<com.example.scribble.model.NoteViewModel> has no zero argument constructor
    at java.lang.Class.newInstance(Native Method)
    at androidx.lifecycle.ViewModelProvider$NewInstanceFactory.create(ViewModelProvider.kt:186)
    at androidx.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.kt:238) 
    at com.example.scribble.MainActivity.onCreate(MainActivity.kt:31) 
    at android.app.Activity.performCreate(Activity.java:8051) 
    at android.app.Activity.performCreate(Activity.java:8031) 
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1329) 
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3608) 
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3792) 
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:103) 
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2210) 
    at android.os.Handler.dispatchMessage(Handler.java:106) 
    at android.os.Looper.loopOnce(Looper.java:201) 
    at android.os.Looper.loop(Looper.java:288) 
    at android.app.ActivityThread.main(ActivityThread.java:7839) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:548) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1003) 

错误核心原因 #

Caused by: java.lang.InstantiationException: java.lang.Class<com.example.scribble.model.NoteViewModel> has no zero argument constructor

解决方案 #

问题出在NoteViewModel的继承类选择上，只需修改一处即可解决：

1. 修改ViewModel继承类：
   由于你的NoteViewModel需要接收Application参数，应该继承AndroidViewModel而非ViewModel——AndroidViewModel是专门为需要Application实例的场景设计的，自带对Application参数的支持。修改后的代码如下：

   class NoteViewModel(application: Application) : AndroidViewModel(application) {
   
       val allNotes: LiveData<List<Note>>
       val repository: NoteRepository
   
       init {
           val dao = NoteDatabase.getDatabase(application).getNotesDao()
           repository = NoteRepository(dao)
           allNotes = repository.allNotes
       }
   
       fun insertNote(note: Note) {
           viewModelScope.launch(Dispatchers.IO) {
               repository.insertNote(note)
           }
       }
   
       fun deleteNote(note: Note) = viewModelScope.launch(Dispatchers.IO) { repository.deleteNote(note) }
   
       fun updateNote(note: Note) = viewModelScope.launch(Dispatchers.IO) { repository.updateNote(note) }
   }
   

2. 保留原实例化代码：
   你原来的ViewModel实例化代码是正确的，AndroidViewModelFactory会自动处理Application参数的传递，无需修改：

   viewModel = ViewModelProvider.AndroidViewModelFactory.getInstance(application).create(NoteViewModel::class.java)
   

修改完成后，AndroidViewModelFactory就能正确识别NoteViewModel的构造函数，成功创建实例，解决启动崩溃问题。

内容的提问来源于stack exchange，提问作者vibhum mohan