无法创建类 ViewModel 的实例

Question

I'm currently working on a countdown app, and whenever I open it, it fails with a message saying Cannot create instance of class com.nailuj29gaming.CountdownViewModel

The stack trace is

    Process: com.nailuj29gaming.countdown.debug, PID: 17800
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.nailuj29gaming.countdown.debug/com.nailuj29gaming.countdown.MainActivity}: java.lang.RuntimeException: Cannot create an instance of class com.nailuj29gaming.countdown.CountdownViewModel
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2750)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2811)
        at android.app.ActivityThread.-wrap12(ActivityThread.java)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1528)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:154)
        at android.app.ActivityThread.main(ActivityThread.java:6316)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:872)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:762)
     Caused by: java.lang.RuntimeException: Cannot create an instance of class com.nailuj29gaming.countdown.CountdownViewModel
        at androidx.lifecycle.ViewModelProvider$NewInstanceFactory.create(ViewModelProvider.java:221)
        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:187)
        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:150)
        at com.nailuj29gaming.countdown.MainActivity.onCreate(MainActivity.kt:23)
        at android.app.Activity.performCreate(Activity.java:6757)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2703)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2811) 
        at android.app.ActivityThread.-wrap12(ActivityThread.java) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1528) 
        at android.os.Handler.dispatchMessage(Handler.java:102) 
        at android.os.Looper.loop(Looper.java:154) 
        at android.app.ActivityThread.main(ActivityThread.java:6316) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:872) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:762) 
     Caused by: java.lang.InstantiationException: java.lang.Class<com.nailuj29gaming.countdown.CountdownViewModel> has no zero argument constructor
        at java.lang.Class.newInstance(Native Method)
        at androidx.lifecycle.ViewModelProvider$NewInstanceFactory.create(ViewModelProvider.java:219)
        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:187) 
        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:150) 
        at com.nailuj29gaming.countdown.MainActivity.onCreate(MainActivity.kt:23) 
        at android.app.Activity.performCreate(Activity.java:6757) 
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119) 
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2703) 
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2811) 
        at android.app.ActivityThread.-wrap12(ActivityThread.java) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1528) 
        at android.os.Handler.dispatchMessage(Handler.java:102) 
        at android.os.Looper.loop(Looper.java:154) 
        at android.app.ActivityThread.main(ActivityThread.java:6316) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:872) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:762) 

I know there is a very similar question, this one , but that is target at Java, and my code is written in Kotlin. My ViewModel class's text is

package com.nailuj29gaming.countdown

import android.app.Application
import androidx.lifecycle.AndroidViewModel
import androidx.lifecycle.LiveData
import androidx.lifecycle.viewModelScope
import kotlinx.coroutines.launch

class CountdownViewModel(application: Application) : AndroidViewModel(application) {
    private val repo: CountdownRepo
    val countdowns: LiveData<List<Countdown>>

    init {
        val countdownDao = AppDatabase.getDatabase(application).countdownDao()
        repo = CountdownRepo(countdownDao)
        countdowns = repo.countdowns

    }

    fun insert(countdown: Countdown) = viewModelScope.launch {
        repo.insert(countdown)
    }

    fun delete(countdown: Countdown) = viewModelScope.launch {
        repo.delete(countdown)
    }
}

I create the ViewModel in the first line of my MainActivity.onCreate() method. The code I used to create it is viewModel = ViewModelProvider(this).get(CountdownViewModel::class.java)

My build.gradle file (at least what matters in this question) is

apply plugin: 'kotlin-kapt'

dependencies {
    def room_version = "2.2.3"
    def lifecycle_version = "2.2.0"
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
    implementation 'androidx.appcompat:appcompat:1.1.0'
    implementation 'androidx.core:core-ktx:1.1.0'
    implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
    testImplementation 'junit:junit:4.13'
    androidTestImplementation 'androidx.test.ext:junit:1.1.1'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'
    implementation 'com.google.code.gson:gson:2.8.6'
    implementation 'androidx.preference:preference:1.1.0'
    implementation 'net.steamcrafted:materialiconlib:1.1.5'
    implementation 'com.google.android.material:material:1.0.0'
    implementation "androidx.room:room-runtime:$room_version"
    kapt "androidx.room:room-compiler:$room_version"
    implementation "androidx.lifecycle:lifecycle-viewmodel-ktx:$lifecycle_version"
    implementation "androidx.room:room-ktx:$room_version"
}

I tried using a factory, but the ViewModelProviders class is deprecated

When I used a delegate, the IDE did not give me an error, but the compiler said

Cannot inline bytecode built with JVM target 1.8 into bytecode that is being built with JVM target 1.6. Please specify proper '-jvm-target' option

My code is compiled into bytecode built with JVM target 1.8, so this error shouldn't show, right?

Answer 1

You need to upgrade to Fragment 1.2.0 or higher, which is what adds a support for the ViewModelProvider constructor as per the Lifecycle 2.2.0 release notes :

You can pass a Fragment or FragmentActivity to the new ViewModelProvider(ViewModelStoreOwner) constructor to achieve the same functionality when using Fragment 1.2.0 .

When using an older version of Fragments (ie, the Fragment 1.1.0 that AppCompat 1.1.0 pulls in), you'll only get the ViewModelProvider$NewInstanceFactory you see in your exception trace, rather than a ViewModel.Factory that supports AndroidViewModel .

Therefore you should add an explicit dependency on Fragment 1.3.5 (the current latest):

implementation 'androidx.fragment:fragment:1.3.5'

Answer 2

This post helped me, as I was facing a similar issue: https://stackoverflow.com/a/60670866/6030520

To address the error you were getting ( Cannot inline bytecode built with JVM target 1.8 into bytecode that is being built with JVM target 1.6. Please specify proper '-jvm-target' option ), you can simply add:

android { 

    ...

    kotlinOptions { jvmTarget = "1.8" }

}

... to your app/build.gradle file.

Answer 3

You should consider this line from the stacktrace

Caused by: java.lang.InstantiationException: java.lang.Class has no zero argument constructor

This means you should pass an argument in the constructor when you buildng your viewmodel

In the link you quoted there is a specific kotlin answer] 1

EDIT: And also control that you have correctly insta intiated the factory as shown in this response or this other one