Question

我目前正在开发一个倒计时应用程序，每次打开它时，它都会失败并显示一条消息，提示Cannot create instance of class com.nailuj29gaming.CountdownViewModel

堆栈跟踪为

    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)

我知道有一个very similar question, this one，但这是针对Java的，我的代码是用Kotlin编写的。我的ViewModel课的文字是

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)
    }
}

我在ViewModel方法的第一行中创建了MainActivity.onCreate()。我用来创建它的代码是viewModel = ViewModelProvider(this).get(CountdownViewModel::class.java)

我的build.gradle文件（至少在此问题中很重要）是

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"
}

我尝试使用工厂，但不推荐使用ViewModelProviders类

当我使用委托时，IDE并没有给我一个错误，但是编译器说

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

我的代码被编译为使用JVM target 1.8构建的字节码，因此不应显示此错误，对吧？

Answer 1

您需要升级到Fragment 1.2.0或更高版本，这为Lifecycle 2.2.0 release notes的ViewModelProvider构造函数增加了支持：

使用Fragment 1.2.0时，可以将Fragment或FragmentActivity传递给新的ViewModelProvider（ViewModelStoreOwner）构造函数以实现相同的功能。

使用较旧版本的Fragments（即AppCompat 1.1.0插入的Fragment 1.1.0）时，您只会得到在异常跟踪中看到的ViewModelProvider$NewInstanceFactory，而不是ViewModel。支持AndroidViewModel的工厂。

因此，您应该添加对片段1.2.1（当前最新版本）的显式依赖项：

implementation 'androidx.fragment:fragment:1.2.1'

Answer 2

这篇文章对我有帮助，因为我面临着类似的问题：https://stackoverflow.com/a/60670866/6030520

要解决您遇到的错误（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），您只需添加：

android { 

    ...

    kotlinOptions { jvmTarget = "1.8" }

}

...到您的app / build.gradle文件。

Answer 3

您应该考虑stacktrace中的这一行

产生者：java.lang.InstantiationException： java.lang.Class有没有零参数构造函数

这意味着您在构建viewmodel

时应在构造函数中传递一个参数。

在您引用的链接中有一个特定的kotlin答案] 1

编辑： 并控制您已按照本response或其他one

的指示正确启动了工厂

无法创建类ViewModel的实例

3 个答案: