ava.lang.RuntimeException：无法创建类com.example.bookkeepers.BookViewModel的实例

Question

我正在html" target="_blank">开发一本新书android应用程序，但当我测试从logcat获取以下异常的代码时，应用程序崩溃

hread.mainRuntimeException：无法启动活动ComponentInfo{hread.java:6662}：java.lang.reflect.RuntimeException：无法创建ethod.invokeBookViewModel类的实例com.android.internal.os.ActivityTaller.runLaunchActive（ActivityTnit.java:547）com.android.internal.os.ActivityTnit.mainLaunchActive（ActivityTnit.java:873）java.lang.ActivityThread.-wrap11（未知来源：0）在com.example.bookkeepers.ActivityThread$H. handleMessage（ActivityTandroidx.lifecycle.）在android.os.Handler.dispatchMessage（Handler.java:106）在android.os.Looper.loop（Looper.java:176）android.app.ActivityTjava.lang.（ActivityTcom.example.bookkeepers/com.example.bookkeepers.MainActivity）java.lang.Mcom.example.bookkeepers.（Native Method）android.app.RuntimeInit$Method odAndArgsChread.perform（RuntimeIhread.java:2843）在android.app.ZygoteIhread.handle（ZygoteIhread.java:2921）引起的：android.app.RuntimeException：无法在hread.java:1639ViewModelProvider$AndroidViewModelFacory. create（ViewModelProvider. java： 208）在androidx. l创建android.app.BookViewModel类的实例

ifecycle。ViewModelProvider。在androidx上获取（ViewModelProvider.java:135）。生命周期。ViewModelProvider。获取（ViewModelProvider.java:103）。实例簿记员。主要活动。在android上创建（MainActivity.kt:30）。应用程序。活动在android上执行创建（Activity.java:7074）。应用程序。活动在android上执行创建（Activity.java:7065）。应用程序。仪器仪表。android上的callActivityOnCreate（Instrumentation.java:1214）。应用程序。ActivityThread。performLaunchActivity（ActivityThread.java:2796）。。。9更多原因：java。lang.reflect。java上的InvocationTargetException。lang.reflect。构造函数。java上的newInstance0（本机方法）。lang.reflect。构造函数。androidx上的newInstance（Constructor.java:334）。生命周期。ViewModelProvider$AndroidViewModelFactory。创建（ViewModelProvider.java:200）。。。还有16个原因：java。lang.RuntimeException：找不到com的实现。实例簿记员。BookRoomDatabase。androidx上不存在BookRoomDatabase\u Impl。房间房间androidx上的GetGenerateImplementation（Room.java:94）。房间RoomDatabase$生成器。在com上构建（RoomDatabase.java:952）。实例簿记员。BookRoomDatabase$Companion。getDatabase（BookRoomDatabase.kt:24）位于com。实例簿记员。BookViewModel。（BookViewModel.kt:12）。。。19个以上

下面是我的BookViewModel类，它扩展了AndroidViewModel类

class BookViewModel(application: Application) : AndroidViewModel(application) {

private val bookDao: BookDao

init {
    val bookDb = BookRoomDatabase.getDatabase(application)

    bookDao = bookDb!!.bookDao()
}


fun insert(book: Book) {
    InsertAsyncTask(bookDao).execute(book)
}

companion object {
    private class InsertAsyncTask(private val bookDao: BookDao) :
        AsyncTask<Book, Void, Void>() {
        override fun doInBackground(vararg books: Book): Void? {
            bookDao.insert(books[0])
            return null
        }

    }
}}

主活动下方。kt级

MainActivity : AppCompatActivity() {

 lateinit var bookViewModel: BookViewModel

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

    fab.setOnClickListener { view ->
        val intent =  Intent(this, NewBookActivity::class.java)
        startActivityForResult(intent, NEW_NOTE_ACTIVITY_REQUEST_CODE)
    }
   bookViewModel = ViewModelProviders.of(this).get(BookViewModel::class.java)
}


override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
    super.onActivityResult(requestCode, resultCode, data)

    if (requestCode == NEW_NOTE_ACTIVITY_REQUEST_CODE && resultCode == Activity.RESULT_OK) {
        val id = UUID.randomUUID().toString()
        val authorName = data!!.getStringExtra(NewBookActivity.NEW_AUTHOR)
        val bookName = data!!.getStringExtra(NewBookActivity.NEW_BOOK)

        val book = Book(id, authorName, bookName)
        bookViewModel.insert(book)

        Toast.makeText(
            applicationContext,
            R.string.saved,
            Toast.LENGTH_LONG
        ).show()

    } else {
        Toast.makeText(
            applicationContext,
            R.string.not_saved,
            Toast.LENGTH_LONG
        ).show()
    }
}

companion object{
    private const val NEW_NOTE_ACTIVITY_REQUEST_CODE = 1
}}

下面的数据库类

  @Database(entities = [Book::class], version = 1)
 abstract class BookRoomDatabase :RoomDatabase() {


abstract fun bookDao():BookDao


companion object {

    private var bookRoomInstance:BookRoomDatabase? = null
    fun getDatabase(context: Context): BookRoomDatabase? {
        if (bookRoomInstance == null)
            synchronized(BookRoomDatabase::class.java){
                if(bookRoomInstance == null){
                    bookRoomInstance = Room.databaseBuilder<BookRoomDatabase>(context.applicationContext,
                        BookRoomDatabase::class.java, "book_database")
                        .build()
                }
            }
    return bookRoomInstance
    }

}}

dao级以下

@Dao
 interface BookDao {
  @Insert
  fun insert(book: Book)
  }

帐面下实体类

@Entity(tableName = "books")
class Book(@PrimaryKey val id: String,

       @ColumnInfo(name = "author")
       val author: String,
       val book:String) {
}

Answer 1

问题是您试图在不提供所需参数的情况下实例化ViewModel应用程序：应用程序。我在这里发布了一个解决这个问题的答案：https://stackoverflow.com/a/65004171/13834895.

Answer 2

它背后的想法是，您正在使用kotlin，并且在您的应用程序build.gradle中，您应该应用kapt plugin应用插件'kotlin-kapt'并使用kapt而不是annotationProcess

implementation "android.arch.lifecycle:extensions:$lifecycle_version"
kapt "android.arch.lifecycle:compiler:$lifecycle_version"

implementation "android.arch.persistence.room:runtime:$room_version"
kapt "android.arch.persistence.room:compiler:$room_version"

检查这个答案https://stackoverflow.com/a/50487165/4614550