For those working with Kotlin, try changing annotationProcessor to kapt in the apps build.gradle

for example:

// Extensions = ViewModel + LiveDataimplementation "android.arch.lifecycle:extensions:1.1.0"kapt "android.arch.lifecycle:compiler:1.1.0"// Roomimplementation "android.arch.persistence.room:runtime:1.0.0"kapt "android.arch.persistence.room:compiler:1.0.0"

also remember to add this plugin

apply plugin: 'kotlin-kapt'

to the top of the app level build.gradle file and do a clean and rebuild (according to https://codelabs.developers.google.com/codelabs/android-room-with-a-view/#6)

In Android Studio, if you get errors when you paste code or during the build process, select Build >Clean Project. Then select Build > Rebuild Project, and then build again.

UPDATE

If you have migrated to androidx

def room_version = "2.3.0" // check latest version from docsimplementation "androidx.room:room-runtime:$room_version"kapt "androidx.room:room-compiler:$room_version"

UPDATE 2 (since July 2021)

def room_version = "2.3.0" // check latest version from docsimplementation "androidx.room:room-ktx:$room_version"kapt "androidx.room:room-compiler:$room_version"

Just use

apply plugin: 'kotlin-kapt'

in app build.gradle

And keep both in dependencies

annotationProcessor "android.arch.persistence.room:compiler:$rootProject.roomVersion"kapt "android.arch.persistence.room:compiler:$rootProject.roomVersion"

EDIT

In newer version don't need to add both dependencies at a time Just use, hope it will work.

kapt 'android.arch.persistence.room:compiler:1.1.1'

I had this error when I missed

@Database(entity="{<model.class>})

Ensure that the entity model specified in the annotation above refers to the particular model class and also ensure that the necessary annotation:

@Entity(tableName = "<table_name>" ...)

is properly defined and you'd be good