Android Room - simple select query - Cannot access database on the main thread

Question

I am trying a sample with Room Persistence Library  I created an Entity    Entity public class Agent        PrimaryKey     public String guid      public String name      public String email      public String password      public String phone      public String licence      Created a DAO class    Dao public interface AgentDao        Query  SELECT COUNT    FROM Agent where email    email OR phone    phone OR licence    licence       int agentsCount String email  String phone  String licence         Insert     void insertAgent Agent agent       Created the Database class    Database entities    Agent class   version   1  public abstract class AppDatabase extends RoomDatabase       public abstract AgentDao agentDao        Exposed database using below subclass in Kotlin   class MyApp   Application          companion object DatabaseSetup           var database  AppDatabase    null            override fun onCreate             super onCreate           MyApp database    Room databaseBuilder this  AppDatabase  class java   MyDatabase   build             Implemented below function in my activity   void signUpAction View view            String email   editTextEmail getText   toString            String phone   editTextPhone getText   toString            String license   editTextLicence getText   toString             AgentDao agentDao   MyApp DatabaseSetup getDatabase   agentDao              1  Check if agent already exists         int agentsCount   agentDao agentsCount email  phone  license           if  agentsCount  gt  0                  2  If it already exists then prompt user             Toast makeText this   Agent already exists    Toast LENGTH LONG  show                      else               Toast makeText this   Agent does not exist  Hurray      Toast LENGTH LONG  show                onBackPressed                      Unfortunately on execution of above method it crashes with below stack trace       FATAL EXCEPTION  main  Process  com example me MyApp  PID  31592 java lang IllegalStateException  Could not execute method for android onClick     at android support v7 app AppCompatViewInflater DeclaredOnClickListener onClick AppCompatViewInflater java 293      at android view View performClick View java 5612      at android view View PerformClick run View java 22288      at android os Handler handleCallback Handler java 751      at android os Handler dispatchMessage Handler java 95      at android os Looper loop Looper java 154      at android app ActivityThread main ActivityThread java 6123      at java lang reflect Method invoke Native Method      at com android internal os ZygoteInit MethodAndArgsCaller run ZygoteInit java 867      at com android internal os ZygoteInit main ZygoteInit java 757   Caused by  java lang reflect InvocationTargetException     at java lang reflect Method invoke Native Method      at android support v7 app AppCompatViewInflater DeclaredOnClickListener onClick AppCompatViewInflater java 288      at android view View performClick View java 5612        at android view View PerformClick run View java 22288        at android os Handler handleCallback Handler java 751        at android os Handler dispatchMessage Handler java 95        at android os Looper loop Looper java 154        at android app ActivityThread main ActivityThread java 6123        at java lang reflect Method invoke Native Method        at com android internal os ZygoteInit MethodAndArgsCaller run ZygoteInit java 867        at com android internal os ZygoteInit main ZygoteInit java 757     Caused by  java lang IllegalStateException  Cannot access database on the main thread since it may potentially lock the UI for a long periods of time      at android arch persistence room RoomDatabase assertNotMainThread RoomDatabase java 137      at android arch persistence room RoomDatabase query RoomDatabase java 165      at com example me MyApp RoomDb Dao AgentDao Impl agentsCount AgentDao Impl java 94      at com example me MyApp View SignUpActivity signUpAction SignUpActivity java 58      at java lang reflect Method invoke Native Method        at android support v7 app AppCompatViewInflater DeclaredOnClickListener onClick AppCompatViewInflater java 288        at android view View performClick View java 5612        at android view View PerformClick run View java 22288        at android os Handler handleCallback Handler java 751        at android os Handler dispatchMessage Handler java 95        at android os Looper loop Looper java 154        at android app ActivityThread main ActivityThread java 6123        at java lang reflect Method invoke Native Method        at com android internal os ZygoteInit MethodAndArgsCaller run ZygoteInit java 867        at com android internal os ZygoteInit main ZygoteInit java 757      Seems like that problem is related to execution of db operation on main thread  However the sample test code provided in above link does not run on a separate thread    Test     public void writeUserAndReadInList   throws Exception           User user   TestUtil createUser 3           user setName  george            mUserDao insert user           List lt User gt  byName   mUserDao findUsersByName  george            assertThat byName get 0   equalTo user            Am I missing anything over here  How can I make it execute without crash  Please suggest

User · Answer

In my opinion the right thing to do is to delegate the query to an IO thread using RxJava.

I have an example of a solution to an equivalent problem I've just encountered.

((ProgressBar) view.findViewById(R.id.progressBar_home)).setVisibility(View.VISIBLE);//Always good to set some good feedback
        Completable.fromAction(() -> {
            //Creating view model requires DB access
            homeViewModel = new ViewModelProvider(this, factory).get(HomeViewModel.class);
        }).subscribeOn(Schedulers.io())//The DB access executes on a non-main-thread thread
        .observeOn(AndroidSchedulers.mainThread())//Upon completion of the DB-involved execution, the continuation runs on the main thread
        .subscribe(
                () ->
                {
                    mAdapter = new MyAdapter(homeViewModel.getExams());
                    recyclerView.setAdapter(mAdapter);
                    ((ProgressBar) view.findViewById(R.id.progressBar_home)).setVisibility(View.INVISIBLE);
                },
                error -> error.printStackTrace()
        );

And if we want to generalize the solution:

((ProgressBar) view.findViewById(R.id.progressBar_home)).setVisibility(View.VISIBLE);//Always good to set some good feedback
        Completable.fromAction(() -> {
            someTaskThatTakesTooMuchTime();
        }).subscribeOn(Schedulers.io())//The long task executes on a non-main-thread thread
        .observeOn(AndroidSchedulers.mainThread())//Upon completion of the DB-involved execution, the continuation runs on the main thread
        .subscribe(
                () ->
                {
                    taskIWantToDoOnTheMainThreadWhenTheLongTaskIsDone();
                },
                error -> error.printStackTrace()
        );

User · Answer

Kotlin Coroutines  Clear  amp  Concise   AsyncTask is really clunky  Coroutines are a cleaner alternative  just sprinkle a couple of keywords and your sync code becomes async        Step 1  add  suspend  to your fun suspend fun roomFun       Int suspend fun notRoomFun        withContext Dispatchers IO              Step 2  launch from coroutine scope private fun myFun         lifecycleScope launch      coroutine on Main         val queryResult   roomFun         coroutine on IO         doStuff         back on Main           Dependencies  adds coroutine scopes for arch components       lifecycleScope  implementation  androidx lifecycle lifecycle-runtime-ktx 2 2 0-alpha04      viewModelScope  implementation  androidx lifecycle lifecycle-viewmodel-ktx 2 2 0-alpha04    -- Updates  08-May-2019  Room 2 1 now supports suspend 13-Sep-2019  Updated to use Architecture components scope

User · Answer

With the Jetbrains Anko library  you can use the doAsync     method to automatically execute database calls  This takes care of the verbosity problem you seemed to have been having with mcastro s answer    Example usage        doAsync            Application database myDAO   insertUser user           I use this frequently for inserts and updates  however for select queries I  reccommend using the RX workflow

User · Answer

Just do the database operations in a separate Thread  Like this  Kotlin    Thread        Do your database  s operations here   start

User · Answer

If you are more comfortable with Async task     new AsyncTask lt Void  Void  Integer gt                       Override                 protected Integer doInBackground Void    voids                        return Room databaseBuilder getApplicationContext                                AppDatabase class  DATABASE NAME                               fallbackToDestructiveMigration                                build                                getRecordingDAO                                getAll                                size                                        Override                 protected void onPostExecute Integer integer                        super onPostExecute integer                       Toast makeText HomeActivity this   Found     integer  Toast LENGTH LONG  show                                    execute

User · Answer

Simply you can use this code for solve it    Executors newSingleThreadExecutor   execute new Runnable                          Override                     public void run                             appDb daoAccess   someJobes     replace with your code                                             Or in lambda you can use this code   Executors newSingleThreadExecutor   execute    - gt  appDb daoAccess   someJobes       You can replace appDb daoAccess   someJobes   with your own code

User · Answer

You cannot run it on main thread instead use handlers  async or working threads   A sample code is available here and read article over room library here     Android s Room Library          Insert and get data using Database Async way     AsyncTask execute new Runnable          Override     public void run                Insert Data         AppDatabase getInstance context  userDao   insert new User 1  James   Mathew                 Get Data         AppDatabase getInstance context  userDao   getAllUsers                If you want to run it on main thread which is not preferred way    You can use this method to achieve on main thread Room inMemoryDatabaseBuilder

User · Answer

You can allow database access on the main thread but only for debugging purpose  you shouldn t do this on production   Here is the reason   Note  Room doesn t support database access on the main thread unless you ve called allowMainThreadQueries   on the builder because it might lock the UI for a long period of time  Asynchronous queries   queries that return instances of LiveData or Flowable   are exempt from this rule because they asynchronously run the query on a background thread when needed

User · Answer

The error message       Cannot access database on the main thread since it may potentially lock the UI for a long periods of time    Is quite descriptive and accurate    The question is how should you avoid accessing the database on the main thread   That is a huge topic  but to get started  read about AsyncTask  click here   -----EDIT----------  I see you are having problems when you run a unit test   You have a couple of choices to fix this    Run the test directly on the development machine rather than on an Android device  or emulator    This works for tests that are database-centric and don t really care whether they are running on a device  Use the annotation  RunWith AndroidJUnit4 class  to run the test on the android device  but not in an activity with a UI  More details about this can be found in this tutorial

User · Answer

An elegant RxJava Kotlin solution is to use Completable fromCallable  which will give you an Observable which does not return a value  but can observed and subscribed on a different thread     public Completable insert Event event        return Completable fromCallable new Callable lt Void gt               Override         public Void call   throws Exception               return database eventDao   insert event                      Or in Kotlin      fun insert event  Event    Completable   Completable fromCallable       database eventDao   insert event      You can the observe and subscribe as you would usually    dataManager insert event       subscribeOn scheduler       observeOn AndroidSchedulers mainThread         subscribe

User · Answer

Database access on main thread locking the UI is the error  like Dale said  --EDIT 2-- Since many people may come across this answer    The best option nowadays  generally speaking  is Kotlin Coroutines  Room now supports it directly  currently in beta   https   kotlinlang org docs reference coroutines-overview html https   developer android com jetpack androidx releases room 2 1 0-beta01 --EDIT 1-- For people wondering    You have other options  I recommend taking a look into the new ViewModel and LiveData components  LiveData works great with Room  https   developer android com topic libraries architecture livedata html Another option is the RxJava RxAndroid  More powerful but more complex than LiveData  https   github com ReactiveX RxJava --Original answer-- Create a static nested class  to prevent memory leak  in your Activity extending AsyncTask  private static class AgentAsyncTask extends AsyncTask lt Void  Void  Integer gt           Prevent leak     private WeakReference lt Activity gt  weakActivity      private String email      private String phone      private String license       public AgentAsyncTask Activity activity  String email  String phone  String license            weakActivity   new WeakReference lt  gt  activity           this email   email          this phone   phone          this license   license              Override     protected Integer doInBackground Void    params            AgentDao agentDao   MyApp DatabaseSetup getDatabase   agentDao            return agentDao agentsCount email  phone  license               Override     protected void onPostExecute Integer agentsCount            Activity activity   weakActivity get            if activity    null                return                     if  agentsCount  gt  0                  2  If it already exists then prompt user             Toast makeText activity   quot Agent already exists  quot   Toast LENGTH LONG  show              else               Toast makeText activity   quot Agent does not exist  Hurray    quot   Toast LENGTH LONG  show                activity onBackPressed                       Or you can create a final class on its own file  Then execute it in the signUpAction View view  method  new AgentAsyncTask this  email  phone  license  execute     In some cases you might also want to hold a reference to the AgentAsyncTask in your activity so you can cancel it when the Activity is destroyed  But you would have to interrupt any transactions yourself  Also  your question about the Google s test example    They state in that web page   The recommended approach for testing your database implementation is writing a JUnit test that runs on an Android device  Because these tests don t require creating an activity  they should be faster to execute than your UI tests   No Activity  no UI

User · Answer

You have to execute request in background  A simple way could be using an Executors    Executors newSingleThreadExecutor   execute       yourDb yourDao yourRequest     Replace this by your request

User · Answer

You can use Future and Callable  So you would not be required to write a long asynctask and can perform your queries without adding allowMainThreadQueries     My dao query -   Query  SELECT   from user data table where SNO   1   UserData getDefaultData      My repository method -  public UserData getDefaultData   throws ExecutionException  InterruptedException        Callable lt UserData gt  callable   new Callable lt UserData gt               Override         public UserData call   throws Exception               return userDao getDefaultData                          Future lt UserData gt  future   Executors newSingleThreadExecutor   submit callable        return future get

User · Answer

With lambda its easy to run with AsyncTask   AsyncTask execute    - gt    run your query here

User · Answer

It s not recommended but you can access to database on main thread with allowMainThreadQueries    MyApp database    Room databaseBuilder this  AppDatabase  class java   MyDatabase   allowMainThreadQueries   build

User · Answer

Update  I also got this message when I was trying to build a query using  RawQuery and SupportSQLiteQuery inside the DAO    Transaction public LiveData lt List lt MyEntity gt  gt  getList MySettings mySettings          return getMyList    -- gt this is ok      return getMyList new SimpleSQLiteQuery  select   from mytable     -- gt  this is an error   Solution  build the query inside the ViewModel and pass it to the DAO   public MyViewModel Application application                list   Transformations switchMap searchParams  params - gt                 StringBuilder sql              sql   new StringBuilder  select                       return appDatabase rawDao   getList new SimpleSQLiteQuery sql toString                           Or     You should not access the database directly on the main thread  for example    public void add MyEntity item         appDatabase myDao   add item         You should use AsyncTask for update  add  and delete operations    Example   public class MyViewModel extends AndroidViewModel        private LiveData lt List lt MyEntity gt  gt  list       private AppDatabase appDatabase       public MyViewModel Application application            super application            appDatabase   AppDatabase getDatabase this getApplication             list   appDatabase myDao   getItems               public LiveData lt List lt MyEntity gt  gt  getItems             return list             public void delete Obj item            new deleteAsyncTask appDatabase  execute item              private static class deleteAsyncTask extends AsyncTask lt MyEntity  Void  Void gt             private AppDatabase db           deleteAsyncTask AppDatabase appDatabase                db   appDatabase                      Override         protected Void doInBackground final MyEntity    params                db myDao   delete  params 0                 return null                       public void add final MyEntity item            new addAsyncTask appDatabase  execute item              private static class addAsyncTask extends AsyncTask lt MyEntity  Void  Void gt             private AppDatabase db           addAsyncTask AppDatabase appDatabase                db   appDatabase                      Override         protected Void doInBackground final MyEntity    params                db myDao   add  params 0                 return null                       If you use LiveData for select operations  you don t need AsyncTask

User · Answer

For quick queries you can allow room to execute it on UI thread   AppDatabase db   Room databaseBuilder context getApplicationContext            AppDatabase class  DATABASE NAME  allowMainThreadQueries   build      In my case I had to figure out of the clicked user in list exists in database or not  If not then create the user and start another activity          Override         public void onClick View view                   int position   getAdapterPosition                 User user   new User                String name   getName position               user setName name                AppDatabase appDatabase   DatabaseCreator getInstance mContext  getDatabase                UserDao userDao   appDatabase getUserDao                ArrayList lt User gt  users   new ArrayList lt User gt                 users add user               List lt Long gt  ids   userDao insertAll users                Long id   ids get 0               if id    -1                                user   userDao getUser name                   user setId user getId                               else                               user setId id                              Intent intent   new Intent mContext  ChatActivity class               intent putExtra ChatActivity EXTRAS USER  Parcels wrap user                mContext startActivity intent

User · Answer

As asyncTask are deprecated we may use executor service  OR you can also use ViewModel with LiveData as explained in other answers   For using executor service  you may use something like below   public class DbHelper        private final Executor executor   Executors newSingleThreadExecutor         public void fetchData DataFetchListener dataListener           executor execute    - gt                    Object object   retrieveAgent agentId                   new Handler Looper getMainLooper    post    - gt                            dataListener onFetchDataSuccess object                                             Main Looper is used  so that you can access UI element from onFetchDataSuccess callback

User · Answer

For all the RxJava or RxAndroid or RxKotlin lovers out there  Observable just db             subscribeOn Schedulers io               subscribe   db - gt     database operation

[android] Android Room - simple select query - Cannot access database on the main thread

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