crash - 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 > 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<User> 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.

Answer 1

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()