Room: Cannot access database on the main thread si

Question

In the main activity, I have LiveData which contains members and a click listener. If I click on a member, then his ID is passed with intent.putExtra. That ID is later passed on to the method open in this activity. With this activity, I want to see the details of a member. In my MemberInfo activity, I marked a line where my problem lies. It shows me this error: Cannot access database on the main thread since it may potentially lock the UI for a long period of time.

My DAO consists this code:

@Query("SELECT * FROM member_table WHERE MemberID=:id")
Member getMemberInfo(long id);

This is my main activity:

public class MemberMainActivity extends AppCompatActivity implements MemberListAdapter.MemberClickListener{

private MemberViewModel mMemberViewModel;
private List<Member> mMember;

void setMember(List<Member> members) {
    mMember = members;
}

public static final int NEW_MEMBER_ACTIVITY_REQUEST_CODE = 1;

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_member);
    Toolbar toolbar = findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    FloatingActionButton fab = findViewById(R.id.fab);
    fab.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Intent intent = new Intent(MemberMainActivity.this, NewMemberActivity.class);
            startActivityForResult(intent, NEW_MEMBER_ACTIVITY_REQUEST_CODE);
        }
    });

    RecyclerView recyclerView = findViewById(R.id.recyclerviewcard_member);
    final MemberListAdapter adapter = new MemberListAdapter(this);
    recyclerView.setAdapter(adapter);
    recyclerView.setLayoutManager(new LinearLayoutManager(this));

    mMemberViewModel = ViewModelProviders.of(this).get(MemberViewModel.class);

    mMemberViewModel.getAllMember().observe(this, new Observer<List<Member>>() {
        @Override
        public void onChanged(@Nullable final List<Member> members) {
            mMember = members;
            // Update the cached copy of the words in the adapter.
            adapter.setMember(members);
        }
    });

}

public void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);

    if (requestCode == NEW_MEMBER_ACTIVITY_REQUEST_CODE && resultCode == RESULT_OK) {
        Member member = new Member(data.getStringExtra(NewMemberActivity.EXTRA_REPLY), data.getStringExtra(NewMemberActivity.EXTRA_REPLY2));
        mMemberViewModel.insert(member);
    } else {
        Toast.makeText(
                getApplicationContext(),
                R.string.empty_not_saved,
                Toast.LENGTH_LONG).show();
    }
}

public void onMemberClick(int position) {
    Member member = mMember.get(position);
    Intent intent = new Intent(getApplicationContext(),MemberInfo.class);
    intent.putExtra("MemberID", member.getId());
    MemberInfo.open(this, member.getId());

}

}

This is my activity:

public class MemberInfo extends AppCompatActivity {

public static void open(Activity activity, long memberid) {
    Intent intent = new Intent(activity, MemberInfo.class);
    intent.putExtra("MemberID", memberid);
    activity.startActivity(intent);
}

private List<Member> mMember;
private MemberViewModel mMemberViewModel;


void setMember(List<Member> members){
    mMember = members;
}

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_memberinfo);

    Log.i("okay", "memberinfo");
    Intent intent = getIntent();
    if (intent != null && intent.hasExtra("MemberID")) {
        long memberid = intent.getLongExtra("MemberID", -1);
        // TODO: get customer details based on customer id
        TextView firstname = findViewById(R.id.layout_memberfirstname);
        TextView surname = findViewById(R.id.layout_membersurname);
        TextView balance = findViewById(R.id.layout_memberbalance);
        -------------Member member = MemberRoomDatabase.getDatabase().memberDao().getMemberInfo(memberid);-------------
        firstname.setText(member.getFirstname());
        surname.setText(member.getSurname());

    }
    else {
        Toast.makeText(
                getApplicationContext(),
                R.string.empty_not_saved,
                Toast.LENGTH_LONG).show();
    }
}
}

I thought that maybe it is because I'm missing a AsyncTask method. I tried this, but this also didn't work:

private static class insertMemberInfoAsyncTask extends AsyncTask<Member, Void, Void> {

    private MemberDao mAsyncTaskDao;

    insertMemberInfoAsyncTask(MemberDao dao) {
        mAsyncTaskDao = dao;
    }

    @Override
    protected Void doInBackground(Member... params) {
        Member member = params[0];
        mAsyncTaskDao.getMemberInfo(member.getId());
        return null;
    }
}

public Member getMemberInfo(long id) {
    mAllMember = mMemberDao.getAllMember();
    Member member = mMemberDao.getMemberInfo(id);
    new insertMemberInfoAsyncTask(mMemberDao).execute(member);
    return member;
}

I think I use the method wrong. Can anybody help me?

Answer 1

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() or using LiveData.

My dao query:-

@Query("SELECT * from user_data_table where SNO = 1")
UserData getDefaultData();

My repository method:-

public UserData getDefaultData() throws ExecutionException, InterruptedException {

    Callable<UserData> callable = new Callable<UserData>() {
        @Override
        public UserData call() throws Exception {
            return userDao.getDefaultData();
        }
    };

    Future<UserData> future = Executors.newSingleThreadExecutor().submit(callable);

    return future.get();
}

Answer 2

Using Future and Callables can be an alternative here. By using Future and Callable you can get rid of AsyncTask and forcing your queries to the main thread.

The syntax would be as follow -

@Throws(ExecutionException::class, InterruptedException::class)
private fun canContinue(id: String): UserData{

    val callable = Callable { userDao.getDefaultData() }

    val future = Executors.newSingleThreadExecutor().submit(callable)

    return future!!.get()
}

And, don't forget the null check for the data returned. Because it might be null

Answer 3

One option is to update your query to this:

@Query("SELECT * FROM member_table WHERE MemberID=:id")
LiveData<Member> getMemberInfo(long id);

(or similar, using Flowable). This avoids the need to manually create your own AsyncTask.

Returning the LiveData wrapper around the Member type automatically signals to Room that the query can/should be performed asynchronously. Per https://developer.android.com/training/data-storage/room/accessing-data (my emphasis):

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.