Room: Cannot access database on the main thread since it may potentially lock the UI for a long period of time

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

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.

Answer 2

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();
}