Question

My application has a SQLite database to contain a list of highways. It shows them all in a list. It first tries to query the database for the roads. If the query doesn’t return any, it calls another method that downloads a list from a remote server, and populates the database. After that returns, it immediately queries the database again.

That’s how it should work. How it actually works is, the first query always returns nothing. It goes straight to downloading a fresh list, it inserts the list into the database, and queries again. The second query always returns the correct result. The strange thing is, I can repeat the operation without even exiting the application. Using the adb shell, I can read the SQLite3 database on the emulator. The data shown in the database is exactly as expected. But the application is behaving as though the data isn’t there? Is there some behaviour I’m not aware of? Here’s the code.

RoadsDataSource.java

public class RoadsDataSource {

    private DataStorage data;

    private static Context context;

    public RoadsDataSource() {
        this.data = new DataStorage(RoadsDataSource.context);
    }

    private List<Road> getRoads(Integer state) {
        List<Road> roads = loadRoadsFromDb(state);
        if (roads.isEmpty()) {
            Request api = new Request(RoadsDataSource.context);
            Roads apiRoads = api.fetchRoads(state);
            this.data.storeRoads(apiRoads);
            roads = loadRoadsFromDb(state);
        }
        return roads;
    }

    private List<Road> loadRoadsFromDb(Integer state) {
        SQLiteQueryBuilder query = new SQLiteQueryBuilder();
        query.setTables(Queries.ROAD_STATE_MATCHES);
        Cursor results = query.query(
                this.data.getWritableDatabase(),
                new String[] {Tables.ROADS + "." + Tables.Roads.ID, Tables.ROADS + "." + Tables.Roads.TYPE, Tables.ROADS + "." + Tables.Roads.NUMBER}, 
                Queries.ROADS_BY_STATE,
                new String[] {state.toString()}, null, null, null 
        );

        List<Road> roads = new ArrayList<Road>();

        results.moveToFirst();
        while (!results.isAfterLast()) {
            roads.add(new Road(results.getInt(0), results.getString(1), results.getInt(2)));
            results.moveToNext();
        }
        results.close();

        System.out.println(roads.size());
        return roads;
    }
}

DataStorage.java

public class DataStorage extends SQLiteOpenHelper {
    public void storeRoads(Roads roads) {
        SQLiteDatabase db = this.getWritableDatabase();
        for (Road road : roads.getRoads()) {
            ContentValues roadRow = new ContentValues();
            roadRow.put(Tables.Roads.ID, road.getId());
            roadRow.put(Tables.Roads.TYPE, road.getType());
            roadRow.put(Tables.Roads.NUMBER, road.getNumber());
            try {
                db.insertOrThrow(Tables.ROADS, null, roadRow);
            } catch (SQLException e) {
            }
            ContentValues linkRow = new ContentValues();
            linkRow.put(Tables.StatesRoads.STATE_ID, roads.getState());
            linkRow.put(Tables.StatesRoads.ROAD_ID, road.getId());
            try {
                db.insertOrThrow(Tables.STATES_ROADS, null, linkRow);
            } catch (SQLException e) {
            }
        }
    }
}

Answer 1

I’ve had huge issues in the past with not closing the database handle – generally I do all db operations inside an SQLiteOpenHelper subclass, keeping a reference to the db, opening and closing it atomically.