Android: SQLite Query Return 0 Rows Without Reload?

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

Mo Kargas is right. Your db helper should look more like this http://code.google.com/p/android-notes/source/browse/trunk/src/com/bitsetters/android/notes/DBHelper.java?r=10

This may fix your issue though Replace

SQLiteDatabase db = this.getWritableDatabase();

with this

SQLiteDatabase db;
try {
    db = dbHelper.getWritableDatabase();
} catch (SQLiteException e) {
   db = dbHelper.getReadableDatabase();
}

Answer 2

过去，我有很多不关闭数据库句柄的问题-通常，我在SQLiteOpenHelper子类中执行所有数据库操作，保留对数据库的引用，以原子方式打开和关闭它。