Android多线程下安全访问数据库 DatabaseManager

public class DatabaseManager {

    private AtomicInteger mOpenCounter = new AtomicInteger();

    private static DatabaseManager instance;

    private static SQLiteOpenHelper mDatabaseHelper;

    private SQLiteDatabase mDatabase;

    public static synchronized void initializeInstance(SQLiteOpenHelper helper) {

        if (instance == null) {

            instance = new DatabaseManager();

            mDatabaseHelper = helper;

        }

    }

    public static synchronized DatabaseManager getInstance() {

        if (instance == null) {

            throw new IllegalStateException(DatabaseManager.class.getSimpleName() +

                    " is not initialized, call initializeInstance(..) method first.");

        }

        return instance;

    }

    public synchronized SQLiteDatabase openDatabase() {

        if(mOpenCounter.incrementAndGet() == 1) {

            // Opening new database

            mDatabase = mDatabaseHelper.getWritableDatabase();

        }

        return mDatabase;

    }

    public synchronized void closeDatabase() {

        if(mOpenCounter.decrementAndGet() == 0) {

            // Closing database

            mDatabase.close();

        }

    }}