Problems with concurrent access to sqlitedatabase on Android?

I am making some tests with my database connection factory on Android.

public class ConnectionFactory {

    private Context context;
    private SQLiteDatabase w;

    private static DatabaseManager manager;

    public ConnectionFactory(Context context) {
        this.context = context;
    }

    public synchronized SQLiteDatabase getDatabase(){
        if(manager == null){
            DatabaseManager.init(new DatabaseHelper(context));
            manager = DatabaseManager.getInstance();
        }

        return w == null || !w.isOpen() ? w = manager.getDatabase() : w;
    }

    public synchronized void close() {
        if(w != null && w.isOpen()){
            manager.close();
        }
    }

}

my DatabaseManager (I made this class for scenarios of concurrent access to database)

public class DatabaseManager {

    private static SQLiteOpenHelper helper;
    private static DatabaseManager instance;
    private static int COUNTER = 0;

    private SQLiteDatabase database;

    private DatabaseManager(){}

    public static synchronized void init(SQLiteOpenHelper first) {
        if (instance == null) {
            helper = first;

            instance = new DatabaseManager(){

                @Override
                protected void finalize() throws Throwable {
                    helper.close();
                    COUNTER = 0;

                    helper = null;
                }

            };

        }
    }

    public static synchronized DatabaseManager getInstance() {
        if (instance == null) {
            throw new IllegalStateException("O método init deve ser chamado antes deste método.");
        }

        return instance;
    }

    public synchronized SQLiteDatabase getDatabase() {
        COUNTER++;

        if(COUNTER == 1) {
            Log.i("SIAEP", "====================== ABRINDO UMA NOVA REFERENCIA DE SQLITEDATABASE ======================");
            database = helper.getWritableDatabase();
        }

        Log.i("SIAEP", "====================== DATABASE MANAGER COUNTER 'getDatabase': "+ COUNTER +" ======================");

        return database;
    }

    public synchronized void close() {
        if(COUNTER != 0)
            COUNTER--;

        if(COUNTER == 0) {

            if(database.inTransaction()){
                database.endTransaction();
            }

            Log.i("SIAEP", "====================== FECHANDO A REFERENCIA DE SQLITEDATABASE ======================");

            database.close();
        }

        Log.i("SIAEP", "====================== DATABASE MANAGER COUNTER 'close': "+ COUNTER +" ======================");
    }


}

my class of test:

public class TesteActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        Worker t1 = new Worker(this);
        Worker t2 = new Worker(this);

        t1.start();
        t2.start();
    }

    public static class Worker extends Thread {

        private Activity instance;

        public Worker(Activity instance) {
            this.instance = instance;
        }

        @Override
        public void run() {

            ConnectionFactory factory = new ConnectionFactory(instance);

            for (int i = 0; i < 100; i++) {
                SQLiteDatabase database = factory.getDatabase();
                factory.close();
            }
        }
    }

}

this works perfectly, but when I use transactions, I receive some exceptions:

    @Override
    public void run() {

        ConnectionFactory factory = new ConnectionFactory(instance);

        for (int i = 0; i < 100; i++) {
            SQLiteDatabase database = factory.getDatabase();

            database.beginTransaction();
            database.setTransactionSuccessful();
            database.endTransaction();

            factory.close();
        }
    }

excetion:

E/ACRA ( 9850): java.lang.IllegalStateException: attempt to re-open an already-closed object: SQLiteDatabase: /data/data/seduc.ma.com.br.siaepmovel/databases/database.sqlite E/ACRA ( 9850): at android.database.sqlite.SQLiteClosable.acquireReference(SQLiteClosable.java:55) E/ACRA ( 9850): at android.database.sqlite.SQLiteDatabase.beginTransaction(SQLiteDatabase.java:503) E/ACRA ( 9850): at android.database.sqlite.SQLiteDatabase.beginTransaction(SQLiteDatabase.java:416) E/ACRA ( 9850): at seduc.ma.com.br.siaepmovel.test.TesteActivity$Worker.run(TesteActivity.java:45)

any idea why?