java - 为什么Fragment无法连接数据库

Question

我有 2 个类似的项目。第一个，我可以连接数据库。然而，在第二个我不能。

它们之间的区别:

第一个项目:我在 mainActivity 中使用 dbconnection

第二个项目:我在fragment中使用dbconnection，然后将数据传递给cardAdapter

fragment :

public class Frag1 extends Fragment {
    private RecyclerView rv;
    private CardAdapter adapter;
    private EditText editText;
    private ImageButton imageButton;
    private DbConnection db= new DbConnection(getActivity());

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {


      View view =inflater.inflate(R.layout.fragment_frag1, container,false);
        rv= (RecyclerView)view.findViewById(R.id.rv);
        editText=view.findViewById(R.id.editText);
        rv.setHasFixedSize(true);
        rv.setLayoutManager(new LinearLayoutManager(this.getActivity()));


        final ArrayList<Word> words = new ArrayList<>() ;
        dbCopy();

        final ArrayList<Word> wordArrayList = new WordData().AllWords(db);

        adapter= new CardAdapter(getActivity(),wordArrayList);
        rv.setAdapter(adapter);

        return view;
    }
    public void dbCopy(){
        DatabaseCopyHelper helper = new DatabaseCopyHelper(getActivity())  ;
        try {
            helper.createDataBase();
        } catch (IOException e) {
            e.printStackTrace();
        }
        helper.openDataBase();
    }
}

卡适配器:

public class CardAdapter extends RecyclerView.Adapter<CardAdapter.CardTasarimTutucu> {
    private Context mContext;
    private ArrayList<Word> words;


    public CardAdapter(Context mContext, ArrayList<Word> words) {
        this.mContext = mContext;
        this.words =  words;
    }


    public class CardTasarimTutucu extends RecyclerView.ViewHolder{
        private CardView rowCard;
        private CheckedTextView rowText;

        public CardTasarimTutucu(@NonNull View itemView) {
            super(itemView);
            this.rowCard = itemView.findViewById(R.id.rowCard);
            this.rowText = itemView.findViewById(R.id.rowText);
        }
    }
    @NonNull
    @Override
    public CardTasarimTutucu onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View v = LayoutInflater.from(parent.getContext()).inflate(R.layout.card_design,parent,false);
        return new CardTasarimTutucu(v);
    }

    @Override
    public void onBindViewHolder(@NonNull final CardTasarimTutucu holder, int position) {
        String word = words.get(position).getMean();
        holder.rowText.setText(word);
        holder.rowText.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if (holder.rowText.isChecked()) {
// set cheek mark drawable and set checked property to false

                    holder.rowText.setCheckMarkDrawable(null);
                    holder.rowText.setChecked(false);
                } else {
// set cheek mark drawable and set checked property to true

                    holder.rowText.setCheckMarkDrawable(R.drawable.ic_check_black_24dp);
                    holder.rowText.setChecked(true);
                }

            }
        });

    }

    @Override
    public int getItemCount() {
        return words.size();
    }

}

DatabaseCopyHelper

public class DatabaseCopyHelper extends SQLiteOpenHelper  {


    //The Android's default system path of your application database.
    String DB_PATH =null;

    private static String DB_NAME = "dictionary.db";

    private SQLiteDatabase myDataBase;

    private final Context myContext;

    /**
     * Constructor
     * Takes and keeps a reference of the passed context in order to access to the application assets and resources.
     * @param context
     */
    public DatabaseCopyHelper(Context context) {

        super(context, DB_NAME, null, 1);
        this.myContext = context;
        DB_PATH="/data/data/"+context.getPackageName()+"/"+"databases/";


    }

    /**
     * Creates a empty database on the system and rewrites it with your own database.
     * */
    public void createDataBase() throws IOException{

        boolean dbExist = checkDataBase();

        if(dbExist){
            //do nothing - database already exist
        }else{

            //By calling this method and empty database will be created into the default system path
            //of your application so we are gonna be able to overwrite that database with our database.
            this.getReadableDatabase();

            try {

                copyDataBase();

            } catch (IOException e) {

                throw new Error("Error copying database");

            }
        }

    }

    /**
     * Check if the database already exist to avoid re-copying the file each time you open the application.
     * @return true if it exists, false if it doesn't
     */
    private boolean checkDataBase(){

        SQLiteDatabase checkDB = null;

        try{
            String myPath = DB_PATH + DB_NAME;
            checkDB = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READONLY);

        }catch(SQLiteException e){

            //database does't exist yet.

        }

        if(checkDB != null){

            checkDB.close();

        }

        return checkDB != null ? true : false;
    }

    /**
     * Copies your database from your local assets-folder to the just created empty database in the
     * system folder, from where it can be accessed and handled.
     * This is done by transfering bytestream.
     * */
    private void copyDataBase() throws IOException{

        //Open your local db as the input stream
        InputStream myInput = myContext.getAssets().open(DB_NAME);


        // Path to the just created empty db
        String outFileName = DB_PATH + DB_NAME;

        //Open the empty db as the output stream
        OutputStream myOutput = new FileOutputStream(outFileName);

        //transfer bytes from the inputfile to the outputfile
        byte[] buffer = new byte[1024];
        int length;
        while ((length = myInput.read(buffer))>0){
            myOutput.write(buffer, 0, length);
        }

        //Close the streams
        myOutput.flush();
        myOutput.close();
        myInput.close();

    }

    public void openDataBase() throws SQLException{

        //Open the database
        String myPath = DB_PATH + DB_NAME;
        myDataBase = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READONLY);


    }

    @Override
    public synchronized void close() {

        if(myDataBase != null)
            myDataBase.close();

        super.close();

    }

    @Override
    public void onCreate(SQLiteDatabase db) {

    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

    }


    @RequiresApi(api = Build.VERSION_CODES.JELLY_BEAN)
    @Override
    public void onOpen(SQLiteDatabase db) {
        super.onOpen(db);
        db.disableWriteAheadLogging();
    }
    //return cursor

}

DbConnection

public class DbConnection extends SQLiteOpenHelper {

    public DbConnection (@Nullable Context context) {

        super(context, "dictionary.db", null, 1);


    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL("CREATE TABLE IF NOT EXISTS \"Words\" (\n" +
                "\t\"Id\"\tINTEGER NOT NULL PRIMARY KEY AUTOINCREMENT,\n" +
                "\t\"Name\"\tTEXT,\n" +
                "\t\"Mean\"\tTEXT\n" +
                ");");
        db.execSQL("CREATE TABLE IF NOT EXISTS \"MyList\" (\n" +
                "\t\"Id\"\tINTEGER NOT NULL,\n" +
                "\t\"Name\"\tTEXT,\n" +
                "\t\"Mean\"\tTEXT,\n" +
                "\tFOREIGN KEY(\"Id\") REFERENCES \"Words\"(\"Id\"),\n" +
                "\tPRIMARY KEY(\"Id\")\n" +
                ");");


    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        db.execSQL("DROP TABLE IF EXISTS Words");
        db.execSQL("DROP TABLE IF EXISTS MyList");
        onCreate(db);

    }
}

这些是相同的:

dbConnection
databaseCopyHelper
word(object)
wordData(includes sql query)
dbCopy method
database

where is my fault?

Answer 1

您有可能在 getActivity() 中得到 null，因为它需要在创建 Fragment 的 Activity 阶段调用。

这是我给你的建议

public class Frag1 extends Fragment {
private RecyclerView rv;
private CardAdapter adapter;
private EditText editText;
private ImageButton imageButton;
private DbConnection db;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {


    View view = inflater.inflate(R.layout.fragment_frag1, container, false);
    rv = (RecyclerView) view.findViewById(R.id.rv);
    editText = view.findViewById(R.id.editText);
    rv.setHasFixedSize(true);
    rv.setLayoutManager(new LinearLayoutManager(this.getActivity()));




    return view;
}

@Override
public void onActivityCreated(@Nullable Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);
    db = new DbConnection(getActivity());
    final ArrayList<Word> words = new ArrayList<>();
    dbCopy();

    final ArrayList<Word> wordArrayList = new WordData().AllWords(db);

    adapter = new CardAdapter(getActivity(), wordArrayList);
    rv.setAdapter(adapter);
}

public void dbCopy() {
    DatabaseCopyHelper helper = new DatabaseCopyHelper(getActivity());
    try {
        helper.createDataBase();
    } catch (IOException e) {
        e.printStackTrace();
    }
    helper.openDataBase();
}

}