android - 打开 Assets 数据库文件时出现 java.lang.NullPointerException

Question

我在 assets 文件夹中有一个数据库文件。当我尝试从 DBHelper 类打开此文件时，它会在 copyDataBase() 方法下面的行中抛出 "java.lang.NullPointerException"，

myInput = myContext.getAssets().open(DB_NAME,AssetManager.ACCESS_STREAMING);

在我的 Activity 中:

private AssetsDBHelper db;
@Override
    protected void onStart() {
        super.onStart();
        db = new AssetsDBHelper(this);
    }

    @Override
    protected void onStop() {
        super.onStop();
        if(db!=null)
        {
            db.close();
        }
    }

AssetsDBHelper 类:

public class AssetsDBHelper extends SQLiteOpenHelper {

    public static final String DB_PATH ="/data/data/com.innodea.money/databases/";
    public static final String DB_NAME ="stocksdb";
    public static final int DB_VERSION = 1;
    public static final String TABLE_STOCKS = "stocks";
    public static final String COL_ID="_id";
    public static final String COL_SYMBOL="symbol";
    public static final String COL_NAME="name";
    public static final String COL_EXCHANGE="exchange";

    private Context myContext;
    private SQLiteDatabase myDataBase;

    public AssetsDBHelper(Context context) {
        super(context, DB_NAME, null, DB_VERSION);
        Log.i("AssetsDBHelper","AssetsDBHelper constructor");

        Log.i("AssetsDBHelper","before createDataBase");
        try {
            createDataBase();
            Log.i("AssetsDBHelper","after createDataBase");
        } catch (IOException e) {
            e.printStackTrace();
        }
        this.myContext = context;
    }

    public void createDataBase() throws IOException {
        Log.i("AssetsDBHelper","inside createDataBase");
        boolean dbExist = checkDataBase();

        Log.i("AssetsDBHelper","dbExist->"+dbExist);
        if (!dbExist) {
            Log.i("AssetsDBHelper","dbExist not exist.So get database from assets");
            this.getReadableDatabase();
            // close up the database after we have created it.
            this.close();

            try {
                copyDataBase();
            } catch (IOException e) {
                throw new Error("Error copying database");
            }

        }
    }

    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 true if DB exists,else return false
        return checkDB != null ? true : false;
    }


    private void copyDataBase() throws IOException {
        Log.i("AssetsDBHelper","inside copyDatabase");

        // Open your local db as the input stream
        InputStream myInput=null;
        // Open the empty db as the output stream
        OutputStream myOutput=null;

        try {
            Log.i("AssetsDBHelper","1");
            myInput = myContext.getAssets().open(DB_NAME,AssetManager.ACCESS_STREAMING);
            Log.i("AssetsDBHelper","2");

            File dir = new File(DB_PATH);
            dir.mkdirs();
            Log.i("AssetsDBHelper","3");

            // Path to the just created empty db
            String outFileName = DB_PATH + DB_NAME;
            Log.i("AssetsDBHelper",outFileName);
            myOutput = new FileOutputStream(outFileName);
            Log.i("AssetsDBHelper","4");
            // transfer bytes from the inputfile to the outputfile
            byte[] buffer = new byte[1024];
            Log.i("AssetsDBHelper","5");
            int length;
            while ((length = myInput.read(buffer)) > 0) {
                Log.i("AssetsDBHelper","6");
                myOutput.write(buffer, 0, length);
            }
            Log.i("AssetsDBHelper","write to "+outFileName);
        } catch (Exception e) {
            e.printStackTrace();
            Log.i("AssetsDBHelper","copyDatabase error->"+e.toString());
        }
        finally
        {
            // Close the streams
            if(myOutput!=null)
            {
                myOutput.flush();
                myOutput.close();
            }

            if(myInput!=null)
            {
                myInput.close();
            }
        }
    }

    public SQLiteDatabase openDataBase() throws SQLException {
        // Open the database
        String myPath = DB_PATH + DB_NAME;
        try {
            myDataBase = SQLiteDatabase.openDatabase(myPath, null,SQLiteDatabase.OPEN_READONLY);
            Log.i("AssetsDBHelper", "Database Opened");
        } catch (Exception e) {
            e.printStackTrace();
            Log.i("AssetsDBHelper", "Database not Opened.Error->"+e.toString());
        }
        return myDataBase;
    }

    @Override
    public synchronized void close() {
        if (myDataBase != null)
            myDataBase.close();
        super.close();
    }

    public Cursor getWordMatches(String query, String[] columns) {
        String selection = COL_SYMBOL + " MATCH ?";
        String[] selectionArgs = new String[] {query+"*"};
        return query(selection, selectionArgs, columns);
    }

    private Cursor query(String selection, String[] selectionArgs, String[] columns) {
        SQLiteQueryBuilder builder = new SQLiteQueryBuilder();
        builder.setTables(TABLE_STOCKS);

        Cursor cursor=null;
        try {
            cursor = builder.query(openDataBase(),columns, selection, selectionArgs, null, null, null);

            if (cursor == null) {
                return null;
            } else if (!cursor.moveToFirst()) {
                cursor.close();
                return null;
            }
        } 
        catch (SQLException e) {
            e.printStackTrace();
        }
        return cursor;
    }

    @Override
    public void onCreate(SQLiteDatabase db) {

    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        // TODO Auto-generated method stub

    }

}

Answer 1

myContext 在你得到异常的语句执行时还没有赋值。所以像这样向上移动分配:

public AssetsDBHelper(Context context) {
    super(context, DB_NAME, null, DB_VERSION);
    this.myContext = context;
    // ...etc...