android - 尝试访问从 Assets 到数据\数据\的复制数据库中的 DatabaseHelper 时出现空指针异常

Question

我有一个数据库助手类(下面的代码)。这个助手的类任务是将数据库从我的应用程序附带的 Assets 文件夹复制到我的应用程序的 data\data... 中，以便我可以使用它。

一旦我将数据库放入数据\数据(我能够做到)。我想添加它并执行 CRUD 操作，并且该数据库将保留在应用程序中，直到用户删除该应用程序。

但是，一旦复制完成并且我的数据库位于 data\data.. 我尝试在不同的 Activity (我想从中添加数据的 Activity )中创建 DatabaseHelper 的实例当我这样做时，代码到达的那一刻我得到了一个空指针异常错误DatabaseHelper dbh = new DatabaseHelper(this);我知道造成这种情况的可能原因可能是上下文错误或数据库为空...但在我的情况下似乎一切正常。

下面是代码。

DatabaseHelper.java

public class DatabaseHelper extends SQLiteOpenHelper
{   
    String DB_PATH = null;
// assign the database givens, such as name and context.
private static String DB_NAME = "offline";
private SQLiteDatabase myDataBase;
private final Context myContext;

static int count = 0;

/**
 * Constructor Takes and keeps a reference of the passed context in order to
 * access to the application assets and resources.
 * 
 * @param context
 */
public DatabaseHelper(Context context)
{
    super(context, DB_NAME, null, 1);
    this.myContext = context;
    DB_PATH = "/data/data/" + context.getPackageName() + "/" + "databases/";
}

// ///////////////////////////////////////////////////////////////////////////////////////

public void createDataBase() throws IOException
{
    boolean dbExist = checkDataBase();

    if (dbExist)
    {
        // do nothing - database already exist
    } else
    {
        this.getWritableDatabase();

        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_READWRITE);
    } catch (SQLiteException e)
    {
        // database does't exist yet.
    }
    if (checkDB != null)
    {
        checkDB.close();
    }
    return checkDB != null ? true : false;
}

// ///////////////////////////////////////////////////////////////////////////////////////

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_READWRITE);
}

// ///////////////////////////////////////////////////////////////////////////////////////

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

    super.close();
}

// return cursor
public Cursor query(String table, String[] columns, String selection,
        String[] selectionArgs, String groupBy, String having,
        String orderBy)
{
    return myDataBase.query("myTable", null, null, null, null, null, null);

}

// //////////////////////////////////////////////////////////////////////////////////

@Override
public void onCreate(SQLiteDatabase db)
{

}

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

}

// //////////////////////////////////////////////////////////////////////////////////
public long insertPuzzle(String q,String a,String c1,String c2,String c3,String c4)
{               
            ContentValues initialValues = new ContentValues();
                initialValues.put("q", q);
                initialValues.put("a",a);
                initialValues.put("c1",c1);
                initialValues.put("c2",c2);
                initialValues.put("c3",c3);
                initialValues.put("c4",c4);
                return myDataBase.insert("myTable", null, initialValues);

}

每当我在我的 Activity 中做

DatabaseHelper db1 = new DatabaseHelper(this);
db1.openDatabase();
db1.insertPuzzle("q","a","1","2","3","4");

我收到空指针错误。

希望对你有帮助，问候，

Answer 1

把这个文件放到你的包里

public class DBAdapter extends SQLiteOpenHelper {

private static String DB_PATH = "";
private static final String DB_NAME = "temp.sqlite";//your database name with extention any
private SQLiteDatabase myDataBase;
private final Context myContext;

private static DBAdapter mDBConnection;

/**
 * Constructor
 * Takes and keeps a reference of the passed context in order to access to the application assets and resources.
 * @param context
 */
private DBAdapter(Context context) {
    super(context, DB_NAME, null, 1);
    this.myContext = context;
    DB_PATH = "/data/data/"
            + context.getApplicationContext().getPackageName()
            + "/databases/";
    // The Android's default system path of your application database is
    // "/data/data/mypackagename/databases/"
}

/**
 * getting Instance
 * @param context
 * @return DBAdapter
 */
public static synchronized DBAdapter getDBAdapterInstance(Context context) {
    if (mDBConnection == null) {
        mDBConnection = new DBAdapter(context);
    }
    return mDBConnection;
}

/**
 * Creates an 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 following method
        // 1) an empty database will be created into the default system path of your application
        // 2) than we 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();
}

/**
 * Open the database
 * @throws SQLException
 */
public SQLiteDatabase openDataBase() throws SQLException {
    String myPath = DB_PATH + DB_NAME;
    myDataBase = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READWRITE);       
    return myDataBase;
}

/**
 * Close the database if exist
 */
@Override
public synchronized void close() {
    if (myDataBase != null)
        myDataBase.close();
    super.close();
}

/**
 * Call on creating data base for example for creating tables at run time
 */
@Override
public void onCreate(SQLiteDatabase db) {
}

/**
 * can used for drop tables then call onCreate(db) function to create tables again - upgrade
 */
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
}
}

Activity 的 onCreate 方法

private DBAdapter dba;
     @Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.profile);
              dba = DBAdapter.getDBAdapterInstance(this);
    try {
        dba.createDataBase();
    } catch (IOException e) {
        Log.e("log_tag", e.getLocalizedMessage());
    }
}

插入代码

    SQLiteDatabase sqldb = dba.openDataBase();
    ContentValues cv = new ContentValues();
    cv.put("field", fieldvalue);
    sqldb.insert(table, nullColumnHack, cv);
    sqldb.close();
    dba.close();
    cv = null;