1

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应用程序附带的资产文件夹中的数据库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 data\data (我能够).. 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在 data\data 中有数据库.. 我尝试在不同的活动中创建 DatabaseHelper 的实例(我想从中添加数据的那个).. 当我这样做时, ,当代码到达 DatabaseHelper dbh = new DatabaseHelper(this); 时,我得到一个空指针异常错误 我知道这可能的原因可能是错误的上下文或空数据库......但在我的情况下似乎一切都好。

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);

}

每当我在我的活动中

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

我得到空指针错误。

希望你能有所帮助,问候,

4

1 回答 1

1

将此文件放入您的包中

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) {
}
}

活动 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());
    }
}

并用于插入代码

    SQLiteDatabase sqldb = dba.openDataBase();
    ContentValues cv = new ContentValues();
    cv.put("field", fieldvalue);
    sqldb.insert(table, nullColumnHack, cv);
    sqldb.close();
    dba.close();
    cv = null;
于 2012-11-11T08:58:06.260 回答