Question

我尝试连接SQLite。

public class Database {

private Statement stmt;
private Connection conn;

private final String DB_NAME = "db.sqlite";
private final String DB_PATH = "/data/data/com.Simple_Diary/databases/";

public void Database()
{
    connect();
}

/**
 * Connect to the database
 */
public void connect()
{
    try {
        copyDataBase();
        Class.forName("org.sqlite.JDBC").newInstance();
        conn = DriverManager.getConnection("jdbc:sqlite:" + DB_PATH + DB_NAME);
        stmt = conn.createStatement();
    }
    catch (Exception e) {}
}

/**
 * Query
 * @param sql
 * @return ResultSet Result object
 */
public ResultSet query(String sql)
{
    try {
        return stmt.executeQuery(sql);
    }
    catch (SQLException | NullPointerException e) {
        return null;
    }
}

/**
 * Copy database file from assets to "databases" directory
 */
private void copyDataBase()
{
    try {
        MainActivity ma = new MainActivity();

        InputStream mInput = ma.getAssets().open(DB_NAME);
        String outFileName = DB_PATH + DB_NAME;
        OutputStream mOutput = new FileOutputStream(outFileName);
        byte[] mBuffer = new byte[1024];
        int mLength;
        while ((mLength = mInput.read(mBuffer)) > 0){
            mOutput.write(mBuffer, 0, mLength);
        }
        mOutput.flush();
        mOutput.close();
        mInput.close();
    }
    catch (IOException e) {}
}

}

 try {
        Database db = new Database();
        ResultSet rs = db.query("SELECT * FROM categories");

        String str = "its null!";

        if(rs != null) {
            while (rs.next()) {
                str += rs.getString("title") + "\n";
            }
        }
        dbtext.setText(str);
    } catch(SQLException e){
        dbtext.setText("err: " + e.toString());
    }

但变量“rs”返回null，因为变量“conn”返回null。我在这个网站上为同样的问题读了很多答案，但我无法解决这个问题...... 我正在使用intelliJ。我添加了sqlite-jdbc jar作为库，但它没有帮助。

Answer 1

您应该使用SQLiteOpenHelper类。这封装了很多工作。您需要做的就是

@Override
public void onCreate(SQLiteDatabase db) 
{
  // put DDL here to create and init your tables, if needed.
}

然后使用insertOrThrow或rawQuery（query，args）等方法。

如果您需要更多帮助，请与我联系。

Android：SQLite Connection返回null

1 个答案: