Database connectivity in single class

Question

Hi,
I am currently developing an application which reads and writes to an access db. For my current problem I use three tables which reference to each other:
Table 1: organization (id, name)
Table 2: departments(id, name, id_of_organization)
Table 3: personel (id, surname, firstname, id_of_department, id_of_organization)
Since I do not like managing the database connectivity in each and every class I wrote a class that manages database connectivity and has one public function which accepts a string as argument ( the SQL string) and returns a ResultSet after executing the query. Every class that uses the database has an instance of the "database class".
When listing the personell I would like to list the names of departments and organization, not the id's. The listing is accomplished by moving through the resultset in a while loop (while(rs.next()) )and retrieving the data using the getString() method. In the while loop I also retrieve the other data using the id's.
The problem is that as soon as I retrieve department or organization data the ResultSet containing the personell is closed (even though it is a mere copy of the original ResultSet of the "database class").......
My application (very much shortened of course ;-))
The "database class" DbConnect.class
/* Header, initialize connection and so on*/
public ResultSet Query(String queryIn)
// execute Select Query and return Result as ResultSet
try
rs = stmt.executeQuery(queryIn);
rs.first();
catch(SQLException esql)
JOptionPane.showMessageDialog(null, "Fehler in SQL Verbindung: " + esql.getMessage());
return rs;
What I do later:
/* some code
DbConnect database = new DbConnect();
ResultSet personell = database.Query("SELECT * FROM personell");
while (personell.next())
ResultSet department = database.Query("SELECT name FROM department WHERE id = " + personell.getLong("department"));
System.out.println("Name: " + personell.getString("name") ....... and so on, never gets executed since the SQL exception "ResultSet closed" is fired.
What am I doing wrong?

Answer 1

I agree with your method of doing db access through a single helper class. However there is one glaring problem. After each statement is done you must close the statement and the connection. Now you really want to do this in the Query() method, but you can't because you expose the ResultSet to the world and require clients to close the statement and connection that they can't see.
The solution is to package the result set into another collection, then close the statement and connection. Here's code that I use for executing arbitrary queries with a dynamic number of parameters. I use a map in this example to store each rows column values, but you can also use a List of Arrays or a List or Lists or a List of Value Objects.
   * General purpose Query executor method.
   * Statements to be executed MUST return result sets.
   * A list entry is created for each row which contains a Map of
   * column values, keys are the column names, therefore each computed column in
   * the query must be named.
   * @param sql The sql to execute
   * @param connection An open DB Connection
   * @param params Array of params to be applied to the statement. The number of
   * parameter holders in the SQL statement must match the number of entries in this array.
   * @param list instance of a List, that will be populated with results.
   * @throws SQLException
  private static final void executeQuery(final String sql,
                                         final Connection connection,
                                         final Object[] params,
                                         final List list ) throws SQLException {
    PreparedStatement statement = null;
    try {
      statement = connection.prepareStatement(sql);
      /* populate any parameters */
      for (int i = 0,j = 1; i < params.length; i++,j++) {
        statement.setObject(j,params);
ResultSet rs = statement.executeQuery();
/* Get meta data for column names and #columns */
ResultSetMetaData rsmd = rs.getMetaData();
int columns = rsmd.getColumnCount();
/* Process each row */
while (rs.next()) {
HashMap map = new HashMap(columns);
list.add(map);
/* Process each column */
for (int i = 1; i <= columns; i++) {
map.put(rsmd.getColumnName(i),rs.getObject(i));
} // for
} // while
} // try
finally {
if(statement!=null) statement.close();
if(connection!=null) connection.close();
} // executeQuery()