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JavaSercer Pages The code for creating (Domino hosting) the PreparedStatement

JavaSercer Pages The code for creating the PreparedStatement or Statement object and executing the statement is enclosed in a try/finally block. This is important, because if something fails (due to an invalid SQL statement, for instance), the JDBC methods throw an SQLException . You want the exception to be handled by the application using the SQLCommandBean, but first you must make sure that all JDBC resources are released and the Connection object is returned to the pool. Using a try block with a finally clause but no catch clause gives this behavior. If an exception is thrown, the finally clause is executed, and then the exception is automatically thrown to the object that called the executeQuery( ) method. In the finally clause, the ResultSet object and either the PreparedStatement or Statement object are closed. It should be enough to close the statement object according to the JDBC specification (closing the statement should also close the ResultSet associated with the statement), but doing it explicitly doesn’t hurt and makes the code work even with a buggy JDBC driver. Example 17.7 shows a part of the setValues( ) method. Example 17.7. The SQLCommandBean’s setValues( ) Method private void setValues(PreparedStatement pstmt, Vector values) throws SQLException { for (int i = 0; i < values.size( ); i++) { try { Value v = (Value) values.elementAt(i); // Set the value using the method corresponding to // the type. // Note! Set methods are indexed from 1, so we add // 1 to i if (v instanceof BigDecimalValue) { pstmt.setBigDecimal(i + 1, v.getBigDecimal( )); } else if (v instanceof BooleanValue) { pstmt.setBoolean(i + 1, v.getBoolean( )); } ... } catch (UnsupportedConversionException e) { // Can not happen here since we test the type first } } } The setValue( ) method loops through all elements in the Vector with values. For each element, it tests which Value subclass it is and uses the corresponding JDBC method to set the value for the PreparedStatement object. You may wonder why a PreparedStatement is used here, since it's used only once. It's true that a PreparedStatement is intended to be reused over and over again to execute the same SQL statement with new values. But it offers a convenient solution to the problem of different syntax for values of type date/time and numbers when represented by a string literal. When a PreparedStatement is used, the variable values in the SQL statement can be represented by Java variables of the appropriate types without worrying about what literal representation a certain JDBC driver supports. So even though it's used only once, a PreparedStatement still has an advantage over a regular Statement. The toVector( ) method is shown in Example 17.8. Example 17.8. The SQLCommandBean's toVector( ) Method private Vector toVector(ResultSet rs) throws SQLException, UnsupportedTypeException { Vector rows = new Vector( ); while (rs.next( )) { Row row = new Row(rs); rows.addElement(row); } return rows; } This method simply walks through the ResultSet and adds a new Row object for each row to a Vector that it then returns. As you will see later, the Row constructor reads all column values and creates a Column object for each. page 245
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