20110211 ResultSet Learning

A table of data representing a database result set, which is usually generated by executing a statement that queries the database.

A ResultSet object maintains a cursor pointing to its current row of data. Initially the cursor is positioned before the first row. The next method moves the cursor to the next row, and because it returns false when there are no more rows in the ResultSet object, it can be used in a while loop to iterate through the result set.

A default ResultSet object is not updatable and has a cursor that moves forward only. Thus, you can iterate through it only once and only from the first row to the last row. It is possible to produce ResultSet objects that are scrollable and/or updatable. The following code fragment, in which con is a valid Connection object, illustrates how to make a result set that is scrollable and insensitive to updates by others, and that is updatable. See ResultSet fields for other options.

       Statement stmt = con.createStatement(
                                      ResultSet.TYPE_SCROLL_INSENSITIVE,
                                      ResultSet.CONCUR_UPDATABLE);
       ResultSet rs = stmt.executeQuery("SELECT a, b FROM TABLE2");
       // rs will be scrollable, will not show changes made by others,
       // and will be updatable

The ResultSet interface provides getter methods (getBoolean, getLong, and so on) for retrieving column values from the current row. Values can be retrieved using either the index number of the column or the name of the column. In general, using the column index will be more efficient. Columns are numbered from 1. For maximum portability, result set columns within each row should be read in left-to-right order, and each column should be read only once.
For the getter methods, a JDBC driver attempts to convert the underlying data to the Java type specified in the getter method and returns a suitable Java value. The JDBC specification has a table showing the allowable mappings from SQL types to Java types that can be used by the ResultSet getter methods.


Column names used as input to getter methods are case insensitive. When a getter method is called with a column name and several columns have the same name, the value of the first matching column will be returned. The column name option is designed to be used when column names are used in the SQL query that generated the result set. For columns that are NOT explicitly named in the query, it is best to use column numbers. If column names are used, the programmer should take care to guarantee that they uniquely refer to the intended columns, which can be assured with the SQL AS clause.

	/**
	 * getMetaData and fill List<Map<String, Object>>result with ResultSet Data
	 * @param rs
	 * @param pageLength
	 * @return
	 * @throws SQLException
	 */
	private List<Map<String, Object>> getDetails(ResultSet rs) throws HoffException {
		ResultSetMetaData rsmd = null;
		List<Map<String, Object>> results = new ArrayList<Map<String, Object>>();
		try {
			if(rs==null){
				return results;
			}
			rsmd = rs.getMetaData();
			int columnCount = rsmd.getColumnCount();
			int n = 0;
			while (rs.next()) {
				n++;
				Map<String, Object> rowMap = new HashMap<String, Object>();
				for (int i = 1; i <= columnCount; i++) {
					String colName = rsmd.getColumnName(i);
					if (Util.valueOf(colName).trim().length() == 0) {
						colName = String.valueOf(i);
					}
					rowMap.put(colName, rs.getObject(i));
				}
				results.add(rowMap);
				if (n > FETCH_SIZE) {
					break;
				}
			}

			return results;
		} catch (SQLException sqe) {
			logger.error("SQLException while getting paginated details");
			throw new HoffException(ErrorLevel.ERROR, ErrorType.DATABASE_ERROR, sqe);
		}
	}

A set of updater methods were added to this interface in the JDBC 2.0 API (JavaTM 2 SDK, Standard Edition, version 1.2). The comments regarding parameters to the getter methods also apply to parameters to the updater methods.

The updater methods may be used in two ways:

to update a column value in the current row. In a scrollable ResultSet object, the cursor can be moved backwards and forwards, to an absolute position, or to a position relative to the current row. The following code fragment updates the NAME column in the fifth row of the ResultSet object rs and then uses the method updateRow to update the data source table from which rs was derived.

       rs.absolute(5); // moves the cursor to the fifth row of rs
       rs.updateString("NAME", "AINSWORTH"); // updates the 
          // NAME column of row 5 to be AINSWORTH
       rs.updateRow(); // updates the row in the data source

to insert column values into the insert row. An updatable ResultSet object has a special row associated with it that serves as a staging area for building a row to be inserted. The following code fragment moves the cursor to the insert row, builds a three-column row, and inserts it into rs and into the data source table using the method insertRow.

       rs.moveToInsertRow(); // moves cursor to the insert row
       rs.updateString(1, "AINSWORTH"); // updates the 
          // first column of the insert row to be AINSWORTH
       rs.updateInt(2,35); // updates the second column to be 35
       rs.updateBoolean(3, true); // updates the third column to true
       rs.insertRow();
       rs.moveToCurrentRow();

A ResultSet object is automatically closed when the Statement object that generated it is closed, re-executed, or used to retrieve the next result from a sequence of multiple results.

The number, types and properties of a ResultSet object's columns are provided by the ResulSetMetaData object returned by the ResultSet.getMetaData method.