Why to need close the result set and statement

Question

why to need close the result set and statement

Answer 1

It's best to explicitly close every ResultSet, Statement, and Connection in the narrowest scope possible.
These should be closed in a finally block.
Since each close() method throws SQLException, each one should be in an individual try/catch block to ensure that a failure to close one won't ruin the chances for all the others.
You can capture this in one nice utility class, like this:
package db;
import java.sql.*;
import java.util.ArrayList;
import java.util.Map;
import java.util.LinkedHashMap;
import java.util.List;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
* Created by IntelliJ IDEA.
* User: MD87020
* Date: Feb 16, 2005
* Time: 8:42:19 PM
* To change this template use File | Settings | File Templates.
public class DatabaseUtils
     * Logger for DatabaseUtils
    private static final Log logger = LogFactory.getLog(DatabaseUtils.class);
    /** Private default ctor to prevent subclassing and instantiation */
    private DatabaseUtils() {}
     * Close a connection
     * @param connection to close
    public static void close(Connection connection)
        try
            if ((connection != null) && !connection.isClosed())
                connection.close();
        catch (SQLException e)
            logger.error("Could not close connection", e);
     * Close a statement
     * @param statement to close
    public static void close(Statement statement)
        try
            if (statement != null)
                statement.close();
        catch (SQLException e)
            logger.error("Could not close statement", e);
     * Close a result set
     * @param rs to close
    public static void close(ResultSet rs)
        try
            if (rs != null)
                rs.close();
        catch (SQLException e)
            logger.error("Could not close result set", e);
     * Close both a connection and statement
     * @param connection to close
     * @param statement to close
    public static void close(Connection connection, Statement statement)
        close(statement);
        close(connection);
     * Close a connection, statement, and result set
     * @param connection to close
     * @param statement to close
     * @param rs to close
    public static void close(Connection connection,
                             Statement statement,
                             ResultSet rs)
        close(rs);
        close(statement);
        close(connection);
     * Helper method that maps a ResultSet into a map of columns
     * @param rs ResultSet
     * @return map of lists, one per column, with column name as the key
     * @throws SQLException if the connection fails
    public static final Map toMap(ResultSet rs) throws SQLException
        List wantedColumnNames = getColumnNames(rs);
        return toMap(rs, wantedColumnNames);
     * Helper method that maps a ResultSet into a map of column lists
     * @param rs ResultSet
     * @param wantedColumnNames of columns names to include in the result map
     * @return map of lists, one per column, with column name as the key
     * @throws SQLException if the connection fails
    public static final Map toMap(ResultSet rs, List wantedColumnNames)
        throws SQLException
        // Set up the map of columns
        int numWantedColumns    = wantedColumnNames.size();
        Map columns             = new LinkedHashMap(numWantedColumns);
        for (int i = 0; i < numWantedColumns; ++i)
            List columnValues   = new ArrayList();
            columns.put(wantedColumnNames.get(i), columnValues);
        while (rs.next())
            for (int i = 0; i < numWantedColumns; ++i)
                String columnName   = (String)wantedColumnNames.get(i);
                Object value        = rs.getObject(columnName);
                List columnValues   = (List)columns.get(columnName);
                columnValues.add(value);
                columns.put(columnName, columnValues);
        return columns;
     * Helper method that converts a ResultSet into a list of maps, one per row
     * @param rs ResultSet
     * @return list of maps, one per row, with column name as the key
     * @throws SQLException if the connection fails
    public static final List toList(ResultSet rs) throws SQLException
        List wantedColumnNames  = getColumnNames(rs);
        return toList(rs, wantedColumnNames);
     * Helper method that maps a ResultSet into a list of maps, one per row
     * @param rs ResultSet
     * @param wantedColumnNames of columns names to include in the result map
     * @return list of maps, one per column row, with column names as keys
     * @throws SQLException if the connection fails
    public static final List toList(ResultSet rs, List wantedColumnNames)
        throws SQLException
        List rows = new ArrayList();
        int numWantedColumns = wantedColumnNames.size();
        while (rs.next())
            Map row = new LinkedHashMap();
            for (int i = 0; i < numWantedColumns; ++i)
                String columnName   = (String)wantedColumnNames.get(i);
                Object value = rs.getObject(columnName);
                row.put(columnName, value);
            rows.add(row);
        return rows;
      * Return all column names as a list of strings
      * @param rs query result set
      * @return list of column name strings
      * @throws SQLException if the query fails
    public static final List getColumnNames(ResultSet rs) throws SQLException
        ResultSetMetaData meta  = rs.getMetaData();
        int numColumns = meta.getColumnCount();
        List columnNames = new ArrayList(numColumns);
        for (int i = 1; i <= numColumns; ++i)
            columnNames.add(meta.getColumnName(i));
        return columnNames;
}Anybody who lets the GC or timeouts or sheer luck handle their resource recovery for them is a hack and gets what they deserve.
Do a search on problems with Oracle cursors being exhausted and learn what the root cause is. That should convince you.
scsi-boy is 100% correct.
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