Hey Lew,

Thanks for your comment. To provide you with some (simplified) code:

The superclass looks like this:
(this can be abstract, although subclasses are singletons ;) )

-------------------------------------

package nl.nedcar.apollo.server.dao;

import java.sql.Connection;
import java.sql.SQLException;
import java.sql.Statement;

import javax.naming.InitialContext;
import javax.naming.NamingException;
import javax.sql.DataSource;

public abstract class AbstractDAO {

    private DataSource    dataSource;
    private Connection    connection;
    private final static String JNDI_NAME = "yourJndiName";

    protected AbstractDAO() {
        InitialContext initial;
        try {
            initial = new InitialContext();
            DataSource dataSource = (DataSource) initial.lookup(JNDI_NAME);
            Logger.debug(this, "Obtained ref to DataSource " + JNDI_NAME);
            this.dataSource = dataSource;
        }
        catch (NamingException e) {
            e.printStackTrace();
        }
    }

    private Connection getConnection() throws SQLException {
        return dataSource.getConnection();
    }

    protected void releaseConnection() throws SQLException {
        if(connection != null) {
            connection.close();
        }
        connection = null;
    }

    protected Statement getStatement() throws SQLException  {
        if(connection != null) {
            RuntimeException rt = new
RuntimeException(this.getClass().getName() + ": Creating new statement
while previous query was not " + "properly closed. \nClosing previous
connection...");
            releaseConnection();
            throw rt;
        }
        connection = getConnection();
        return connection.createStatement();
    }

}

The singletons subclasses look like:

-------------------------------------

package nl.nedcar.apollo.server.dao;

import java.sql.ResultSet;

public class FirstDAO extends AbstractDAO {

    private static FirstDAO theInstance;

    public static synchronized FirstDAO getInstance() {
        if(theInstance == null) {
            theInstance = new FirstDAO();
        }
        return theInstance;
    }

    public String getSomethingFromDatabase() throws Exception {
        try {
            ResultSet s = getStatement().executeQuery("select something from
users");
            if(s.next()) {
                return s.getString("username");
            }
            return null;
        }
        finally {
            releaseConnection();
        }
    }

}

-------------------------------------

calls are made like:

String somethingFromDb =
FirstDAO.getInstance().getSomethingFromDatabase();

So, the problem occurs when multiple clients are performing a call at
the moment:
client1 holds the connection, while client2 is attempting to use the
very same connection.