The Servlet which loads the Singleton class should be loaded during the server startup

ConfigServlet.java

public class ConfigServlet extends HttpServlet
{	
	@Override
	public void init() throws ServletException
	{
	  super.init();
	  SingletonDBConnection.getInstance();
	}
}

web.xml

<servlet>
   <servlet-name>StartUpServlet</servlet-name>
   <servlet-class>com.mugil.tutor.ConfigServlet</servlet-class>
   <load-on-startup>1</load-on-startup>
</servlet>

DBConnection.java

public class DBConnection
{
	public Connection getDBConnection()
	{
		Connection connection = null;

		try
		{
			connection = DriverManager.getConnection(
					"HOST_NAME", "USER_NAME", "PASSWORD");

		}
		catch (SQLException e)
		{
			e.getLocalizedMessage();	
			System.out.println("Connection Failed! Check output console");
			e.printStackTrace();
			return null;
		}
		return connection;
	}
}

SingletonDBConnection.java

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

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

public class SingletonDBConnection
{
	private static SingletonDBConnection singleInstance;
	private static DataSource dataSource;
	private static Connection dbConnect;
	
	private SingletonDBConnection()
	{
		try
		{
			Context initContext = new InitialContext();
			Context envContext  = (Context) initContext.lookup("java:/comp/env");
			dataSource		   = (DataSource) envContext.lookup("jdbc/testdb");
			
			try
			{
				dbConnect  = dataSource.getConnection();
			}
			catch (SQLException e)
			{
				e.printStackTrace();
			} 	
		}
		catch (NamingException e)
		{
			e.printStackTrace();
		}
	}
	
	public static SingletonDBConnection getInstance()
	{
		if(singleInstance == null)
		{
			synchronized (SingletonDBConnection.class)
			{
				if(singleInstance == null)
				{
					singleInstance = new SingletonDBConnection();
				}
			}
		}

		return singleInstance;
	}
	
	public static Connection getConnInst()
	{
		try
		{
			dbConnect = dataSource.getConnection();
		}
		catch (SQLException e1)
		{
			e1.printStackTrace();
		}
		
		if(dbConnect == null)
		{
			try
			{
				Context initContext = new InitialContext();
				Context envContext  = (Context) initContext.lookup("java:/comp/env");
				dataSource		    = (DataSource) envContext.lookup("jdbc/testdb");
				
				try
				{
					dbConnect  = dataSource.getConnection();
				}
				catch (SQLException e)
				{
					e.printStackTrace();
				} 	
			}
			catch (NamingException e)
			{
				e.printStackTrace();
			}
		}
		
		return dbConnect;		 
	}
}

ListUsers.java

public List<User> getUsersList()
	{
		Connection conn;
		Statement  stmt = null;
		ResultSet  rs;
		List<User> arrUsersList = new ArrayList<User>();
		conn = SingletonDBConnection.getInstance().getConnInst();
		
		String strSQL = "SELECT UserId, UserName, Gender, UserLocation " +
						"  FROM tblusers";
		
		try
		{
			stmt = conn.createStatement();			
			rs = stmt.executeQuery(strSQL);
			
			while(rs.next())
			{
				User objUser = new User();
				objUser.setUserId(rs.getString("UserId"));
				objUser.setUserName(rs.getString("UserName"));
				objUser.setUserGender(rs.getString("Gender"));
				objUser.setUserLocation(rs.getString("UserLocation"));
				arrUsersList.add(objUser);
			}
		}
		catch (SQLException e)
		{
			e.printStackTrace();
		}
		finally
		{
			if(conn != null) try { conn.close(); } catch (SQLException e) { e.printStackTrace(); }
		}
		
		
		return arrUsersList;
	}