How do I implement a DAO manager using JDBC and connection pools?

Question

My problem is as follows. I need a class that works as a single point to a database connection in a web system, so to avoid having one user with two open connections. I need

Answer 1

I think that if you want to do a simple DAO pattern in plain JDBC you should keep it simple:

      public List listCustomers() {
            List list = new ArrayList<>();
            try (Connection conn = getConnection();
                 Statement s = conn.createStatement();
                 ResultSet rs = s.executeQuery("select * from customers")) { 
                while (rs.next()) {
                    list.add(processRow(rs));
                }
                return list;
            } catch (SQLException e) {
                throw new RuntimeException(e.getMessage(), e); //or your exceptions
            }
        }

You can follow this pattern in a class called for example CustomersDao or CustomerManager, and you can call it with a simple

CustomersDao dao = new CustomersDao();
List customers = dao.listCustomers();

Note that I'm using try with resources and this code is safe to connections leaks, clean, and straightforward, You probably don't want to follow the full DAO pattern with Factorys, interfaces and all that plumbing that in many cases don't add real value.

I don't think that it's a good idea using ThreadLocals, Bad used like in the accepted answer is a source of classloader leaks

Remember ALWAYS close your resources (Statements, ResultSets, Connections) in a try finally block or using try with resources