Adding a DAO

In that spirit, let’s add another component that will pay dividends in future flexibility. Right now, your servlet is creating the car list each time a request comes in. This really isn’t optimal. Servlets should deal with the mechanics of the HTTP request/response cycle. They shouldn’t perform persistence tier tasks.

We aren’t quite ready to install a database (that happens in the next chapter), but we can lay the groundwork by creating a Data Access Object (DAO). A DAO is a layer of abstraction—it hides the actual persistence specifics behind a common interface.

The DAO we create in this chapter still stores the DTO objects in a simple ArrayList. In the next chapter, the DAO will pull car data from a database that uses JDBC. In the chapter after that, it will use Hibernate (an Object/Relational Mapper) to do the same thing. By getting the DAO in place now, however, we’ll be able to make these implementation changes without affecting presentation-tier code. Loose coupling and high cohesion comes to the rescue again.