What methods the DAO layer should implement?

Question

I'm being writing web application right now and got stuck at the stage of designing the DAO layer. Already have surfed through variaty of articles on that subject but still haven't reached the clarity. And my question is: What kind of methods are allowed to be declared and implemented in DAO classes? Are they just limited set of CRUD operations (create, read, update, delete)? Is it considered a good practice to extend this set with some custom methods fitting in your concrete needs in developing a business logic layer? For example: I have an entity class named User which fields fully reflect a corresponding database table. Let's suppose I'm about to validate user's login and password while authorization. What approach would be more appropriate in that situation? Should I call generic CRUD method List<User> findAll() to retrieve all users and validate these concrete login and password in method of business logic (something like boolean validateUser(String login, String password, List<User> users)). Or should I add some not CRUD method like boolean checkUser(String login, String password) directly in DAO class?

Answer 1

I'm being writing web application right now and got stuck at the stage of designing the DAO layer.

Are you writing it by hand using plain old servlets or using a web framework (e.g Spring MVC) to ease your pain?

1. And my question is: What kind of methods are allowed to be declared and implemented in DAO classes? Are they just limited set of CRUD operations (create, read, update, delete)?**

Generally speaking, yes -- the methods should be limited to CRUD operations.

2. Is it considered a good practice to extend this set with some custom methods fitting in your concrete needs in developing a business logic layer?**

Within reason, yes.

For example: I have an entity class named User which fields are fully reflect a corresponding database table. Let's suppose I'm about to validate user's login and password while authorization. What approach would be more appropriate in that situation? Should I call generic CRUD method List findAll() to retrieve all users and validate these concrete login and password in method of business logic (something like boolean validateUser(String login, String password, List users)). Or should I add some not CRUD method like boolean checkUser(String login, String password) directly in DAO class?

In addition to the standard CRUD methods that your DAO classes are expected to have, you can add a helper method like: User findUser(String login) whose job is to return a populated User object for the specified login parameter, or null if the login is non-existent.

User findUser(String login) should leverage List<User> findAll() which should already exist with the rest of the CRUD methods in the DAO class. It could be implemented as follows:

public User findUser(String login) {
    User user = null;
    final SearchCriteria criteria = buildSearchCriteria(login); // construct a search criteria from the login parameter
    List<User> users = findAll(criteria);  
    if (null != users) {
       assert (users.size() == 1) : "More than one user was matched - login must be unique.";
       user = users.get(0);
    }
    return user;
}

To summarize, you only need 2 methods to implement the authorization logic:

1. User findUser(String login) in your DAO layer and;

2. boolean checkUser(String login, String password) which will be in your frontend layer. If you are not using any web framework, this method will be implemented in your servlet otherwise this method will go inside your controller class (if you are using an MVC framework).

Answer 2

short answer is no: dont add any business logic in dao layer. Just let every tier has its own responsibility, so when someone else (or even you) needs changes they will know where to look.

EDIT: Other answers:

Q:What kind of methods are allowed to be declared and implemented in DAO classes?

A:Methods that allow you to access database objects or their properties. i.e. public User getOldUsers(), public boolean isUserExist(Long userId)etc...

Q:Are they just limited set of CRUD operations (create, read, update, delete)?

A:Yes additionally you can control persistence or transaction properties

Q:Generic CRUDS?

A:Almost all the projects I work on we use generic CRUDS (AbstractDao classes) and add additional methods