Should a Model class (in MVC) use static method or instance method?

Question

In term of a MVC framework, should I use a static method or instance method?

e.g. Assume a Users class, and a method getUserById() which return a User class, which one is better choice?

Users users = new Users();
User ret = users.getUserById(123);

or

User ret = Users.getUserById(123);

Assume there is no instance variable in the class Users, which one is a better choice?

Answer 1

I would lean toward instance variable. Simply because it will be easier to write tests for. Plus, a lot of the current server technologies (Spring, JavaEE, etc) support injecting beans/resource very well. Which better supports this rather than static methods.

Answer 2

Defiantly nope. Actually you should look at DAO (Data Access Object) pattern.

Model classes itself are responsible only for transferring information from one logic instance to another and should contain only geter and setter methods.

DAO classes are responsible for storing updating or retrieving info form some Data Source (database).Here is example of DAO pattern:

public class BookDAO {

  private PreparedStatement saveStmt;
  private PreparedStatement loadStmt;

  public DBBookDAO(String url, String user, String pw) {
    Connection con = DriverManager.getConnection(url, user, pw);
    saveStmt = con.prepareStatement("INSERT INTO books(isbn, title, author) "
                                   +"VALUES (?, ?, ?)");
    loadStmt = con.prepareStatement("SELECT isbn, title, author FROM books "
                                   +"WHERE isbn = ?");
  }

  public Book loadBook(String isbn) {
    Book b = new Book();
    loadStmt.setString(1, isbn);
    ResultSet result = loadStmt.executeQuery();
    if (!result.next()) return null;

    b.setIsbn(result.getString("isbn"));
    b.setTitle(result.getString("title"));
    b.setAuthor(result.getString("author"));
    return b;
  }

  public void saveBook(Book b) {
    saveStmt.setString(1, b.getIsbn());
    saveStmt.setString(2, b.getTitle());
    saveStmt.setString(3, b.getAuthor());
    saveStmt.executeUpdate();
  }
}