DAO 实现：使一个 DAO 对象成为其他 DAO 的属性

Question

如何使一个 DAO 对象成为其他 DAO 的属性？

假设我有一个带有部门的 Employee 对象属性

public class Employee {
     public Department;

      //setter and getters
  }

我有这个 EmployeeDAO 和 DepartmentDAO 接口以及相应的实现

我有 DAOFactory

public abstract class DAOFactory {

// db connection instantiation here

public IEmployeeDAO getEmployeeDAO() {
    return new EmployeeDAOImpl(this);
}

public IDepartmentDAO getDepartmentDAO() {
    return new DepartmentDAOImpl(this);
}

}

我有一个 servlet 实例化这个 DAOfactory

public class EmployeeController extends HttpServlet {

public EmployeeController() {
    super();
    DBUtils dbInstance = DBUtils.getInstance("mysql");
    System.out.println("DAOFactory successfully obtained: " + dbInstance);

    // Obtain UserDAO.
    employeeDAO = dbInstance.getEmployeeDAO();
    departmentDAO = dbInstance.getDepartmentDAO();
    jobDAO = dbInstance.getJobDAO();

}
protected void doGet(HttpServletRequest request,
        HttpServletResponse response) throws ServletException, IOException {

            employees = employeeDAO.findAll();

            request.setAttribute("employees", employees);

}

我的问题是，当我调用 employeeDAO 的 findAll 方法时，如何将 Department 对象映射到 employeeDAO 或其实现中？

我在尝试映射结果时遇到了这样的事情：

    private  Employee map(ResultSet rs) throws SQLException {
    Employee employee = new Employee();

    employee.setEmployeeID(rs.getInt("EMPLOYEE_ID"));
    employee.setFirstName(rs.getString("FIRST_NAME"));
    employee.setLastName(rs.getString("LAST_NAME"));

    Department department = new DepartmentDAOImpl().getDepartmentByID(rs
            .getInt("DEPARTMENT_ID"));

    employee.setDepartment(department);

    return employee;
}

但我认为这是一种错误的做法。有人可以帮我解决这个问题吗？

Answer 1

您 EmployeeDAOImpl 依赖 IDepartmentDAO。不要直接实例化一个，而是将其声明为依赖项，然后让构造 EmployeeDAOImpl 的代码找出如何解决它。

假设

interface IEmployeeDAO {
    Employee load(long id);
}
interface IDepartmentDAO  {
    Department load(long id);
}

因为接口需要在构造函数中使用所需的 dao

class EmployeeDAOImpl implements IEmployeeDAO {

    private final DAOFactory factory;
    private final IDepartmentDAO departmentDAO;

    public EmployeeDAOImpl(DAOFactory factory, IDepartmentDAO departmentDAO) {
        this.factory = factory;
        this.departmentDAO = departmentDAO;
    }
    ...

现在您可以在任何地方使用它。例如

@Override
public Employee load(long id) {
    ...
    long departmentId = ....
    Department department = departmentDAO.load(departmentId);
    employee.department = department;
    return employee;
}

你的 DAOFactory 知道你使用哪种实现，现在可以通过添加一个简单的参数来提供依赖关系

public IEmployeeDAO getEmployeeDAO() {
    return new EmployeeDAOImpl(this, getDepartmentDAO());
}

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