What are the DAO, DTO and Service layers in Spring Framework?

Question

I am writing RESTful services using spring and hibernate. I read many resource in internet, but they did not clarify my doubts. Please explain me in details what are DAO

Answer 1

DAO - Data Access Object:

An object that provides a common interface to perform all database operations like persistence mechanism.

public interface GenericDao {
  public T find(Class entityClass, Object id);
  public void save(T entity);
  public T update(T entity);
  public void delete(T entity);
  public List findAll(Class entityClass);
}

See this example : Spring – DAO and Service layer

DTO - Data Transfer Object:

An object that carries data between processes in order to reduce the number of method calls means you combine more than one POJO entities in service layer.

For example a GET request /rest/customer/101/orders is to retrieve all the orders for customer id 101 along with customer details hence you need combine entity Customer and entity Orders with details.