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

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, DTO and Service layers in spring framework? And why usage of these layers is required in spring to develop RESTfull API services.

Solution 1:

First off, these concepts are Platform Agnostic and are not exclusive to Spring Framework or any other framework, for that matter.

Data Transfer Object

DTO is an object that carries data between processes. When you're working with a remote interface, each call it is expensive. As a result you need to reduce the number of calls. The solution is to create a Data Transfer Object that can hold all the data for the call. It needs to be serializable to go across the connection. Usually an assembler is used on the server side to transfer data between the DTO and any domain objects. It's often little more than a bunch of fields and the getters and setters for them.

Data Access Object

A Data Access Object abstracts and encapsulates all access to the data source. The DAO manages the connection with the data source to obtain and store data.

The DAO implements the access mechanism required to work with the data source. The data source could be a persistent store like an RDBMS, or a business service accessed via REST or SOAP.

The DAO abstracts the underlying data access implementation for the Service objects to enable transparent access to the data source. The Service also delegates data load and store operations to the DAO.

Service

Service objects are doing the work that the application needs to do for the domain you're working with. It involves calculations based on inputs and stored data, validation of any data that comes in from the presentation, and figuring out exactly what data source logic to dispatch, depending on commands received from the presentation.

A Service Layer defines an application's boundary and its set of available operations from the perspective of interfacing client layers. It encapsulates the application's business logic, controlling transactions and coordinating responses in the implementation of its operations.

Recommended References

Martin Fowler has a great book on common Application Architecture Patterns named Patterns of Enterprise Application Architecture. There is also, Core J2EE Patterns that worth looking at.

Solution 2:

DAO - Data Access Object:

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

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