Generic DAO Pattern - Subclass Entities

[Joe Harry]

Guys,

I use the Generic DAO pattern to model my DAO layer. I have a domain object (application level) termed as Content. A Content may be an Article, Question, Answer or any other object which is persistable in the database. All these persistable entities extent the Content which is just a marker object. Now in my persistence layer, I would like to have a method that returns a Question entity based on the id passed to it. The DAO (for the Content domain) looks like:

public class ContentDAOImpl extends GenericJPADAO < Content, Long > implements IContentDAO {
	
	/**
	 * {@inheritDoc}
	 */
	public Content getRandomQuestion(final long technologyId) {
		return super.findById(technologyId);
	}

        .......
}

But here when I call the getRandomQuestion by passing the technologyId, I should get a Question object in return. I'm thinking of using the JDK dynamic proxy in this scenario. Any suggestions?

[Joe Harry]

What I want to acheive is that, get a ContentDAOImpl which has Question as the entity type at run time instead of having Content as defined above. What I'm literrally doing here is clubbing Content related entities to a single DAO class.

Any ideas?

[Joe Harry]

Speaking in terms of design patterns, I'm working out on an alternative to the Anemic DAO pattern. I really do not want to have one DAO for each entity that I have. But rather one DAO per Domain. One Domain in my application consists of many Entities and my application has many such Domains. I want to club this idea together with the Generic DAO pattern.

[Joe Harry]

Joe Harry wrote:Guys,

I use the Generic DAO pattern to model my DAO layer. I have a domain object (application level) termed as Content. A Content may be an Article, Question, Answer or any other object which is persistable in the database. All these persistable entities extent the Content which is just a marker object. Now in my persistence layer, I would like to have a method that returns a Question entity based on the id passed to it. The DAO (for the Content domain) looks like:

public class ContentDAOImpl extends GenericJPADAO < Content, Long > implements IContentDAO {
	
	/**
	 * {@inheritDoc}
	 */
	public Content getRandomQuestion(final long technologyId) {
		return super.findById(technologyId);
	}

        .......
}

But here when I call the getRandomQuestion by passing the technologyId, I should get a Question object in return. I'm thinking of using the JDK dynamic proxy in this scenario. Any suggestions?

I want to make this part dynamic. I should be able to pass Question, or rather anything that extends Content.

public class ContentDAOImpl extends GenericJPADAO < Content, Long > implements IContentDAO {

[Joe Harry]

Any suggestions?

[Joe Harry]

Here is my complete source:

The Generic DAO interface (has methods to define the basic CRUD operations):

public interface IGenericBaseDAO < T, ID  extends Serializable > {

    /**
     *
     * @param entity
     */
    void save(T entity);

    /**
     *
     * @param entity
     */
    void delete(T entity);

    /**
     *
     * @param entity
     */
    //void update(T entity);

    /**
     *
     * @param id
     * @return
     */
    T findById(Long id);
}

The abstract class that implements the above interface: Each DAO will then extend this abstract class by specifying the actual Entity type on which it intends to operate.

public abstract class GenericJPADAO < T, ID extends Serializable > implements IGenericBaseDAO < T, ID > {

    protected Class<T> entityClass;

    @PersistenceContext
    protected EntityManager entityManager;

//    public GenericJPADAO(Class<? extends Content> class1) {
//    	this.entityClass = (Class<T>) class1;
//    }
    
    public GenericJPADAO() {
    	this.entityClass = (Class<T>) ((ParameterizedType) getClass()
                .getGenericSuperclass()).getActualTypeArguments()[0];    	
    }

   /**
    * {@inheritDoc}
    */
    public void save(T entity) {
        entityManager.persist(entity);
    }

   /**
    * {@inheritDoc}
    */
    public void delete(T entity) {
        entityManager.remove(entity);
    }

   /**
    * {@inheritDoc}
    */
    public T findById(Long id) {
        return entityManager.find(entityClass, id);
    }
}

The DAO implementation:

public class ContentDAOImpl extends GenericJPADAO < Content, Long > implements IContentDAO {
	
	/**
	 * {@inheritDoc}
	 */
	public Content getRandomQuestion(final long technologyId) {
		return super.findById(technologyId);
	}
	
	/**
	 * {@inheritDoc}
	 */
	public List < Content > getAllAnswersForQuestion(final long questionId) {
		return null;				
	}
	
	/**
	 * {@inheritDoc}
	 */
	public List < Question > getAllQuestionsForExam() {
		return null;		
	}
}

I want to experiment the getRandomQuestion method which actually should return a Question which extends from Content. I do not want to have new DAO's for Question. I also have some other Entity which IS-A Content (for example., Answer, SearchableContent and so on) which are logically grouped under Content. I really do not want to clog my persistence layer with a bunch of DAO's but rather one chic DAO (on the Content domain) which gives me what I need for all Content types and which I can instantiate based on the method that I'm going to call. Hope my question is much clear now for any suggestions from you guys.

[Michele Cozart]

Did you ever get an answer on this? Thanks