Implement Generic Abstract Entity class with dao interface and implemantation

Question

Hey I would like to have generic model/entity class that would download by id of type long element from db. The method for that is like this:

public class GenericModel   {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    public Long id;

    public static GenericModel getBy(Long id) {
        return JPA.em().find(GenericModel.class, id);
    }
}

But in Generic model that will be extended by child model class I'have to dynamically declare entity class name to find in db.

I would like to have generic entity class where I will have common methods like getById(). And that generic class will be extended by concret entity class. I will not have to write in each model class the same method, cause it will be inherited from generic class –

How can I achive this?

Here is my DAO interface. I am not quite sure of it:

public interface GenericModelDao<T> {
    public void add(T entityClass);
    public void update(T entityClass);
    public void delete(long id);
    public T get(long id);
    public List<T> get();
}

And My DAO implementation class of this interface

 @Repository
public class GenericModelDaoImpl <T extends GenericModel> implements GenericModelDao {
    public Class<T> entityClass;

    GenericModelDaoImpl(){
        setEntityClass(((Class) ((ParameterizedType) getClass().getGenericSuperclass()).getActualTypeArguments()[0]));
    }
    public void setEntityClass(Class<T> entityClass) {
        this.entityClass = entityClass;
    }
    @Autowired
    private SessionFactory sessionFactory;
    private Session getCurrentSession() {
        return sessionFactory.getCurrentSession();
    }

    @Override
    public T get(long id) {
        return (T) getCurrentSession().get(entityClass, id);
    }
    @Override
    public void delete(long id) {
        T entityClass = get(id);
            getCurrentSession().delete(entityClass);
    }
    @Override
    public List<T> get() {
        return getCurrentSession().createQuery("from " + entityClass ).list();
    }
    @Override
    public void add(Object entityClass) {
        getCurrentSession().save(entityClass);
    }
    @Override
    public void update(Object entityClass) {
        getCurrentSession().update(entityClass);
    }
}

And my GenericModel Class

@MappedSuperclass
public abstract class GenericModel<T extends GenericModel> {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    public Long id;

    public long getId() {
        return id;
    }

    public void setId(long id) {
        this.id = id;
    }

}

Please give me some extra help once more :D

Answer 1

You cannot use generics parameters in a static method (see https://stackoverflow.com/a/936951/1643132 for more details), so you will have to remove static keyword from getBy() method.

As a first step, you can introduce generics in your GenericModel class:

public abstract class GenericModel<T extends GenericModel> {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    public Long id;

    public T getBy(Long id) {
        return JPA.em().find(????, id);
    }
}

The problem is, something like T.class will not work (in getBy() method). Thanks to java.lang.reflect.ParameterizedType, you can retrieve T class at runtime. So update you GenericModel to :

@MappedSuperclass
public abstract class GenericModel<T extends GenericModel> {

    private Class<T> entityClass;

    GenericModel(){
        entityClass = ((Class) ((ParameterizedType) getClass().getGenericSuperclass()).getActualTypeArguments()[0]);
    }

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    public Long id;

    public T getBy(Long id) {
        return JPA.em().find(entityClass, id);
    }

    public Long getId() {
        return id;
    }
}

Your child entity may look like:

@Table(name="child_model")
@javax.persistence.Entity
public class ChildModel extends GenericModel<ChildModel> {

    @Column
    private String data;

    public String getData() {
        return data;
    }

    public void setData(String data) {
        this.data = data;
    }
}

Answer 2

Here is my working code. Hopes helps someone else :D

Generic Model/Entity class

import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.MappedSuperclass;

@MappedSuperclass
public abstract class GenericModel<T extends GenericModel<?>> {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    private String name;

    public long getId() {return id;}
    public void setId(long id) {this.id = id;}
    public String getName() {return name;}
    public void setName(String name) {this.name = name;}

}

GenericModelDao interface

import java.util.List;

public interface GenericModelDao<T> {
    public void add(T entityClass);
    public void update(T entityClass);
    public void delete(long id);
    public T get(long id);
    public T getByName(String name);
    public List<T> get();
}

GenericModelDaoImpl class

import java.lang.reflect.ParameterizedType;
import java.util.List;

import javax.persistence.MappedSuperclass;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Repository;

import model.GenericModel;

@Repository
@MappedSuperclass
public abstract class GenericModelDaoImpl <T extends GenericModel> implements GenericModelDao<T> {
    GenericModelDaoImpl(){
        setEntityClass(((Class) ((ParameterizedType) getClass().getGenericSuperclass()).getActualTypeArguments()[0]));
    }

    public Class<T> entityClass;

    public Class<T> getEntityClass() {
        return entityClass;
    }
    public void setEntityClass(Class<T> entityClass) {
        this.entityClass = entityClass;
    }
    @Autowired
    private SessionFactory sessionFactory;
    private Session getCurrentSession() {
        return sessionFactory.getCurrentSession();
    }

    @Override
    public void add(T entityClass) {
        getCurrentSession().save(entityClass);
    }
    @Override
    public void update(T entityClass) {
        getCurrentSession().update(entityClass);
    }
    @Override
    public void delete(long id) {
        getCurrentSession().delete(get(id));
    }
    @Override
    public T get(long id) {
        return (T) getCurrentSession().get(entityClass, id);
    }
    @Override
    public T getByName(String name) {
        return (T) getCurrentSession().createQuery("FROM e " + entityClass.getSimpleName() + " WHERE name:=name").setParameter("name", name);
    }
    @Override
    public List<T> get() {
        return getCurrentSession().createQuery("FROM " + entityClass.getSimpleName() ).list();
    }
}

My working class example: Customer Model/Entity class

import javax.persistence.Entity;
import javax.persistence.Table;

@Entity
@Table(name="common__Customer")
public class Customer extends GenericModel<Customer> {}

CustomerDao interface

import com.common.model.Customer;
public interface CustomerDao  extends GenericModelDao<Customer> {}

CustomerDaoImpl class

import org.springframework.stereotype.Repository;
import com.common.model.Customer;

@Repository
public class CustomerDaoImpl extends GenericModelDaoImpl<Customer> implements CustomerDao {}