Generics example: the Data Access Object

package be.bzbit.framework.domain.repository;

import java.io.Serializable;

import java.util.List;
import java.util.Map;

/**
 * Generic Repository, providing basic CRUD operations
 *
 * @author Jurgen Lust
 * @author $LastChangedBy: jlust $
 *
 * @version $LastChangedRevision: 257 $
 *
 * @param <T> the entity type
 * @param <ID> the primary key type
 */
public interface GenericRepository<T, ID extends Serializable> {
    //~ Methods ----------------------------------------------------------------

    /**
     * Get the Class of the entity.
     *
     * @return the class
     */
    Class<T> getEntityClass();

    /**
     * Find an entity by its primary key
     *
     * @param id the primary key
     * @return the entity
     */
    T findById(final ID id);

    /**
     * Load all entities.
     *
     * @return the list of entities
     */
    List<T> findAll();

    /**
     * Find entities based on an example.
     *
     * @param exampleInstance the example
     * @return the list of entities
     */
    List<T> findByExample(final T exampleInstance);

    /**
     * Find using a named query.
     *
     * @param queryName the name of the query
     * @param params the query parameters
     *
     * @return the list of entities
     */
    List<T> findByNamedQuery(
        final String queryName,
        Object... params
    );

    /**
     * Find using a named query.
     *
     * @param queryName the name of the query
     * @param params the query parameters
     *
     * @return the list of entities
     */
    List<T> findByNamedQueryAndNamedParams(
        final String queryName,
        final Map<String, ?extends Object> params
    );

    /**
     * Count all entities.
     *
     * @return the number of entities
     */
    int countAll();

    /**
     * Count entities based on an example.
     *
     * @param exampleInstance the search criteria
     * @return the number of entities
     */
    int countByExample(final T exampleInstance);

    
    /**
     * save an entity. This can be either a INSERT or UPDATE in the database.
     * 
     * @param entity the entity to save
     * 
     * @return the saved entity
     */
    T save(final T entity);

    /**
     * delete an entity from the database.
     * 
     * @param entity the entity to delete
     */
    void delete(final T entity);
}

package be.bzbit.framework.domain.repository.jpa;

import java.io.Serializable;
import java.lang.reflect.ParameterizedType;
import java.util.List;
import java.util.Map;

import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;

import org.hibernate.Criteria;
import org.hibernate.Session;
import org.hibernate.criterion.Criterion;
import org.hibernate.criterion.Example;
import org.hibernate.criterion.Projections;
import org.springframework.beans.factory.annotation.Required;

import be.bzbit.framework.domain.repository.GenericRepository;

/**
 * JPA implementation of the GenericRepository. Note that this implementation
 * also expects Hibernate as JPA implementation. That's because we like the
 * Criteria API.
 * 
 * @author Jurgen Lust
 * @author $LastChangedBy: jlust $
 * 
 * @version $LastChangedRevision: 257 $
 * 
 * @param <T>
 *            The persistent type
 * @param <ID>
 *            The primary key type
 */
public class GenericJpaRepository<T, ID extends Serializable> implements
		GenericRepository<T, ID> {

	// ~ Instance fields
	// --------------------------------------------------------

	private final Class<T> persistentClass;
	private EntityManager entityManager;

	// ~ Constructors
	// -----------------------------------------------------------

	@SuppressWarnings("unchecked")
	public GenericJpaRepository() {
		this.persistentClass = (Class<T>) ((ParameterizedType) getClass()
				.getGenericSuperclass()).getActualTypeArguments()[0];
	}

	public GenericJpaRepository(final Class<T> persistentClass) {
		super();
		this.persistentClass = persistentClass;
	}

	// ~ Methods
	// ----------------------------------------------------------------

	/**
	 * @see be.bzbit.framework.domain.repository.GenericRepository#countAll()
	 */
	@Override
	public int countAll() {
		return countByCriteria();
	}

	/**
	 * @see be.bzbit.framework.domain.repository.GenericRepository#countByExample(java.lang.Object)
	 */
	@Override
	public int countByExample(final T exampleInstance) {
		Session session = (Session) getEntityManager().getDelegate();
		Criteria crit = session.createCriteria(getEntityClass());
		crit.setProjection(Projections.rowCount());
		crit.add(Example.create(exampleInstance));

		return (Integer) crit.list().get(0);
	}

	/**
	 * @see be.bzbit.framework.domain.repository.GenericRepository#findAll()
	 */
	@Override
	public List<T> findAll() {
		return findByCriteria();
	}

	/**
	 * @see be.bzbit.framework.domain.repository.GenericRepository#findByExample(java.lang.Object)
	 */
	@SuppressWarnings("unchecked")
	@Override
	public List<T> findByExample(final T exampleInstance) {
		Session session = (Session) getEntityManager().getDelegate();
		Criteria crit = session.createCriteria(getEntityClass());
		final List<T> result = crit.list();  
		return result; 
	}

	/**
	 * @see be.bzbit.framework.domain.repository.GenericRepository#findById(java.io.Serializable)
	 */
	@Override
	public T findById(final ID id) {
		final T result = getEntityManager().find(persistentClass, id);
		return result;
	}

	/**
	 * @see be.bzbit.framework.domain.repository.GenericRepository
         * #findByNamedQuery(java.lang.String, java.lang.Object[])
	 */
	@SuppressWarnings("unchecked")
	@Override
	public List<T> findByNamedQuery(final String name, Object... params) {
		javax.persistence.Query query = getEntityManager().createNamedQuery(
				name);

		for (int i = 0; i < params.length; i++) {
			query.setParameter(i + 1, params[i]);
		}

		final List<T> result = (List<T>) query.getResultList();
		return result;
	}

	/**
	 * @see be.bzbit.framework.domain.repository.GenericRepository
         * #findByNamedQueryAndNamedParams(java.lang.String, java.util.Map)
	 */
	@SuppressWarnings("unchecked")
	@Override
	public List<T> findByNamedQueryAndNamedParams(final String name,
			final Map<String, ? extends Object> params) {
		javax.persistence.Query query = getEntityManager().createNamedQuery(
				name);

		for (final Map.Entry<String, ? extends Object> param : params
				.entrySet()) {
			query.setParameter(param.getKey(), param.getValue());
		}

		final List<T> result = (List<T>) query.getResultList();
		return result;
	}

	/**
	 * @see be.bzbit.framework.domain.repository.GenericRepository#getEntityClass()
	 */
	@Override
	public Class<T> getEntityClass() {
		return persistentClass;
	}

	/**
	 * set the JPA entity manager to use.
	 *
	 * @param entityManager
	 */
	@Required
	@PersistenceContext
	public void setEntityManager(final EntityManager entityManager) {
		this.entityManager = entityManager;
	}

	public EntityManager getEntityManager() {
		return entityManager;
	}

	/**
	 * Use this inside subclasses as a convenience method.
	 */
	protected List<T> findByCriteria(final Criterion... criterion) {
		return findByCriteria(-1, -1, criterion);
	}

	/**
	 * Use this inside subclasses as a convenience method.
	 */
	@SuppressWarnings("unchecked")
	protected List<T> findByCriteria(final int firstResult,
			final int maxResults, final Criterion... criterion) {
		Session session = (Session) getEntityManager().getDelegate();
		Criteria crit = session.createCriteria(getEntityClass());

		for (final Criterion c : criterion) {
			crit.add(c);
		}

		if (firstResult > 0) {
			crit.setFirstResult(firstResult);
		}

		if (maxResults > 0) {
			crit.setMaxResults(maxResults);
		}

		final List<T> result = crit.list();
		return result;
	}

	protected int countByCriteria(Criterion... criterion) {
		Session session = (Session) getEntityManager().getDelegate();
		Criteria crit = session.createCriteria(getEntityClass());
		crit.setProjection(Projections.rowCount());

		for (final Criterion c : criterion) {
			crit.add(c);
		}

		return (Integer) crit.list().get(0);
	}


	/**
	 * @see be.bzbit.framework.domain.repository.GenericRepository#delete(java.lang.Object)
	 */
	@Override
	public void delete(T entity) {
		getEntityManager().remove(entity);
	}

	/**
	 * @see be.bzbit.framework.domain.repository.GenericRepository
         * #save(java.lang.Object)
	 */
	@Override
	public T save(T entity) {
		final T savedEntity = getEntityManager().merge(entity);
		return savedEntity;
	}
}

package org.bejug.dao;

import org.bejug.model.Person;

import be.bzbit.framework.domain.repository.GenericRepository;

public interface PersonRepository extends GenericRepository<Person, Long> {
	//we inherit the basic CRUD operations from the GenericRepository
}

package org.bejug.dao;

import org.bejug.model.Person;

import be.bzbit.framework.domain.repository.jpa.GenericJpaRepository;

public class PersonJpaRepository extends GenericJpaRepository<Person, Long>
		implements PersonRepository {
	//we inherit the basic CRUD operations from the GenericRepository
}