MainDao.java

package com;

import java.util.List;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.EntityTransaction;
import javax.persistence.Persistence;
import javax.persistence.TypedQuery;
import javax.persistence.criteria.CriteriaBuilder;
import javax.persistence.criteria.CriteriaQuery;
import javax.persistence.criteria.Root;

public abstract class MainDao<T> {
	protected abstract Class<T> getEntityType();

	public List<T> read() {
		EntityManagerFactory emf = Persistence
				.createEntityManagerFactory("Jp_Test");
		EntityManager em = emf.createEntityManager();
		EntityTransaction tx = em.getTransaction();
		tx.begin();
		CriteriaBuilder cb = em.getCriteriaBuilder();
		CriteriaQuery<T> cq = cb.createQuery(getEntityType());
		Root<T> t2 = cq.from(getEntityType());
		cq.select(t2);
		TypedQuery<T> q = em.createQuery(cq);
		List<T> ue = null;
		try {
			ue = q.getResultList();
		} catch (RuntimeException e) {
			tx.rollback();
		} finally {
			em.close();
		}
		return ue;

	}

}