EntityManager JPA null

null
java.lang.NullPointerException
	at br.com.crweb.model.persistence.GenericDAO.save(GenericDAO.java:29)
	at br.com.crweb.teste.TesteHibernateConnection.main(TesteHibernateConnection.java:21)

<?xml version="1.0" encoding="UTF-8" ?>
<persistence 
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://java.sun.com/xml/ns/persistence
						http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd"
	version="2.0" xmlns="http://java.sun.com/xml/ns/persistence">
	<persistence-unit name="pu_requisicao" transaction-type="RESOURCE_LOCAL">
		<provider>org.hibernate.ejb.HibernatePersistence</provider>
		<non-jta-data-source>java:/comp/env/jdbc/requisicaoDataSource</non-jta-data-source>
	
		<properties>

			<property name="hibernate.hbm2ddl.auto" value="update" />
			<property name="hibernate.cache.use_query_cache" value="true" />
			<property name="hibernate.show_sql" value="true" />
			<property name="hibernate.format_sql" value="true" />
		
		</properties>
		
	</persistence-unit>
</persistence>

package br.com.crweb.model.persistence;

import java.io.Serializable;
import java.util.List;

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

import org.springframework.stereotype.Repository;
import org.springframework.transaction.annotation.Transactional;

import br.com.crweb.model.util.JPAUtil;

public class GenericDAO<T> implements IDAO<T>, Serializable{
	
	private EntityManager em;
	private T obj;

	public GenericDAO(T obj) {
		this.setObj(obj);
	}
	
	public GenericDAO() {
		em = JPAUtil.abreConexao();
	}

	public void save(T obj) throws Exception {
		em.getTransaction().begin();
		em.persist(obj);
		em.getTransaction().commit();
		em.close();
	}

	public void update(T obj) throws Exception {
		em.getTransaction().begin();
		em.merge(obj);
		em.getTransaction().commit();
		em.close();
	}

	public void delete(T obj) throws Exception {
		em.getTransaction().begin();
		em.remove(obj);
		em.getTransaction().commit();
		em.close();
	}

	@SuppressWarnings("unchecked")
	public T findById(Integer id) throws Exception {
		return  (T)em.find(this.getClass(), id);
	}

	@SuppressWarnings("unchecked")
	public List<T> findAll() throws Exception {
		Query query = em.createQuery("from " + getClass().getName() + "t");
		//Query query2 = em.createQuery("select u from Usuario u");
		return query.getResultList();
	}
	public T getObj() {
		return obj;
	}
	public void setObj(T obj) {
		this.obj = obj;
	}

}

<?xml version="1.0" encoding="UTF-8"?>
<Context reloadable="true">
	<Resource name="jdbc/RequisicaoDataBase"
		auth="Container"
		type="javax.sql.DataSource"
		maxAtive="100"
		maxIdle="30"
		maxWait="10000"
		username="postgres"
		password="postgres"
		driverClassName="org.postgresql.Driver"
		url="jdbc:postgresql://localhost:5432/requisicaoDB?autoReconnect=true"/>	
</Context>

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://www.springframework.org/schema/beans 
						http://www.springframework.org/schema/beans/spring-beans-3.2.xsd">

	<bean id="dataSource" class="org.springframework.jndi.JndiObjectFactoryBean">
		<property name="jndiName" value="java:comp/env/jdbc/RequisicaoDataBase" />
		<property name="lookupOnStartup" value="true" />
		<property name="proxyInterface" value="javax.sql.DataSource" />
	</bean>
</beans>

package br.com.crweb.model.util;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

public class JPAUtil {
	
	private static EntityManagerFactory entityManagerFactory;
	private static EntityManager em;
	
	static{
		entityManagerFactory = Persistence.createEntityManagerFactory("pu_requisicao");
		em = entityManagerFactory.createEntityManager();
	}
	
	public static EntityManager abreConexao(){
		return em;
	}
	
}