Spring JPA - Injecting transaction manager vs injecting entity manager

Question

If I wanted manage transactions programmatically, what is the difference between starting the transaction by injecting a PlatformTransactionManager vs directly injecting EntityMangerFactory/EntityManager and getting transaction from Entitymanager

public class MyDAO  {
@PersistenceContext(unitName="test") EntityManager em;

JpaTransactionManager txnManager = null;
public void setTxnManager(JpaTransactionManager mgr) {
  txnManager = mgr;
}

public void process(Request request) throws Exception {
  TransactionStatus status =
     txnManager.getTransaction(new DefaultTransactionDefinition());
  try {
     em.persist(request);
     txnManager.commit(status);
  } catch (Exception up) {
     txnManager.rollback(status);
     throw up;
  }
}

As apposed to injecting EntityManager directly

public class MyDAO {
    @PersistenceContext(unitName="test")
    EntityManager em;

    public void process(Request request) throws Exception {
      EntityTransaction txn = em.getTransaction();
      try {
         em.persist(request);
         txn.commit();
      } catch (Exception up) {
         txn.rollback();
         throw up;
      }
    }

where as spring config snippet looks like this

 <beans>
    <bean id="MyDAO" class="com.xyz.app.dao.MyDAO">

    <context:annotation-config />

    <bean id="entityManagerFactory" class="org.springframework.orm.jpa.LocalContainerE ntityManagerFactoryBean">
        <property name="persistenceUnitName" value="persistence" />
        <property name="dataSource" ref="dataSourceProvider" />
        <property name="jpaVendorAdapter" ref="jpaVendorAdapter" />
    </bean>

    <bean id="transactionManagerJpa" class="org.springframework.orm.jpa.JpaTransactionM anager">
          <property name="entityManagerFactory" ref="entityManagerFactory"/>
    </bean>
 </beans>

Answer 1

Transaction managers should not be injected into DAOs, because a DAO has no way to tell whether they're one participant in a larger transaction or not.

I think the transaction manager belongs with the service layer, not the persistence layer. The services know about use cases and units of work. They orchestrate other services, DAOs and model objects to fulfill their use cases.