EntityManager & Servlets

[tornike]

Hi,
I was trying to follow the examples that are given in the Geronimo documentation to understand JPA. In lot of sources I have read that EntityManager is not thread safe and it should not be injected into the servlet. (container managed one) However in the example of ejb-JPA http://cwiki.apache.org/GMOxDOC21/container-managed-persistence-with-jpa.html
it is being injected in the servlet.  
[
public class Test extends javax.servlet.http.HttpServlet
 implements javax.servlet.Servlet {

 static final long serialVersionUID = 1L;

 @PersistenceContext(unitName="AccountUnit")
 private EntityManager em;
]
I have tested the example works without problems so are there conditions when it can be done or am I understanding it the wrong way?
Thanks.

[Fredrik Jonson-3]

In <25221488.post@...> tornike wrote:

>  EntityManager is not thread safe and it should not be injected into the
>  servlet. (container managed one) However in the example of ejb-JPA
>  http://cwiki.apache.org/GMOxDOC21/container-managed-persistence-with-jpa.html
>
>  I have tested the example works without problems so are there conditions
>  when it can be done or am I understanding it the wrong way?

I'd say the example is wrong or at least careless. So I'm also interested
if someone thinks otherwise.

If I would have done it I'd inject a EntityManagerFactory in the servlet and
acquired a new EntityManager from the pool for each request to doGet method:

public class Example extends HttpServlet {
  @PersistenceUnit
  private EntityManagerFactory emf;

  @Override
  protected void doGet(HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException {
      EntityManager em = emf.createEntityManager();
      // do stuff

--
Fredrik Jonson

[tornike]

Yes, but if it's in their example maybe it's OK for Geronimo to use container managed EntityManager with the servlet and there won't be any threading issues. If it's true then we wouldn't need any EJB's in the project and simply communicate to JPA entities even when using container managed EntityManager. As far as I know this method is discouraged but if it's ok to do in Geronimo it would greatly simplify many projects.

Fredrik Jonson-3 wrote:

In <[e-mail]> tornike wrote:

>  EntityManager is not thread safe and it should not be injected into the
>  servlet. (container managed one) However in the example of ejb-JPA
>  http://cwiki.apache.org/GMOxDOC21/container-managed-persistence-with-jpa.html
>
>  I have tested the example works without problems so are there conditions
>  when it can be done or am I understanding it the wrong way?

I'd say the example is wrong or at least careless. So I'm also interested
if someone thinks otherwise.

If I would have done it I'd inject a EntityManagerFactory in the servlet and
acquired a new EntityManager from the pool for each request to doGet method:

public class Example extends HttpServlet {
  @PersistenceUnit
  private EntityManagerFactory emf;

  @Override
  protected void doGet(HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException {
      EntityManager em = emf.createEntityManager();
      // do stuff

--
Fredrik Jonson

[Jay D. McHugh-3]

Sorry for the late reply to this.

Geronimo should not allow you to add container managed entity managers
to servlets.  They would not be thread safe.

I haven't looked at the example in a while - but I believe it is using a
stateless session bean to access the entity manager.  That is perfectly
acceptable (and correct)...

I just took a look at the example and it does in fact use a stateless
bean (AccountBean).  So, that is why it is able to use container managed
persistence.

I don't think that it would actually make projects simpler to move the
data access logic into each of the servlets that need it.  For me at
least, centralizing the logic into session beans means that I am forced
to consider exactly what I will be doing with the data and only write
the code once to do it.

Hope that helps,

Jay

tornike wrote:

> Yes, but if it's in their example maybe it's OK for Geronimo to use container
> managed EntityManager with the servlet and there won't be any threading
> issues. If it's true then we wouldn't need any EJB's in the project and
> simply communicate to JPA entities even when using container managed
> EntityManager. As far as I know this method is discouraged but if it's ok to
> do in Geronimo it would greatly simplify many projects.
>
>
>
> Fredrik Jonson-3 wrote:
>> In <25221488.post@...> tornike wrote:
>>
>>>  EntityManager is not thread safe and it should not be injected into the
>>>  servlet. (container managed one) However in the example of ejb-JPA
>>>
>>> http://cwiki.apache.org/GMOxDOC21/container-managed-persistence-with-jpa.html
>>>
>>>  I have tested the example works without problems so are there conditions
>>>  when it can be done or am I understanding it the wrong way?
>> I'd say the example is wrong or at least careless. So I'm also interested
>> if someone thinks otherwise.
>>
>> If I would have done it I'd inject a EntityManagerFactory in the servlet
>> and
>> acquired a new EntityManager from the pool for each request to doGet
>> method:
>>
>> public class Example extends HttpServlet {
>>   @PersistenceUnit
>>   private EntityManagerFactory emf;
>>
>>   @Override
>>   protected void doGet(HttpServletRequest request, HttpServletResponse
>> response)
>>     throws ServletException, IOException {
>>       EntityManager em = emf.createEntityManager();
>>       // do stuff
>>
>> --
>> Fredrik Jonson
>>
>>
>>
>

[Fredrik Jonson-3]

Jay D. McHugh wrote:

>  Geronimo should not allow you to add container managed entity
>  managers to servlets. They would not be thread safe.

Then please examine the example again. You're correct that there's a
stateless bean acting as a accessor in _part_ of the example. But
there is also a servlet class called Test - search for Test.java in
that page.

http://cwiki.apache.org/GMOxDOC21/container-managed-persistence-with-jpa.html

You'll see that the servlet Test has a injected EntityManager. Now, I
didn't notice before but the servlet actually only accesses the entity
manager in a UserTransaction which it looks up per request. I'm not
sure if and how the transaction would make access to the single
instance entity manager thread safe though?

The blog linked below seems to suggest that even when using user
transactions - which on the other side seems very servlet injection
friendly - it is still wrong to share a entity manager instance in
servlets.

http://weblogs.java.net/blog/ss141213/archive/2005/12/dont_use_persis_1.html

--
Fredrik Jonson

[tornike]

Thanks for replying to my question.
I have figured out how EJB works with servlets now, I am not a very experienced developer and I learn through examples. I inject session beans (@EJB) into my servlets and database manupulations are kept in session beans.
While reading about these issues I came across Java 6EE where as I understood EJBs are introduced in web container as web beans. Should be easier to figure out for newcomers like me:)
Thanks

Jay D. McHugh-3 wrote:

Sorry for the late reply to this.

Geronimo should not allow you to add container managed entity managers
to servlets.  They would not be thread safe.

I haven't looked at the example in a while - but I believe it is using a
stateless session bean to access the entity manager.  That is perfectly
acceptable (and correct)...

I just took a look at the example and it does in fact use a stateless
bean (AccountBean).  So, that is why it is able to use container managed
persistence.

I don't think that it would actually make projects simpler to move the
data access logic into each of the servlets that need it.  For me at
least, centralizing the logic into session beans means that I am forced
to consider exactly what I will be doing with the data and only write
the code once to do it.

Hope that helps,

Jay

tornike wrote:
> Yes, but if it's in their example maybe it's OK for Geronimo to use container
> managed EntityManager with the servlet and there won't be any threading
> issues. If it's true then we wouldn't need any EJB's in the project and
> simply communicate to JPA entities even when using container managed
> EntityManager. As far as I know this method is discouraged but if it's ok to
> do in Geronimo it would greatly simplify many projects.
>
>
>
> Fredrik Jonson-3 wrote:
>> In <[e-mail]> tornike wrote:
>>
>>>  EntityManager is not thread safe and it should not be injected into the
>>>  servlet. (container managed one) However in the example of ejb-JPA
>>>
>>> http://cwiki.apache.org/GMOxDOC21/container-managed-persistence-with-jpa.html
>>>
>>>  I have tested the example works without problems so are there conditions
>>>  when it can be done or am I understanding it the wrong way?
>> I'd say the example is wrong or at least careless. So I'm also interested
>> if someone thinks otherwise.
>>
>> If I would have done it I'd inject a EntityManagerFactory in the servlet
>> and
>> acquired a new EntityManager from the pool for each request to doGet
>> method:
>>
>> public class Example extends HttpServlet {
>>   @PersistenceUnit
>>   private EntityManagerFactory emf;
>>
>>   @Override
>>   protected void doGet(HttpServletRequest request, HttpServletResponse
>> response)
>>     throws ServletException, IOException {
>>       EntityManager em = emf.createEntityManager();
>>       // do stuff
>>
>> --
>> Fredrik Jonson
>>
>>
>>
>