What is persistence contexts collision?

I'm trying to learn more on persistence in EJB, specially about persistence context. I know than a persistence context is created when a method than requires a transaction have been called and no persistence context existed before. I also know than a persistence context is destroyed when there is no method in action than requires a transaction, except if the EntityManager scope is extended. In this case, the persistence context will be destroyed when the session bean than contains the EntityManager is going to be destroyed. I read "Pro EJB 3 Java Persistence API" by Mike Keith and Merrick Schincariol. In this e-book, it mention than two persistence contexts can collide. Collisions between persistence contexts happen when we use different kinds of EntityManager. This very special part of the book brought to me a lot of questions. I want to know more about persistence context, so these are my questions :
- First of all, how many persistence contexts from the same persistence unit (EntityManagerFactory) can exist at the same time? If more than one is possible, in which context it is possible and how the propagation works when it happen?
- Second, when I'm using a new EntityManager while a persistence context created by the same persistence unit exist, is this new EntityManager will be related to the persistence context than already exists?
- Finally, what is a collision of persistence contexts? When two persistence contexts collide? What are the consequences?
- For example, if I use a stateful session bean with an extended scope EntityManager, then I call a method of this stateful session bean than call a method of a stateless session bean, and the last method use a transaction scope EntityManager, can I get a collision of two persistence contexts? I mean, will the persistence context in the stateful session bean collide with the persistence context in the stateless session bean?
Thank you in advance from your answer.
Dominic
P.S. You can answer to me in french if it's easier for you.

morphian wrote:
Hi,
I'm a total noob. I've been reading and following examples from books but I haven't read anything about how persistence really works? How does it help my session beans to access or make transactions to the database?
How does it actually work? What if I just make direct queries from my session beans without the help of the EntityManager or the Peristence API? What's the difference?
You have to code the stuff that EntityManager and Persistence are doing for you.
Like this for example:
Customer requestCustomer = em.find(Customer.class.customerId);Does it mean that the Persistence API keeps the object alive in the Customer Entity? For how long? Until the application is closed? Complicated. Not enough info.
If so, what happens if the user opens the application again and looks for that object?
Open the app and the object will be refreshed from that data store. Hence the word "persistent".
Why can't I just make a direct query to see if that customerId exists in the database?
You can.
I hope you guys don't mind my questions... I just really want to learn this stuff and I gotta let things out of my head. Hope you can help. Thanks!This isn't the way to do it, but it's a start.
%

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Is Persistence context thread safe? nop

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it is said entity manager is not thread safe, so it should not be an instance variable. But in the code above, the variable "em" is an instance variable of stateless bean B, Does it make sense to put it there using pc annotation? is it then thread safe when it is injected (manager) by container?
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So , could you please give me any guide on this problem? make me clear.
any is appreciated. thank you
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ext
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sistenceContextPropagationInterceptor.java:57)     Regards,
Paata.

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What is persistence? How does it work?

Hi,
I'm a total noob. I've been reading and following examples from books but I haven't read anything about how persistence really works? How does it help my session beans to access or make transactions to the database? How does it actually work? What if I just make direct queries from my session beans without the help of the EntityManager or the Peristence API? What's the difference?
Like this for example:
Customer requestCustomer = em.find(Customer.class.customerId);Does it mean that the Persistence API keeps the object alive in the Customer Entity? For how long? Until the application is closed? If so, what happens if the user opens the application again and looks for that object?
Why can't I just make a direct query to see if that customerId exists in the database?
I hope you guys don't mind my questions... I just really want to learn this stuff and I gotta let things out of my head. Hope you can help. Thanks!

morphian wrote:
Hi,
I'm a total noob. I've been reading and following examples from books but I haven't read anything about how persistence really works? How does it help my session beans to access or make transactions to the database?
How does it actually work? What if I just make direct queries from my session beans without the help of the EntityManager or the Peristence API? What's the difference?
You have to code the stuff that EntityManager and Persistence are doing for you.
Like this for example:
Customer requestCustomer = em.find(Customer.class.customerId);Does it mean that the Persistence API keeps the object alive in the Customer Entity? For how long? Until the application is closed? Complicated. Not enough info.
If so, what happens if the user opens the application again and looks for that object?
Open the app and the object will be refreshed from that data store. Hence the word "persistent".
Why can't I just make a direct query to see if that customerId exists in the database?
You can.
I hope you guys don't mind my questions... I just really want to learn this stuff and I gotta let things out of my head. Hope you can help. Thanks!This isn't the way to do it, but it's a start.
%

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What is persistence contexts collision?

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