问题
Based on this post http://www.adam-bien.com/roller/abien/entry/ejb_3_1_killed_the I use in my app @Named @Stateless bean for communication with a database (injecting EntityManager here) and display information on a jsf page. It's great facilitation since Java EE 5, but I have one question.
Is it secure to use such beans for maintaining a user session (shopping cart etc)? I read a book about ejb 3.0 and I know that the same stateless bean could be used with many clients.
What's the best approach to use a managed bean with all ejb features (transactions, thread safety etc)? I mean any other way than managed bean + ejb interface with implementation + ejb injection as in Java EE 5?
I use GlassFish 3.1 WebProfile
回答1:
Stateless beans cannot maintain shopping carts or session; that's what "stateless" means.
You need either a stateful EJB or to do it in the web tier. Those are the only places where session is maintained.
回答2:
Adding to the advice of duffymo; there are some additional considerations for using stateful session beans vs using the HTTP session.
The HTTP session basically has a map like structure. It's directly available to all threads (requests) that are part of the session. This makes manipulating several items a relatively unsafe action. It's possible to synchronize on the session itself, but this is a risky operation that can potentially dead-lock your entire application. The HTTP session does allow you to declare event listeners, which fire upon any kind of modification of the http session.
The Stateful session bean of course has a bean structure. It has a kind of auto synchronization feature, as only thread can be active in the bean at the same time. Via annotations you can declare if other threads wait (and if so, for how long) or immediately throw an exception in the face of concurrent access.
Where there is normally only one http session per user, a single user can make use of multiple stateful session beans at the same time. A particular advantage of stateful session beans is that they have a mechanism to passivate their state after some timeout, which can free up your server's memory (at the cost of disk space of course). Stateful session beans do not directly have the kind of event listeners that the http session does have.
I think that originally the "session" aspect of stateful session beans was to maintain a session with remote non-web clients (Swing, another AS, etc). This is much like the http session was created to maintain a session with remote web clients. Since a non-web client can request and hold on to multiple proxies for stateful session beans, the web analogy is actually more akin to that of the recently introduced conversation scope.
In the case of remote web clients talking to a server, where the server internally talks to a stateful session bean, the concepts greatly overlap. The remote web client only knows about the http session (via the JSESSIONID) and nothing about the stateful session bean's session. So if the http session was lost, you typically would not be able to connect the remote client with the specific stateful session bean again. The HTTP session in this case is thus always leading and you might as well store your shopping cart items inside a single (http) session scoped bean.
There is one specific case where stateful session beans come in handy for internal communication, and that's if you need JPA's extended persistence context. This can be used if e.g. locks on entities need to last between requests (which is possibly handy for a shopping cart if you have limited stock and don't want to confront the user with an "out of stock" message as soon as he actually checks out).
来源：https://stackoverflow.com/questions/6328794/can-i-use-ejb-stateless-bean-with-cdi-to-maintain-user-session