Why Stateless session beans?

问题:

I was reading about stateless session bean and couldn't understand it's use.

Excerpt from sun tutorial below

"..Because stateless session beans can support multiple clients, they can offer better scalability for applications that require large numbers of clients"

Where stateless session bean is being used? what kind of applications use it?

What mechanism has been being used before the advent of 'stateless session bean' to support multiple clients in the similar contexts?

Can anyone please provide some details?

thank you!

回答1:

To be honest, it is hard to find any reasonable use case for SLSBs. Since they don't hold any state (as the name imposes), they should be inherently thread-safe. Even though they are pooled by the container.

On the other hand it is tempting to use them as a safe temporary storage as they are guaranteed to be thread-safe (thanks to pooling), you don't need any synchronization or thread-safe collections. But consider the following pseude-code:

@Stateless
public class Slsb {
  private int counter;

  public void increment() {
    ++counter;
  }

  public int getCounter() {
    return counter;
  }
}

Client-side:

@Resource
private Slsb slsb;

public void clientMethod() {
  slsb.increment();
  slsb.increment();
  slsb.getCounter();  //???
}

This code (despite its vulgarity) is perfectly fine and it does not require AtomicInteger for instance.

What result do you expect? Actually, any non-negative value is possible... Any call to slsb might be served by different instance of Slsb and in the meantime your (previously used) instance might have been used to serve different clients. Conclusion: storing state in SLSB is wrong, but for some reason SLSBs are pooled to avoid threading issues when changing the state (?!?). Personally I much prefer singleton services (Spring-like) and I have never got the SLSB idea. And yes, I am aware of singleton EJBs in EJB 3.1.

回答2:

Having used EJB 3.0, in my opinion Stateless Session beans are there as to complete the Enterprise Bean landscape. They indeed are there to setup a Facade to the rest of your business logic. People often suggest SLSB's to be threadsafe, but this is misleading to say the least.

They are definitely not thread safe when their codepath includes calling non-threadsafe code (eg. a shared non threadsafe cache).The only guarantee that SLSLB give is that a single SLSB instance is used by at most one thread at the same time. This basically boils down to SLSB's having synchronized method access, and that there will be multiple instances to serve client calls. But having a SLSB method calling code from a shared non-thread safe class from these multiple instance could still wreak havoc and would render the SLSB in question non-threadsafe.

Since EE contexts (transactions , security resources etc) are bound to the thread already I see no need for SLSB over say Spring Singletons. They do complement Statefull Session beans in an EJB-only application.

In my opinion the route they choose with SLSB's and the new lock concurrency settings for EJB 3.1 is an attempt to dumb down the programmer and have the Mighty Container serve your needs. Do yourself a favor and go read Java Concurrency in Practice and start using singletons combined with stock java thread concurrency constructs. (synchronized, volatile, concurrent collections etc.)

回答3:

First Stateless session beans (SLSB) are a server side technology - you don't use them within swing code for example.

But they are for example used as a "Facade" for many clients that connect to central servers. SLSB contain business logic and can then e.g. call into databases.

As SLSB can be pooled, you do not need one SLSB per client, but only one per simultaneous client. When the SLSB is not used, it is put back into the pool and can then be used for the next client.

As the SLSBs are 'hosted' in a container, they are thread safe and the container does a lot of the heaving lifting for you: transactions, security, resource injection and so on.

回答4:

In contrary to what most answers here let you believe, statelessness has nothing to do with threadsafety of the class itself. That's absolutely not the primary goal of @Stateless. The developer itself is still responsible for that the class representing a @Stateless EJB doesn't have any instance variables declared (i.e. no state). The container isn't responsible for that part. Basically, the developer must say "Hey container, here's a stateless business service class, I'll annotate it with @Stateless so that you can use it as a stateless EJB" and thus not the other way round or so.

If you want state, then use a @Stateful which will be newly recreated every time the client obtains it (so, if the client is e.g. a view scoped JSF managed bean, then the EJB will live as long as that bean, or if the client is e.g. a session scoped CDI managed bean, then the EJB will live as long as that bean, etc). Or, use a @Singleton which is basically application scoped and actually thread locked.

Statelessness and pooling has more to do with threadsafety of transactions. You probably already know that a single method call on a @Stateless counts by default as a single full transaction. However, that transaction in turn requires a thread lock on the specific EJB instance, because of all the sensitive pre- and post processing work. So the EJB could basically block all other clients wanting to call the same method until the transaction is committed. That is exactly why they are cloned and pooled on demand.

Do note that a @Singleton is not pooled and is by default actually thread locked. You should by now understand that a @Singleton is by default absolutely not faster than a @Stateless when (ab)used as a "stateless EJB". See also a.o. Java EE 7 tutorial "Managing concurrent access in a singleton session bean".

See also:

• When is it necessary or convenient to use Spring or EJB3 or all of them together?
• JSF request scoped bean keeps recreating new Stateful session beans on every request?
• When using @EJB, does each managed bean get its own @EJB instance?
• EJB @Asynchronous to retrieve real time inserted row in JSF seems to be thread locked

回答5:

From a non-EJB technology specific standpoint, Stateless Session Beans are infrastructure code that obviously does not hold any state but passes objects that have state. State can beheld by your Stateful Session Beans or Domain POJOs in other implementations outside an EJB. As the comment above states, it is an entry point or facade to your business layer.

In a java web application, you can design by layer such as Controller, Service class (SLSB or just a plain Java interace) then DAO or whatever to call a database/backend.

EJB however does some automatic lifting of boiler plate such as transactions, security and such.

回答6:

Stateless object will enable you to loosely couple itself from the client and thus allows you to scale out easily.

Stateless session beans (SLB) are server side (EJB) components which are used to abstract your business logic. Because of this very nature of stateless ness you can easily deploy your SLBs in a different container which is not running on top of the same JVM. And as per your requirement you can have one or more containers running these beans.