Java – What use are EJBs

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I'm currently learning Jave-EE, having plenty of C++ experience and having learned Java SE. I don't understand the purpose of Enterprise Java Beans; can someone clarify this for me. I'm not interested in legacy uses: this is in the context of EJB-3.1 and Java-EE 6.

It seems that some people use them to contain the business logic, for implementing the business layer of the conventional 3-layer architecture. That separates the domain logic from the domain objects, leading to an anemic domain model. But that goes against all my OOD instincts; I agree with Martin Fowler that it is an anti-pattern. Should I relax my objections to an anemic domain model? Or do EJBs have other uses?

Best Solution

As indicated by several other answers, EJBs are perfect for implementing the service layer. They are a very modern, lightweight kind of bean in Java EE. Despite the name you cannot compare them with the draconian heavy weight EJB2 beasts that were in J2EE. Everyone agrees those were a disaster, but it's not 2002 anymore.

Ever since EJB3 (2006), EJB beans have been a perfectly fine technology.

They help a lot here by providing declarative transactions (every entry method automatically starts a transaction if one is not already in progress, although this can be changed if desired), pooling, security, locking, remoting and then some. See the following answers for some additional details:

Transactions have been explained here, but to add to this: it's not something that's only needed for highly complex, highly secure systems. I would go as far to state it's a basic requirement even when only dealing with databases. If I process a simple order, I want that the inventory and the order are both updated or both not at all. This is as basic as having PKs and FKs in your database to ensure integrity.

EJBs make it trivial to manage transactions. Without EJBs there's a lot of boilerplate code for starting, committing or rolling-back the tx.

One should also not underestimate the benefits of pooling and stubs that EJB provides. It means a bean can have a lot of EJBs injected, and you don't have to worry about them being instantiated each and every time such a bean is created. This would otherwise especially be troublesome when not all EJBs would be used every time.

Because of pooling however, only very lightweight stubs are injected, which are more akin to a kind of URLs that point to an actual instance. These cost next to nothing in terms of memory or cpu overhead to inject.

EJBs also feature annotations to declare them being Singletons, arrange their locking behavior (write locks/read locks), declaring one should be initiated at startup, allow them to manage a so-called extended persistence context (a persistence context not scoped to a TX), etc.

These are all concerns you don't want in your slim entities. In many architectures, a User object for instance is a simple data entity that I want to send across layers. I don't want my User instance to have a sendMsg() method and have a JMS resource as a dependency, so that message sending can suddenly be done from some client. I'm not really sure why people think this is somehow 'natural' and 'OOP'.

In the real world I also don't invoke a sendMsg operation on my friend Joe whenever I want to send him a postcard. Instead, I address a card and bring it to the postoffice or put it in a postbox.

I also don't invoke a bake() operation on a cake. Instead, I put the cake in an oven, etc.