An Honest Look at Seam – Part 1: How Seam builds on Spring

I have been spending a lot of time with JBoss Seam, and I must say, I am very excited about what I have seen. My goal in these next few posts is to talk about why I think these features of Seam are game changers and what they are saying about the direction the JEE Spec is going.

Before I dive into the topic for this post, one of my coworkers gave a brief introduction to Seam a few weeks ago. I would recommend reading it first if you are completely unfamiliar with Seam. If you are looking for a more thorough introduction (other than the reference docs), I give my recommendation goes to Seam in Action by Dan Allen. It has been my guide in my own exploration of Seam and, in spite of it’s Seam 2.0/2.1 focus, it is extremely relevant to working with Seam 2.2 today. With that out of the way, let us begin with some background.

I am a big fan of the Spring Framework. The many “glue code” utility classes and the Spring IoC Container have been staples in my applications since I started working with the Java Platform. Spring provided a way to do POJO programming in a world that was under the tyranny of the ugly EJB 2 programming model. They created an alternative to a broken standard that was very welcome. But this post is about Seam, so what’s all the Spring talk for? One of the key features, or rather the key feature, of Seam is what the Seam guys like to call Contextual Components. For those of you with a Spring background like me, a Seam Component is analogous to a Spring Bean. The contextual piece is what makes it interesting.

In Spring, you usually assume that a bean is going to be a singleton and that it is going to be stateless. That is the way of things with Spring. The common case is that everything is constructed at startup and all wired together before your application services it’s first request. You get all that glue code and DI goodness right up front. There is another way to handle your beans that is not as widely used. You can declare the scope of a bean to be Prototype, which means Spring will construct a new instance of that bean for you each time you ask your ApplicationContext for it.

Seam has taken this a step further. Instead of beans being limited to being either Stateless application singletons or construction prototypes, Seam let’s you scope components to rich set of contexts. See here for the complete list. This makes the Seam context not just a repository of pre-built service objects, but a scoped application state manager that is managed as a central concern of the framework (which is a fancy way of saying Seam takes care of the plumbing so you don’t have to). Now, instead of having a DAO object that encapsulates your data access methods, you can have a Data Mediator that not only knows how to get at an entity instance and save changes back to the database, but it also tracks the instance in the proper scope for you, so you only have to read it once from the database, even across multiple http requests. It is extremely powerful.

The other piece that Seam adds is called Outjection. No, that isn’t a typo. As part of an operation invoked on a Seam component, it not only gets all it’s dependencies injected, scoped as appropriate for the context of the components involved, but it also can send new components back to the context to be used by other parts of the application, managing the scope all the way.

What does this mean practically? No more leaky Http Sessions!

Now, if you are wondering if this is relevant to where JEE is going, take a look at JSR 299. CDI, or “Web Beans” as it has been called, is largely based on what I just explained to you. This JSR is part of JEE 6 and will available in the next update to your favorite application server. You can get this functionality now with Seam 2.2, or if you are required to “stick with the specs” when you do things, go ahead and download Weld, the JSR 299 reference implementation. Weld is what JBoss AS 6 is using to implement CDI and what Seam 3.0 will be built on.

In my next post, I’ll talk about the power of the new Conversation Context that Seam and JSR 299 introduce.