Introduction to JBoss Seam - JBoss Seam 1.0.1 正式版英文参考手册

JSF and EJB 3.0 are two of the best new features of Java EE 5. EJB3 is a brand new component model for server side business and persistence logic. Meanwhile, JSF is a great component model for the presentation tier. Unfortunately, neither component model is able to solve all problems in computing by itself. Indeed, JSF and EJB3 work best used together. But the Java EE 5 specification provides no standard way to integrate the two component models. Fortunately, the creators of both models foresaw this situation and provided standard extension points to allow extension and integration of other solutions.

Seam unifies the component models of JSF and EJB3, eliminating glue code, and letting the developer think about the business problem.

Seam provides a built-in JavaScript remoting layer for EJB3 components. AJAX clients can easily call server-side components and subscribe to JMS topics, without the need for an intermediate action layer.

Optionally, Seam integrates transparent business process management via jBPM. You won't believe how easy it is to implement complex workflows using jBPM and Seam.

Seam even allows definition of presentation tier conversation flow by the same means.

JSF provides an incredibly rich event model for the presentation tier. Seam enhances this model by exposing jBPM's business process related events via exactly the same event handling mechanism, providing a uniform event model for Seam's uniform component model.

Seam provides a uniform component model. A Seam component may be stateful, with the state associated to any one of a number of contexts, ranging from the long-running business process to a single web request.

There is no distinction between presentation tier components and business logic components in Seam. It is possible to write Seam applications where "everything" is an EJB. This may come as a surprise if you are used to thinking of EJBs as coarse-grained, heavyweight objects that are a pain in the backside to create! However, EJB 3.0 completely changes the nature of EJB from the point of view of the developer. An EJB is a fine-grained object - nothing more complex than an annotated JavaBean. Seam even encourages you to use session beans as JSF action listeners!

Unlike plain Java EE or J2EE components, Seam components may simultaneously access state associated with the web request and state held in transactional resources (without the need to propagate web request state manually via method parameters). You might object that the application layering imposed upon you by the old J2EE platform was a Good Thing. Well, nothing stops you creating an equivalent layered architecture using Seam - the difference is that you get to architect your own application and decide what the layers are and how they work together.

We are all used to the concept of declarative transaction management and J2EE declarative security from EJB 2.x. EJB 3.0 even introduces declarative persistence context management. These are three examples of a broader problem of managing state that is associated with a particular context, while ensuring that all needed cleanup occurs when the context ends. Seam takes the concept of declarative state management much further and applies it to application state. Traditionally, J2EE applications almost always implement state management manually, by getting and setting servlet session and request attributes. This approach to state management is the source of many bugs and memory leaks when applications fail to clean up session attributes, or when session data associated with different workflows collides in a multi-window application. Seam has the potential to almost entirely eliminate this class of bugs.

Declarative application state management is made possible by the richness of the context model defined by Seam. Seam extends the context model defined by the servlet spec—request, session, application—with two new contexts—conversation and business process—that are more meaningful from the point of view of the business logic.

The notion of Inversion of Control or dependency injection exists in both JSF and EJB3, as well as in numerous so-called "lighweight containers". Most of these containers emphasize injection of components that implement stateless services. Even when injection of stateful components is supported (such as in JSF), it is virtually useless for handling application state because the scope of the stateful component cannot be defined with sufficient flexibility.

Bijection differs from IoC in that it is dynamic, contextual, and bidirectional. You can think of it as a mechanism for aliasing contextual variables (names in the various contexts bound to the current thread) to attributes of the component. Bijection allows auto-assembly of stateful components by the container. It even allows a component to safely and easily manipulate the value of a context variable, just by assigning to an attribute of the component.

Optionally, Seam applications may take advantage of workspace management, allowing users to freely switch between different conversations (workspaces) in a single browser window. Seam provides not only correct multi-window operation, but also multi-window-like operation in a single window!

EJB 3.0 embraces annotations and "configuration by exception" as the easiest way to provide information to the container in a declarative form. Unfortunately, JSF is still heavily dependent on verbose XML configuration files. Seam extends the annotations provided by EJB 3.0 with a set of annotations for declarative state management and declarative context demarcation. This lets you eliminate the noisy JSF managed bean declarations and reduce the required XML to just that information which truly belongs in XML (the JSF navigation rules).

Seam components, being POJOs, are by nature unit testable. But for complex applications, unit testing alone is insufficient. Integration testing has traditionally been a messy and difficult task for Java web applications. Therefore, Seam provides for testability of Seam applications as a core feature of the framework. You can easily write JUnit or TestNG tests that reproduce a whole interaction with a user, exercising all components of the system apart from the view (the JSP or Facelets page). You can run these tests directly inside your IDE, where Seam will automatically deploy EJB components into the JBoss Embeddable EJB3 container.

Seam works in any application server that supports EJB 3.0. You can even use Seam in a servlet container like Tomcat, or in any J2EE application server, by leveraging the new JBoss Embeddable EJB3 container.

However, we realize that not everyone is ready to make the switch to EJB 3.0. So, in the interim, you can use Seam as a framework for applications that use JSF for presentation, Hibernate (or plain JDBC) for persistence and JavaBeans for application logic. Then, when you're ready to make the switch to EJB 3.0, migration will be straightforward.