The Java EE 6 Tutorial, Volume I

Using Scopes

For a web application to use a bean that injects another bean class, the bean needs to be able to hold state over the duration of the user's interaction with the application. The way to define this state is to give the bean a scope. You can give an object any of the scopes described in Table 17–1, depending on how you are using it.

Table 17–1 Scopes


Scope	Annotation	Duration
Request scope	`@RequestScoped`	A user's interaction with a web application in a single HTTP request
Session scope	`@SessionScoped`	A user's interaction with a web application across multiple HTTP requests
Application scope	`@ApplicationScoped`	Shared state across all users' interactions with a web application
Dependent scope	`@Dependent`	The default scope if none is specified; it means that an object exists to serve exactly one client (bean), and has the same lifecycle as that client (bean)
Conversation scope	`@ConversationScoped`	A user's interaction with a JavaServer Faces application, within explicit boundaries controlled by the developer that extend the scope across multiple invocations of the JavaServer Faces life cycle. All long-running conversations are scoped to a particular HTTP servlet session and may not cross session boundaries.

The first three scopes were originally defined by the Java Servlet specification. The last two are defined by JSR-299.

You can also define and implement custom scopes, but that is an advanced topic. Custom scopes are likely to be used by those who implement and extend the CDI specification.

A scope gives an object a well-defined lifecycle context. A scoped object can be automatically created when it is needed and automatically destroyed when the context in which it was created ends. Moreover, its state is automatically shared by any clients that execute in the same context.

JavaBeans components, and Java EE components such as servlets and enterprise beans, do not by definition have a well-defined scope. These components are either:

Singletons, such as Enterprise JavaBeans singleton beans, whose state is shared between all clients
Stateless objects, such as servlets and stateless session beans, which do not contain client-visible state
Objects that must be explicitly created and destroyed by their client, such as JavaBeans components and stateful session beans, whose state is shared by explicit reference passing between clients

If, however, you create a Java EE component that is a managed bean, it becomes a scoped object, which exists in a well-defined lifecycle context.

The web application for the Printer bean will use a simple request and response mechanism, so the managed bean can be annotated as follows:

import javax.inject.Inject;
import javax.enterprise.context.RequestScoped;

@RequestScoped
public class Printer {

       @Inject @Informal Greeting greeting;
    ...

Beans that use session, application, or conversation scope must be serializable, but beans that use request scope do not have to be serializable.